The document discusses the properties, types, and historical development of dental porcelain and ceramic materials, emphasizing porcelain's aesthetic qualities and various crystalline phases that enhance its mechanical and optical properties. It outlines the evolution of ceramic techniques and innovations used in dentistry, from early forms to modern applications like glass-ceramics and hot-pressed ceramics. Additionally, it classifies ceramics based on composition, processing methods, and firing temperatures, detailing the importance of thermal compatibility in metal-ceramic prostheses.

1. INTRODUCTION
A ceramic so white that it was comparable only to snow, so strong that vessels
needed walls only 2–3 mm thick and consequently light could shine through it. So
continuous was the internal structure that a dish, if lightly struck would ring like a
bell.
This is porcelain!
It could be said that the ceramic material known as porcelain holds a special
place in dentistry because, not withstanding the many advances made in composites
and glass–ionomers, it is still considered to produce aesthetically the most pleasing
result. Its colour, translucency and vitality cannot as yet be matched by any material
except other ceramics. Most ceramics for metal-ceramic restorations contain from 15
to 25 vol% leucite as their major crystalline phase, but changes in the leucite volume
fraction can occur during thermal treatment of dental porcelains (Mackert and Evans,
1991a,b). Leucite is a potassium alumino-silicate with a high thermal expansion
coefficient (Mackert et al, 1986a). Materials for all-ceramic restorations use a wider
variety of crystalline phases as reinforcing agents and contain up to 90% by volume of
crystalline phase. The nature, amount, and particle size distribution of the crystalline
phase directly influence the mechanical and optical properties of the material (Morena
et al, 1986b; Kon et al, 1994). The match between the refractive indices of the
crystalline phase and glassy matrix is a key factor for controlling the translucency of
the porcelain. Similarly, the match between the thermal expansion coefficients of the
crystalline phase and glassy matrix is critical in controlling residual thermal stresses
within the porcelain (Mackert, 1988). The first glass-ceramics were developed in the
late 1950s (Stookey, 1959). Glass-ceramics are polycrystalline solids prepared by the
controlled crystallization of glasses" (McMillan, 1979). The crystallization is
achieved when the glass is submitted to a heat treatment during which crystal
nucleation and growth are thermodynamically possible. Proper control of the
crystallization heat treatment is necessary to ensure the nucleation of a sufficient
number of crystals and their growth to an effective size. The dual nature of glass-
ceramic materials confers upon them the esthetic, mechanical, and chemical qualities
of ceramics as well as the ability to be cast and processed as glasses. These
characteristics are of great interest for dental applications. Machinability is another
property desirable for the maximum utility of glassceramics as dental materials. The
ability of glass-ceramics to be machined is closely related to the nature and particle
size of the crystalline phase that develops during the crystallization heat treatment
(Utsumi and Sakka, 1970). Machinable glass-ceramics for industrial as well as dental
applications often contain mica as a major crystalline phase.
Hot-pressed ceramics constitute another application of high technology to
dentistry. This process relies on the application of external pressure at elevated
temperatures to obtain sintering of the ceramic body. Hotpressed ceramics are also
called "heat-pressed" ceramics. Hot-pressing classically helps avoid large pores
caused by non-uniform mixing. It also prevents extensive grain growth or secondary
crystallization, considering the temperature at which sintering is obtained. The
mechanical properties of many ceramic systems are maximized with high density and
1

2. small grain size. Therefore, optimum properties can be obtained by hot-pressing
techniques (Kingery et al, 1976).
In spite of their excellent esthetic qualities and their good biological
compatibility, dental ceramics, like all ceramic materials, are brittle. They are
susceptible to fracture at the time of placement or during function.
2

3. HISTORICAL PERSPECTIVE
Ceramics are the earliest group of inorganic materials to be structurally
modified by man and his early history is principally traced through these materials.
The origin of glazing techniques is probably the most interesting advancement.
Ceramic objects have been constructed for thousands of years. The earlier
techniques usually consisted of shaping the item in clay or soil and then backing it to
fuse the particles together. The initial attempts resulted primarily in coarse and some
what porous products, such as goblets and other forms of pottery. Later developments
led to unite detailed stone ware items.
1774: Nicholas Dubais de Chemant, a surgeon dentist of Paris is credited with
making porcelain dentures.
1791: Dechemont – obtains both French and English patent for dental porcelain.
1792: John Woodforde – manufactured porcelain pastes
1808: Giuseppangelo Fonzi – An Italian dentist produced porcelain metal backed
artificial teeth.
1850: First commercial production of porcelain denture teeth by white.
1860: Introduction of tube tooth and pivot crown in England.
1889: Porcelain inlays and jacket crowns introduced platinum matrix for fusing
porcelain inlays and crowns developed by Land in USA.
1903: Dr. Hugh Avery – Introduced new porcelain inlay technique
1905: Electric porcelain furnace
1908: Dr. A. Eschneider – Baked porcelain jacket crown
1923: Casting of dental porcelain for inlays and crown by lost technique.
1925: Dr. Albert LE Gro’s used porcelain by high fusing method. Jan Adrianasen –
pioneered the technique of building up porcelain with a brush.
1940: Vacuum firing of dental porcelain
1942: Fluorescent dental porcelain introduced
1962: Gold alloy for porcelain bonding were used
1963: Development of dental aluminous porcelain by McLean and Hughes
1968: Use of photosensitive glass ceramic in dentistry by Macclloch
1974: Palladium silver alloy introduced for porcelain fused to metal
1976: Platinum bonded alumina crown was used by McLean and Seed.
1983: High expansion core material by O’Brien
1984: First commercial castable dental glass ceramic
The earliest glazing technique was a Summerian invention made famous about
4000 BC as a Egyptian blue faience. More than 10,000 years ago stone age people
also used ceramics.
As early as the second half of the eighteen century, Fauchard and others
attempted to use porcelain for dental applications. Their efforts, working in the
demanding and potentially destructive intraoral environment, were largely
unsuccessful.
Porcelain was, however, successfully used for dental prosthesis by the end of
the 1800s, when the technique to fire all porcelain jacket crowns on a platinum matrix
was first developed but it was not until the mid 1950s that a dental porcelain was
3

4. developed with a coefficient of thermal expansion similar to that of existing dental
casting alloys.
4

5. TERMINOLOGIES
Alumina core: A ceramic containing sufficient crystalline alumina (Al2O3) to achieve
adequate strength and opacity when used for the production of a core for ceramic
jacket crowns.
Aluminous porcelain: A ceramic composed of a glass matrix phase and 35 vol% of
more of Al2O3.
CAD-CAM ceramic: A machinable ceramic material formulated for the production
of inlays and crowns through the use of a computer aided design, computer aided
machining process.
Castable dental ceramic: A dental ceramic specially formulated to be cast using a
lost wax process.
Ceramic: A compound of metallic and nonmetallic elements.
Ceramic, dental: A compound of metals (such as aluminium, calcium, lithium,
magnesium, potassium, sodium, tin, titanium, and zirconium) and non metals (such as
silicon, boron, fluorine, and oxygen) that may be used as a single structural
component, such as when used in a CAD-CAM inlay, or as one of several layers that
are used in the fabrication of a ceramic based prosthesis. Dental ceramics are
formulated to provide one or more of the following properties, castability,
moldability, injectability, color, opacity, translucency, machinability, abrasion
resistance, strength and toughness.
Note: All porcelains and glass ceramics are ceramics, but not all ceramics are
porcelains or glass ceramics.
Ceramic jacket crown (CJC): An all ceramic crown without a supporting metal
substrate that is made from a ceramic with a substantial crystal content (> 50 vol%)
from which its higher strength and/or toughness is derived. These crowns are
distinguished from porcelain jacket crowns that are made with porcelain to produce an
aesthetic porcelain margin as an alternative to a metal margin on a metal ceramic
crown.
Sintering: The process of heating closely packed particles to achieve interparticle
bonding and sufficient diffusion to decrease the surface area or increase the density of
the structure. For products such as In-Ceram and In-Ceram Spinel, surface contact
sintering and minimal density change are required.
Spinel or Spinelle: A hard crystalline mineral (MgAl2O4) consisting of magnesium
and aluminium. Also, any of a group of mineral oxides of ferrous iron, magnesium,
manganese or zinc.
5

6. Stain: A mixture of one or more pigmented metal oxides and usually a low fusing
glass that when dispersed in an aqueous slurry or monomer medium, applied to the
surface of porcelain or other specialized ceramic dried or light cured and fired, will
modify the shade of the ceramic based restoration. One product is supplied in a light
curable binder. These stain product are also called surface colorants or
characterization porcelains.
Thermal compatibility: The desirable condition of low transient and residual tensile
stress in porcelain adjacent to a metal coping that is associated with a small difference
in the thermal contraction coefficients between the metal and the veneering
porcelains. The contraction coefficient of the metal should be slightly greater than that
of the porcelains so that residual axial and knoop compressive stresses are produced.
This condition will ensure the cooling of metal ceramic prostheses without immediate
crack formation or delayed fracture caused by residual tensile stresses in porcelain.
6

7. CLASSIFICATION OF CERAMICS:
1. By content :
- Regular feldspathic porcelain
- Aluminous porcelain
- Leucite reinforced porcelain
- Glass infiltrated alumina
- Glass infiltrated spinel
2. By use:
- Artificial teeth
- Core ceramic
- Veneer ceramic
3. By processing method :
- Sintering
- Casting
- Machining
4. By their firing temperature :
- High fusing : 1300 c
- Medium fusing : 1100-13000 c
- Low fusing : 850 – 1100 c
- Ultra low fusing : less than 850C
5. By method of firing ;
- Air fired
- Vaccum fired
- Diffusable gas
6. By their area of application :
- Core porcelain
- Body dentin porcelain
- Gingival dentin porcelain
- Incisal enamel
COMPOSITION OF CERAMICS:
1. Feldspar:
 When mixed with metal oxides and fired, it forms a glass phase that is able to
soften and flow slightly
 This softening of glass allows porcelain particles to coalesce together. This is
called sinteringsintering
 Seen in concentration of 75-85 %.
2. Kaolin / clay:
 It acts as the binder.
 When mixed with water , it forms a sticky mass which allows unfired
porcelain to be easily worked and molded.
 On heating it reacts with feldspar and gives rigidity.
 Its white in color and reduces translucency .so its added only in
concentration of 4-5 %.
7

8. 3. Quartz:
 It imparts more strength, firmness and translucency.
 It gives stability of mass during heating by providing a frame work.13-14%
• GLAZES:
It decreases pores on the surface of fired porcelain.
• This increases strength by decreasing the crack propagation.
if glaze is removed by grinding, the transverse strength is half of glazed porcelain.
1. Self glazing:
 External glaze layer is not applied here.
 The completed restorations is subjected to glazing here.
2. Add on glazes:
 External glaze layer is applied here.
 They are uncolored glasses whose fusing temperature is lowered by the
addition of glass modifiers.
Disadvantages: Low chemical durability, difficulty to apply evenly, difficult to
get exact surface characteristics.
6. Colouring agents:
 These coloring pigments are produced by fusing metallic oxides together with
fine glass and feldspar -Ex : iron / nickel oxides- brown , copper oxides-
green, titanium oxide –yellowish brown, cobalt oxide – blue.
7. Opacifying agents:
a. Opacifying agents consists of a metal oxide ground to a very fine particle
size. ex :cerium oxide, titanium oxide, zirconium oxide –most popular.
8. Stains:
 These powder is mixed with water and the wet mix is applied with brush to
the surface of porcelain before glazing.
 Internal staining is preferred as it gives life like results and prevents direct
damage to stains by surrounding environment.
9. Glass former:
Glass formers are silica.
1. Crystalline quartz
2. Crystalline cristobalite
3. Crystalline tridymite
4. Non crystalline fused silica
The vitreous matrix is made of silicate glass. Silica forms sio4 ions with
oxygen and is thus highly charged and fills the space between 4 oxygen atoms.the
tetrahydra must permit sharing of oxygen atoms to permit the formation of sio4
groups thus resulting in polymerization and a three dimensional network.
10. Glass modifiers :
 Potassium oxide, Sodium oxide, Calcium oxide are used as glass modifiers
 They act as fluxes by lowering the softening temperature of a glass
8

9.  When sodium oxide is introduced, instead of bridging the atoms together, it
contributes a oxygen atom which act as a non bridging oxygen and as a result
a gap is produced in the sio4 network. So the silica tetrahydra thus obtained is
able to move more easily at lower temperature than the earlier network.
11. Intermediate oxides :
 Glass modifiers reduces the viscousity of porcelain.
 It needs a high viscosity as well as low firing temperature. This is done by
the addition of Aluminium oxide and boric oxides.
The composition of the ceramic generally corresponds to that of the glasses in table,
except for an increased alkali content. The addition of greater quantities of soda,
potash, and/or leucite is necessary to increase the thermal expansion to a level
compatible with the metal coping. The opaque porcelains also contain relatively large
amounts of metallic oxide opacifiers to conceal the underlying metal and to minimize
the thickness of the opaque layer.
The high contraction porcelains have a greater tendency to devitrify because
of their alkali content. They should not be subjected to repeated firing, because this
may increase the risk for cloudiness within the porcelain, as well as changes in the
thermal contraction behavior. Thus it is obvious that a proper matching of the
properties of the alloy and porcelain is imperative to success. Criteria and test
methods for determining metal porcelain compatibility have been suggested. Testing
methods are focused on the measurement of coefficients of thermal expansion and
contraction, thermal shock resistance, and the strength of the bond, which are
discussed later.
Conventional dental porcelain is a vitreous ceramic based on a silica (SiO2)
network and potash feldspar (K2O.Al2O3.6SiO2) or soda feldspar (Na2O.Al2O3.6SiO2)
or both. Pigments, opacifiers, and glasses are added to control the fusion temperature,
sintering temperature, thermal contraction coefficient, and solubility. The feldspars
used for dental porcelains are relatively pure and colorless. Thus pigments must be
added to produce the hues of natural teeth or the color appearance of tooth-colored
restorative materials that may exist in adjacent teeth.
Silica (SiO2) can exist in four different forms: crystalline quartz, crystalline
cristobalite, crystalline tridymite, and noncrystalline fused silica. Fused silica is a
material whose high-melting temperature is attributed to the three - dimensional
network of covalent bonds between silica tetrahedral, which are the basic structural
the temperature required to sinter the porcelain powder particles together at low
enough temperatures so that the allow to which it is fired does not melt or sustain sag
(flextural creep).
Glass Modifiers:
The sintering temperature of crystalline silica is too high for use in veneering
aesthetic layers bonded to metal substrates. At such temperatures the alloys would
melt. In addition, the thermal contraction coefficient of crystalline silica is too low
for these alloys. Bonds between the silica terahedra can be broken by the addition of
9

10. alkali metal ions such as sodium, potassium, and calcium. These ions are associated
with the oxygen atoms at the corners of the tetrachedra and interrupt the oxygen
silicon bonds. As a result, the three-dimensional silica network contains many linear
chains of silica tetrahedral that are able to move more easily at lower temperatures
than the atoms that are locked into the three-dimensional structures of silica
tetrahedral. This ease of movement is responsible for the increased fluidity
(decreased viscosity), lower softening temperature, and increased thermal expansion
conferred by glass modifiers. Too high a modifier concentration, however, reduces
the chemical durability (resistance to attack by water, acids, and alkalis) of the glass.
In addition, if too many tetrahedral are disrupted, the glass may crystallizer (devitrify)
during porcelain firing operations. Hence, a balance between a suitable melting range
and good chemical durability must be maintained.
Manufactures employs glass modifiers to produce dental porcelains with
different firing temperatures. Dental porcelains are classified according to their firing
temperatures. A typical classification is as follows:
High fusing 13000
C (23720
F)
Medium fusing 11010
- 13000
C ( 20130
- 20720
F)
Low fusing 8500
- 11000
C ( 15620
- 20120
F)
Ultra – low fusing < 8500
C (15620
F)
The medium – fusing and high – fusing types are used for the production of
denture teeth. The low – fusing ultraslow – fusing porcelains are used for crown and
bridge construction. Some of the ultraslow – fusing porcelains are used for titanium
and titanium alloys because of their low contraction coefficients that closely match
those of these metals and because the low firing temperatures reduce the risk for
growth of the metal oxide. However, some of these ultraslow – fusing porcelains
conation enough leucite to raise porcelains. The potential advantage of ultraslow-
fusing veneering ceramics are the reduction in sintering times, decrease in sag
deformation of FPD frameworks, less thermal degradation of ceramic firing ovens,
and less wear of opposing enamel surfaces.
Because commercial dental laboratories do not fabricate denture teeth for
complete denture or removable partial dentures, it has become more common to
classify crown and bridge porcelains as high – fusing (850 - 11000
C) and low – fusing
(> 8500
C). However, this change in classification has not been universally adopted
Thus, to avoid confusion, the sintering temperature range should be identified (at least
initial) in discussions between dentists and dental technicians so that the less –
abrasive benefit claimed for ultraslow – fusing porcelains that were used exclusively
between the 1960s and 1990s.
Because it ensures adequate chemical durability, self – glazing of porcelain is
preferred to an add – on glaze. A thin external layer of glassy material is formed
during a self – glass phase and settling of crystalline particles within the surface for an
applied glaze procedure contains more glass modifiers and thus has a lower firing
10

11. temperature. However, a higher proportion of glass modifiers tends to reduce the
resistance of the applied glazes to leaching by oral fluids.
Another important glass modifier is water, although it is not an intentional
addition to dental porcelain. The hydronium ion, H3O+,
can replace sodium or other
metal i8o0ns in a ceramic that contains glass modifiers. This fact accounts for the
phenomenon of “slow crack growth” of ceramics that are exposed to tensile stresses
and moist environments. It also may account for the occasional long-term failure of
porcelain restorations after several years of service.
Feldspathic Porcelains
Potassium and sodium feldspar are naturally occurring minerals composed
primarily of potash (K2O) and soda (Na2O), respectively. The also contain alumina
(Al2O3) , and silica (SiO2) components. Feldspars are used in the preparation of man
dental porcelains designed for metal-ceramic crowns and many other dental glasses
and ceramics. When potassium feldspar is mixed with various metal oxides and
fired to high temperatures, lit can form leucite and a glass phase that will soften and
flow slightly. The softening of this glass phase during porcelain firing allows the
porcelain powder particles coalesce is called liquid-phase sintering, a process
controlled by diffusion between particles at a temperature sufficiently high to form a
dense solid. The driving force for sintering is the decrease in energy caused by a
reduction in surface area. As explained in the key terms section, section, three dental
products (In-ceram Alumina, spinell, and Zirconia) are slightly sintered to produce
interconnected pore channels that are necessary for subsequent glass infiltration.
Another important property of feldspar is its tendency to form the crystalline
mineral leucite when melted. Leucite is a potassium-aluminum-silicate mineral with a
large coefficient of thermal expansion (20to25ppm/o
C) compared with feldspar
glasses (which have coefficients of thermal expansion less than 10ppm/o
C). When
feldspar is heated at temperatures between 1150 o
C and 1530 it undergoes incongruent
melting to form crystals of leucite in a liquid glass. Incongruent melting is the process
by which one material melts to form a liquid plus a different crystalline material. This
tendency of feldspar to form leucite during incongruent melting is used to advantage
in the manufacture of porcelains for metal bonding. Further information is provided in
the sintering of porcelain section.
Man dental glasses do not contain leucite as a raw material. Since feldspar is
not essential as a precursor to the formation of leucite, as described earlier, these
glasses are modified with additions of leucite to control their thermal contraction
coefficients.
Feldspathic porcelains contain a variety of oxide components, including SiO2
(52-62 wt% ) , AlOO (11-16 wt%),k2O(9-11 wt%), Na2 O (5-7 wt%), and certain
additives, including Li2 O and B2O3. These ceramics are called porcelains because
they contain a glass matrix and one more crystal phases. They cannot be classified as
glass – ceramics because crystal formation does not occur through controlled
nucleation and crystal formation and growth. there are four types of veneering
ceramics. These include (1) low-fusing ceramics (feldspar – based porcelain and
nepheline senate- based porcelain); (2) ultra low-fusing ceramics (porcelains and
11

12. glasses); (3) stains; and (4) glazes ( self – glaze and add – on glaze). The particle type
and size of crystal particles, if present, will greatly influence the potential abrasives of
the ceramic prosthesis.
The thermal expansion coefficients of some ultraslow – fusing ceramics
(sintering temperatures be below 8500
C) and low-fusing ceramics are listed. These
ultra low-fusing ceramics represent an exciting new fail of ceramic core and
veneering materials because of their microstructural features. The ontain either a
well-distributed dispersion of small crystal particles or few or no crystals, depending
on the whether the ceramic is to be used as a veneer or glaze. Initial results of wear
studies are promising in several cases relative to reduced enamel wear caused these
ceramics. These results are summarized in a later section of this chapter (see wear of
Enamel by ceramic Products and Other Restorative Materials).
Other Additives:
Other metallic oxides can be introduced, as indicated in Table 21-1. Boric oxide
(B2O3) behaves as a glass modifier, that is, it decreases viscosity, lowers the softening
temperature and forms its own glass network. Because boric oxide forms a separate
lattice interspersed with the silica lattice, it still interrupts the more rigid silica
network and lowers the softening point of the glass. Alumina is not considered a true
glass former by itself because of the dimensions of the ion and the oxygen/aluminum
ratio. Nevertheless, it can take part in the glass network to alter the softening point
and viscosity.
1. Pigmenting oxides are added to obtain the various shades needed to
simulate natural teeth. These coloring pigments are produced by fusing metallic
oxides together with fine glass and feldspar and then regrinding to a powder.
These powders are blended with the unpigmented powdered frit to provide the
proper hue and chrome. Examples of metallic oxides and their respective color
contributions to oxide (yellowish brown) manganese oxide (lavender), and oxide
(green) titanium oxide (yellowish brown), manganese oxide (lavender), and
cobalt oxide (blue). Opacity may be achieved by the addition of cerium oxide,
zirconium oxide, titanium oxide, or tin oxide.
2. Composition:
Dental porcelains are essentially mixture of fine particles of Feldspar and
quartz. However the general trend towards the use of less kaolin (clay) with an
increase in the feldspar content in order to improve translucency suggests that dental
porcelain should be more correctly described as glasses. The feldspar melts first to
provide a glossy matrix for the quartz. The quartz thus act as a filler to provide
strength. The quartz may be replaced by alumina (Al2O3) such a material is referred to
as alumonous porcelain
Low fusing dental porcelain:
Oxide Weight %
SiO2 69.36
B2O3 7.53
12

13. CaO 1.85
K2O 8.33
Na2O 4.81
Al2O3 8.11
13

14. Medium fusing dental porcelain:
Oxide Weight %
SiO2 64.20
B2O3 2.80
K2O 8.20
Na2O 1.90
Al2O3 19.00
Ci2O 2.1
MgO 0.5
P2O5 0.7
14

15. Composition of dental ceramics for fusing to high temperature alloys:
Compound
Biodent
opaque
BG 2 (%)
Ceramco
opaque 60
(%)
VMK
opaque
131 (%)
Biodent
dentin BD
27 (%)
Ceramco
dentin T 69
(%)
SiO2 52.0 55.0 52.4 56.9 62.2
Al2O3 13.55 11.65 15.15 11.80 13.40
CaO - - - 0.61 0.98
K2O 11.05 9.6 9.9 10.0 11.3
Na2O 5.28 4.75 6.58 5.42 5.37
TiO2 3.01 - 2.59 0.61 -
ZrO2 3.22 0.16 5.16 1.46 0.34
SnO2 6.4 15.0 4.9 - 0.50
Rb2O 0.09 0.04 0.08 0.10 0.06
BaO 1.09 - - 3.52 -
ZnO - 0.26 - - -
UO3 - - - - -
B2O3, CO2
and H2O
4.31 3.54 3.24 9.58 5.85
15

16. PROPERTIES
GENERAL PROPERTIES OF CERAMICS
1. PHYSICAL PROPERTIES OF PORCELAIN
Strength: Porcelain is a material having good strength. However it is brittle and
tends to fracture. The strength of dental porcelain is usually measured by terms of this
flexure strength or modulus of rupture.
Flexure strength: It is a combination of compressive, tensile, as well as shear
strength.
Ground – 11,000 PSI (75.8MPa)
Glazed – 20,465 PSI (141.1 MPa)
1) Compressive strength of porcelain is 48000 psi (321 MPa) tensile strength [5000
psi (35 MPa)]. Tensile strength is low because of the unavoidable surface defects
like porosities and microscopic cracks.
Shear strength: It is low and is due to the lack of ductility caused by the complex
structure of dental porcelain [6000 PSI (110 MPa)]
Inadequate firing weakens porcelain, the firing also decrease strength as more
of the core gets dissolved in the flexure.
2) Surface Hardness: Porcelain is much harder than natural teeth. KHN – 460
(enamel 343).
3) Wear resistance: Porcelain is more resistant to wear than natural teeth.
4) Thermal properties: Porcelain has low thermal conductivity, co-efficient of
thermal expansion is close to that of natural teeth 6.4 to 7.8 x 10-6
/OC
5) Specific gravity: The specific gravity of fired porcelain is usually less, because
of the presence of air voids. It varies from 2.2 to 2.3
6) Dimensional stability: Porcelain is dimensionally stable after firing.
7) Chemical stability: It is insoluble and impermeable to oral fluids. Also it is
resistant to most solvents. However contact with hydrofluoric acid causes
etching of the porcelain surface. A source of this is APF (acidulated phosphate
fluoride) and stannous fluoride; which are used as topical fluorides.
8) Esthetic properties: The esthetic qualities of porcelain are excellent. It is to
match adjacent tooth structure in translucency, color and intensity. In addition,
attempts have also been made to match the fluorescent property of natural teeth
when placed under ultraviolet light.
9) Biocompatibility: It is compatible with the oral tissues. The margins of finishing
line can be even extended to the gingival sulcus
10) Modulous of elasticity: Porcelain has a high modulous of elasticity [10 x 106
PSI (69 GPa)]
11) Optical properties: The colors of commercial premixed dental porcelains are in
the yellow to yellow red range. Usually supplied in blue, yellow, pink, orange,
brown and grey.
The modifiers are added to the opaque and body porcelain during
building of the crown.
16

17. Surface staining: Disadvantages of surface staining are a lowered durability as a
result of high solubility and reduction of translucency. Opaque porcelains have
very low translucency values to mask metal substructure surfaces. Body
porcelain translucency values range between 20% and 35%.
Incisal porcelains have the highest values of translucency and range
between 45% and 50%. Since dental enamel is fluorescent under ultraviolet
light, uranium oxide have been added to produce fluorescence with porcelain.
However because of the low but detectable radioactivity of uranium, newer
formulations contain rare earth oxides such as cerium oxide which produce
fluorescence.
2. BIOLOGICAL PROPERTIES :
 They have excellent biocompatibility.
3. CHEMICAL PROPERTIES :
 It resist attack by chemicals.
 They have to be roughened by etching with hydrofluoric acid or sand blasting
to improve the retention of a cement to the internal surface of the restoration
4. MECHANICAL PROPERTIES:
 Low tensile strength
 Exhibits little plastic deformation
 Have good compressive strength
a) Compressive strength : 50000psi
b) tensile strength : 5000psi
c) Shear strength:16000psi
d) Elastic modulus : 10 × 106
psi
e) Knoop hardness : 460
f) C T E :12 × 10 -6
psi
g) R.I : 1.52 – 1.54
5. THERMAL PROPERTIES:
 They have insulating capacity.
6. OPTICAL PROPERTIES:
 They have good optical properties
 They are translucent because of absence of free electrons.
Strength of porcelains:
Strength of porcelain is decreased by,
1. By the presence of stress concentration areas
2. Porosity, roughness , machine damage
3. Sharp line and point angles
17

18. 4. Interface between bonded structures where elastic modulus of 2 components are
different. More brittle material should have less elastic modulus. So it can transfer
stress to one with high modulus of elasticity.
5. Interface between bonded structure where large difference in thermal coefficient.
Material should have lower coefficient of thermal expansion, so the other has
protective compressive stress.
6. Areas of sharp point contacts on brittle material. Rounding of opposing cusp is
done, so that occlusal contacts are large areas.
Methods of strengthening porcelain:
1. Method of strengthening brittle materials
2. Method of designing components to minimize the stress concentration and tensile
stress.
1. Method of strengthening brittle materials:
Done in 2 ways
1. Development of residual compressive stresses within the surface of the
material
2. Ion exchange mechanism :
This techniques is called chemical tempering. in this procedure, a sodium
containing glass is placed in a bath of molten potassium nitrate , potassium ions in the
bath exchanges place with some of the sodium ions in the surface of the glass article.
The potassium ions being around 35% larger than the sodium ions, squeezes in to the
place formerly occupied by the sodium ions this creates large compressive stresses in
the surface of the glass these residual stresses produce a strengthening effect.
3. Thermal tempering:
This is the most common form of strengthening. This creates residual surface
compressive stresses by rapidly cooling the surface of the object while it is hot and in
the softened state. This rapid cooling produces a skin of rigid glass surrounding a soft
molten core.as the molten core solidifies, it tends to shrink, but the outer skin remains
rigid. The pull of the molten solidifying core as it shrinks, creates residual tensile
stresses in the core and residual compressive stresses within the outer surface.
4. Disruption of crack propagation:
by 3 ways
1. Crack tip interactions
2. Crack tip shielding
3. Crack bridging
1. Crack tip interactions: these occur when obstacles in the microstructure act to
impede the crack motion. These obstacles are second phase particles and act to
deflect the crack out of the crack plane. The re orientation of the crack plane leads
to a reduction of the force being exerted on the crack in the area of deflection.
When the crack is deflected out of plane, the crack is no longer subjected to pure
tensile stresses and will involve some shear displacement thus increasing the
difficulty of crack propogation.
18

19. 2. Crack tip shielding :
By 2 ways
a) Transformation toughening
b) Microcrack toughening
a) Transformation toughening:
This is most commonly associated with the presence of zirconia. Under
unrestrained conditions, zirconia undergoes a high to low temperature phase
transformation which involves 3-5% volume increase. In toughened ceramic, the high
temperature phase of zirconia is constrained at room temperature. Applied tensile
stresses were to advance the crack plane. In the area directly behind the crack tip, the
matrix constrains on zirconia are released, allowing low temperature transformation to
take place. The transformed phase occupies a greater volume in the bulk material
resulting in compressive forces that tend to counteract any advancing crack tip
stresses.
b) Micro crack toughening :
The high coefficient of thermal contraction and volume reduction associated
with the high to low temperature phase transformation of leucite crystals create a
condition which causes the leucite crystals to contract significantly more than the
glass matrix. Compressive forces are created in the glass matrix surrounding the
particles leading to micro cracking in the leucite phase. The residual compressive
stresses in the glass phase around the particles can counteract the tensile stresses
which drive the crack forward.
3. Crack tip bridging:
It occurs when a second phase act as a ligament to make it more difficult for
the crack faces to open. This is better understood by bonded fiber composites. The
fibers act as ligaments which makes it more difficult to open the crack at an applied
stress.
Methods of designing components to minimize stress concentrations and
tensile stress:
1. Minimizing tensile stresses:
In a full coverage metal restoration with porcelain, the metal being of higher
thermal expansion will contract faster than the porcelain as a result the metal is
placed in tension and the porcelain in compression. For partial metal coverage
restorations, the junction between the metal and the porcelain is a potential site for
high stress as, the area with only metal will have no balancing compressive forces.so
ideally full coverage restorations are preferred. Porcelain unsupported by metal is
more subjective to fracture.
Reducing stress raisers:
Stress raisers are discontinuities in ceramic structures and in brittle materials
that cause stress concentration. The design of ceramic restorations should avoid stress
raisers. Abrupt changes in shape/ thickness in the ceramic contour can act as stress
raisers and make the restoration more prone to failure. Notches caused the porcelain
19

20. due to the folds of the underlying platinum foil substrate also is a stress raiser. Sharp
line angles, large changes in the thickness of porcelain are factors leading to stress
concentration. Usually contact points should be avoided and contact areas should be
preferred to avoid localized stress areas.
I. Reinforcement of inner surface by a higher strength ceramic:
Ex. Aluminous core porcelain
Cerestore
Reinforced high alumina crown
II. Reinforcement of inner surface by metal bonding:
Ex. Platinum foil
Gold foil swaged gold coping of 0.90-0.14mm thickness (renaissance system)
Titanium (procera) e.g. Titanium coping
III. Porcelain fusid to metal restorations
Ex. Noble metal alloys – Gold containing alloys
Gold free alloys
Base metal alloys Nickel – chromium alloys
Cobalt – chromium alloys
(rarely used in ceramic bonding)
IV. Designing of restorations:
The design should be such that it should not be subjected to tensile stress. To
avoid stress concentration in porcelain, sharp angles should be avoided and the
porcelain should be of uniform thickness.
Tensile stresses can be avoided by having a favorable occlusion in porcelain
jacket crown. In porcelain fused to metal restoration, the metal should be strong and
ductile not allowing flexing. Contact of opposing tooth or teeth should be either on
porcelain or on metal, but not at the junction.
Fluorescence and Opalescence:
For clinicians who practice esthetic restorative dentistry, particularly in the
field of ceramics, fluorescence is an important physical property. Natural teeth are
fluorescent. In other words, they emit visible light when exposed to ultraviolet light.
Fluorescence adds to the vitality of a restoration and minimizes the metameric effect
between teeth and restorative materials. The components of porcelain consist of
agents that cause them to fluoresce; thus, they also will emit visible light when
exposed to ultraviolet light. It is important that all the basic components of the
porcelain, including the dentins, enamels, stains and even the glazing agents, are
fluorescent. Opalescence is the ability of a translucent material to appear blue in
reflected light and orange-yellow in transmitted light. Opalescence also contributes to
the vitality of a restoration.
MODE OF SUPPLY
Dental porcelains are available as fine powders to be used with liquid I (or
distilled water). The powders and liquid are mixed to form a plastic mass which is
shaped or moulded into a desired shape, it is then fired (or sintered) at a high
temperature in order to fuse the particles together to form a ceramic body which is
esthetically like a natural crown.
20

21. Porcelain is supplied as a kit containing:
1. Fine ceramic powders in different shades:
 Enamel
 Dentin
 Core
2. Special liquid/distilled water
3. Stains of colour pigments
4. Glaze
CERAMIC PROCESSING METHODS
The single unit crown may be a metal ceramic crown (also called a porcelain
fused-to-metal crown), a traditional aluminous porcelain crown based on a core of
aluminous porcelain, or the newer ceramic crowns based on a core of leucite
reinforced porcelain, injection or pressure molded leucite based ceramic, glass
ceramic, sintered aluminous porcelain, sintered aluminum oxide, or glass-ceramic
processed from cast glass. The types of restoration, with their variations, are discussed
in detail in succeeding sections.
The processing stages of the ceramic core for production of ceramic
prostheses are summarized in Table. These seven different processes represent the
main procedures that were available in 2003. the quality of the final ceramic
prosthesis is dependent on each stage of the fabrication process. Machining or
grinding of the core structure is of particular importance since flaws or minute cracks
can be introduced that can possibly be propagated to the point of fracture during
subsequent intraoral stressing cycles. The use of computer aided manufacture (CAM)
bprocesses are most likely to induce such damage, although the ceramics with higher
fracture toughness are less likely to exhibit such damage. It is possible that subsequent
sintering or veneering procedures can reduce the potential for propagation of cracks in
the prostheses while in service. However, insufficient data are available from clinical
studies of ceramics.
The processing procedures for these ceramics are as follows. The feldspathic
porcelain of traditional PFM restorations, some aluminous porcelains (Vitadur-N, Hi-
Ceram), and pure alumina ceramic (Procera AllCeram) are condensed by vibration or
dry-pressed (Procera) and sintered at high temperature. Pressable ceramics (e.g., IPS
Empress, IPS Empress 2, Finesse All-Ceramic, OPC, and OPC-3G), when heated and
subjected to hydrostatic pressure, flow into a mold and after removal and divesting are
then veneered. Cast and cerammed crowns, such as the obsolete product Dicor, are
made using the lost-wax technique. The molten glass is cast into a mold, heat-treated
to form a glass-ceramic, and colored with shading porcelain and surface stains.
For slip cast ceramics (I-Ceram, In-Ceram Spinell, and In-Ceram Zirconia), a
slurry of liquid and particles of alumina, magnesia-alumina silicate (spinel), or
zirconia is placed on a dry refractory die that draws out the water from the slurry. The
slip-cast deposit is sintered on this die, and then it is coated with a slurry of a glass
phase layer. During firing, the glass melts and infiltrates the porous ceramic core.
Translucent porcelain veneers are then fired onto the core to provide the final contour
and color.
21

22. For CAD-CAM processes, the ceramic block materials (Dicor MGC, Vita
Cerec Mk I, and Vita Cerec Mk II) are shaped into inlays or crowns using a CAD-
CAM system (Cerec). CAM refers to computer-aided milling or machining. This
process is sometimes referred to as a CAD-CAM process, where CIM refers to
computer-integrated machining or milling. These blocks can also be used in copy
milling devices (Celay) that mill or machine blocks into core shapes in a manner
similar to that for cutting a key from a key blank, that is, by tracing over a master die
of the shape to be produced out of the ceramic.
SINTERED PORCELAINS
Leucite-reinforced feldspathic porcelain
Optec HSP material (leneric/Pentron, Inc.) is a feldspathic porcelain
containing up to 45 vol% tetragonal leucite (Schmid et al, 1992; Piche et al, 1994;
Denry and Rosenstiel, 1995). The greater leucite content of Optec HSP porcelain
compared with conventional feldspathic porcelain for metal-ceramics leads to a higher
modulus of rupture and compressive strength (Vaidyanathan et al, 1989). The large
amount of leucite in the material contributes to a high thermal contraction coefficient
(Katz, 1989). In addition, the large thermal contraction mismatch between leucite (22
to 25 x 10"6/°C) and the glassy matrix (8 x 10~6/°C) results in the development of
tangential compressive stresses in the glass around the leucite crystals when cooled.
These stresses can act as crack deflectors and contribute to increase the resistance of
the weaker glassy phase to crack propagation. After heat treatment of Optec HSP for
one hour at temperatures ranging from 705 to 980°C, a second metastable phase
identified as sanidine (KAlSi3O8) forms at the expense of the glassy matrix
(Vaidyanathan et al, 1989). The crystallization of sanidine is associated with a
modification of the optical properties of the material from translucent to opaque.
However, sanidine does not appear when the porcelain is heated to 980°C, since
sanidine is metastable in the temperature range 500-925°C. The recipitation of
sanidine has been reported as well upon isothermal heat treatment of conventional
feldspathic porcelain for metal-ceramics (Mackert et al, 1986b; Mackert, 1988;
Barreiro et al, 1989). An isothermal timetemperature- transformation diagram that
makes it possible to predict the amount of leucite and sanidine in samples subjected to
different thermal histories has been established (Barreiro and Vicente, 1993).
Alumina-based porcelain
Aluminous core porcelain is a typical example of strengthening by dispersion
of a crystalline phase (McLean and Kedge, 1987). Alumina has a high modulus of
elasticity (350 GPa) and high fracture toughness (3.5 to 4 MPa.m05). Its dispersion in
a glassy matrix of similar thermal expansion coefficient leads to significant
strengthening of the core. The first aluminous core porcelains contained 40 to 50%
alumina by weight (McLean and Hughes, 1965). The core was baked on a platinum
foil and later veneered with matched-expansion porcelain. Hi-Ceram (Vident,
Baldwin Park, CA) is a
more recent development of this technique. Aluminous core porcelain is now baked
directly onto a refractory die (McLean et al , 1994).
22

23. Magnesia-based core porcelain
Magnesia core ceramic was developed as an experimental material in 1985
(O'Brien, 1985). Its high thermal expansion coefficient (14.5 x 10'6/°C) closely
matches that of body and incisal porcelains designed for bonding to metal (13.5 x
10"6/°C). The flexural strength of unglazed magnesia core ceramic is twice as high
(131 MPa) as that of conventional feldspathic porcelain (65 MPa). The core material
is made by reacting magnesia with a silica glass within the 1100-1150°C temperature
range. This treatment leads to the formation of forsterite (Mg2Si04) in various
amounts, depending on the holding time. The proposed strengthening mechanism is
the precipitation of fine forsterite crystals (O'Brien et al, 1993). The magnesia core
material can be significantly strengthened by glazing, thereby placing the surface
under residual compressive stresses that have to be overcome before fracture can
occur (Wagner et al, 1992).
Zirconia-based porcelain
Mirage II (Myron International, Kansas City, KS) is a conventional feldspathic
porcelain in which tetragonal zirconia fibers have been included. Zirconia undergoes a
crystallographic transformation from monoclinic to tetragonal at 1173°C. Partial
stabilization can be obtained by using various oxides such as CaO, MgO, Y2O3, and
CeO, which allows the high-temperature tetragonal phase to be retained at room
temperature. The transformation of partially stabilized tetragonal zirconia into the
stable monoclinic form can also occur under stress and is associated with a slight
particle volume increase. The result of this transformation is that compressive stresses
are established on the crack surface, thereby arresting its growth. This mechanism is
called transformation toughening. The addition of yttria-stabilized zirconia to a
conventional feldspathic porcelain has been shown to produce substantial
improvements in fracture toughness, strength, and thermal shock resistance (Morena
et al 1986a; Kon et al, 1990). However, other properties, such as translucency and
fusion temperature, can be adversely affected. The modulus of rupture of
commercially available zirconia-reinforced feldspathic dental porcelain (Mirage II) is
not significantly different from that of conventional feldspathic porcelain (Seghi et al,
1990b).
GLASS-CERAMICS
Mica-based
As described earlier, glass-ceramics are obtained by controlled devitrification
of glasses with a suitable composition including nucleating agents. Depending on the
composition of the glass, various crystalline phases can 7(2):134-143 (1996) Crit Rev
Oral Biol Med 137 nucleate and grow within the glass. The advantage of this process
is that the dental restorations can be cast by means of the lost-wax technique, thus
increasing the homogeneity of the final product compared with conventional sintered
feldspathic porcelains.Dicor (Dentsply Inc., York, PA) is a mica-based machinable
glass-ceramic. The machinability of Dicor glass-ceramic is made possible by the
23

24. presence of a tetrasilicic fluormica (KMg25Si4O10F2) as the major crystalline phase
(Grossman and Johnson, 1987). Micas are classified as layer-type silicates. Cleavage
planes are situated along the layers, and this specific crystal structure dictates the
mechanical properties of the mineral itself. Crack propagation is not likely to occur
across the mica crystals and is more probable along the cleavage planes of these
layered silicates (Daniels and Moore, 1975). In the glass-ceramic material, the mica
crystals are usually highly interlocked within the glassy matrix, achieving a "house of
cards" microstructure (Grossman, 1972). The interlocking of the crystals is a key
factor in the fracture resistance of the glass-ceramic, and their random orientation
makes fracture propagation equally difficult in all directions. After being cast, the
Dicor glass is converted into a glass-ceramic by means of a single-step heat treatment
with a six-hour dwell at 1070°C. This treatment facilitates controlled nucleation and
growth of the mica crystals.However, it is critical to re-invest the cast glass
restoration prior to the crystallization heat treatment, to prevent sagging or rounding
of the edges at high temperature. The match in the thermal expansion coefficients of
the glass and the investment is achieved by use of a leucite based gypsum-bonded
investment. The interaction of the glass-ceramic and the investment during the
crystallization heat treatment leads to the formation of calcium magnesium silicate at
the surface of the glass-ceramic (Denry and Rosenstiel, 1993). This crystalline phase
could be formed by decomposition of the mica into magnesium silicate that later
reacts with the gypsum-bonded investment. This surface layer, called the"ceram
layer", has been reported to decrease the strength of glass ceramic crowns
significantly (Campbell and Kelly, 1989; Kelly et al, 1989). The effects of alumina
and zirconia additions on the bending strength of Dicor glass-ceramic have been
investigated. It was found that alumina additions successfully increase the bending
strength of Dicor glass-ceramic, whereas zirconia additions had no effect (Tzengetal,
1993).
Hydroxyapatite-based
Cerapearl (Kyocera, San Diego, CA) is a castable glass ceramic in which the
main crystalline phase is oxyapatite, transformable into hydroxyapatite when exposed
to moisture (Hobo and Iwata, 1985).
Lithia-based:
Glass-ceramics can be obtained from a wide variety of compositions, leading
to a wide range of mechanical and optical properties, depending on the nature of the
crystalline phase nucleating and growing within the glass. Experimental glass-
ceramics in the system Li2O-Al2O3- CaO-SiO2 are currently the object of extensive
research work. The choice of adequate additives is critical in the development of
tougher and higher-strength glassceramics (Anusavice et al. 1994b). Differential
thermal analysis can be efficiently used to determine the heat treatment leading to the
maximum lithium disilicate crystal population in the shortest amount of time, thereby
optimizing the nucleation and crystallization heat treatment of this type of glass-
ceramic (Parsell and Anusavice, 1994).
Machinable ceramics
24

25. Cerec system:The evolution of CAD-CAM systems for the production of machined
inlays, onlays, and crowns led to the development of a new generation of machinable
porcelains.There are two popular systems available for machining all-ceramic
restorations. The best-known is the Cerec system (Siemens, Bensheim, Germany). It
has been marketed for several years, and two materials can be used with this system:
Vita Mark II (Vident, Baldwin Park, CA) and Dicor MGC (Dentsply International,
Inc., York, PA). Vita Mark II contains sanidine (KAlSi3O8) as a major crystalline
phase within a glassy matrix. As explained earlier, the presence of sanidine could
explain the lack of translucency of this material. Dicor MGC is a machinable glass-
ceramic similar to Dicor, with the exception that the material is cast and cerammed by
the manufacturer. Colorants have been added to match tooth color. The glass-ceramic
contains 70 vol% of the crystalline phase (Grossman, 1991). Manufacturer's control
over the processing of this material and the higher volume percent of the crystalline
phase could explain the improved mechanical properties of Dicor MGC compared
with conventional Dicor glass-ceramic. The use of adhesive resinbased cements has
been shown to improve the fracture resistance of all-ceramic crowns (Eden and
Kacicz, 1987; Grossman and Nelson, 1987). Other studies have shown that the overall
fracture resistance of Dicor MGC was independent of cement film thickness (Scherrer
et al., 1994). Presently, the main identified weakness of the Cerec system is the
marginal fit of the restorations (Anusavice, 1993).
Celay system:
The Celay system (Mikrona Technologie, Spreitenbach, Switzerland) uses a
copy-milling technique to manufacture ceramic inlays or onlays from resin analogs.
The Celay system is a mechanical device based on pantographic tracing of a resin
inlay or onlay fabricated directly onto the prepared tooth or onto the master die
(EidenbenzeU/., 1994). As with the Cerec system, the starting material is a ceramic
blank available in different shades. One ceramic material currently available for use
with the Celay system is Vita-Celay (Vident, Baldwin Park, CA). This material
contains sanidine as the major crystalline phase within a glassy matrix. Recently, ln-
Ceram pre-sintered slip-cast alumina blocks (Vident, Baldwin Park, CA) have been
machined with the Celay copy-milling system used to generate copings for crowns
and fixed partial dentures (McLaren and Sorensen, 1995). The alumina copings were
further infiltrated with glass following the conventional
ln-Ceram technique, resulting in a final marginal accuracy within 50 urn.
SLIP-CAST CERAMICS
Alumina-based (n-Ceram)
ln-Ceram (Vident, Baldwin Park, CA) is a slip-cast aluminous porcelain. The
alumina-based slip is applied to a gypsum refractory die designed to shrink during
firing.The alumina content of the slip is more than 90%, with a particle size between
0.5 and 3.5 micrometers. After being fired for four hours at 1100°C, the porous
alumina coping is shaped and infiltrated with a lanthanum-containing glass during a
second firing at 1150°C for four hours. After removal of the excess glass, the
restoration is veneered with matched expansion veneer porcelain (Probster and Diehl,
1992). This processing technique is unique in dentistry and leads to a high-strength
25

26. material due to the presence of densely packed alumina particles and the reduction of
porosity. Two modified porcelain compositions for the In Ceram technique have been
recently introduced. In-Ceram Spinell contains a magnesium spinel (MgAl2O4) as the
major crystalline phase with traces of alpha-alumina, which seems to improve the
translucency of the final restoration. The second material contains tetragonal zirconia
and alumina. A variety of alumina-glass dental composites can be prepared by the
glass-infiltration process. However, research has shown that the fracture toughness of
the composites is relatively insensitive to the volume fraction of alumina in the range
investigated (Wolfrtfll., 1993).
Hot-pressed, injection-molded ceramics
Leucite-based
IPS Empress (Ivoclar USA, Amherst, NY) is a leucite-containing porcelain.
Ceramic ingots are pressed at 1150°C (under a pressure of 0.3 to 0.4 MPa) into the
refractory mold made by the lost-wax technique. This temperature is held for 20
minutes in a specially designed automatic press furnace (Dong et al, 1992). The
ceramic ingots are available in different shades. They are produced by sintering at
1200°C and contain leucite crystals obtained by surface crystallization (Holand et al. ,
1995). The leucite crystals are further dispersed by the hot-pressing step. The final
microstructure of IPS Empress exhibits 40% by volume of tetragonal leucite. The
leucite crystals measure 1-5 um and are dispersed in a glassy matrix. Two finishing
techniques can be used with IPS Empress: a staining technique or a layering technique
involving the application of veneering porcelain. The two techniques lead to
comparable mean flexure strength values for the resulting porcelain composite (Luthy
et al, 1993). The thermal expansion coefficient of the IPS Empress material for the
veneering technique (14.9 x 10"6/°C) is lower than that of the material for the staining
technique (18 x 10~6/°C) to be compatible with the thermal expansion coefficient of
the veneering porcelain. The flexural strength of IPS Empress material was
significantly improved after additional
firings (Dong et al, 1992). The strength increase is attributed to a good dispersion of
the fine leucite crystals as well as the tangential compressive stresses arising from the
thermal contraction mismatch between the leucite crystals and the glassy matrix.
Spinel-based
Alceram (Innotek Dental Corp, Lakewood, CO) is a material for injection-
molded technology and contains a magnesium spinel (MgAl2O4) as the major
crystalline phase (McLean and Kedge, 1987). This system was initially introduced as
the "shrink-free" Cerestore system, which relied on the conversion of alumina and
magnesium oxide to a magnesium aluminate spinel. One of the recognized advantages
of this system was the excellent marginal fit of the restorations (Wohlwend et al,
1989).
CONDENSATION (COMPACTION)
The process of packing the powder particles together and removing excess
water is known as condensation. Proper condensation gives dense packing and reduce
the shrinkage of porcelain and minimize porosity in the fired porcelain.Condensation
procedure is followed in application of core, dentin and enamel porcelain either in
26

27. porcelain jacket crown or porcelain fused to metal.The porcelain powder is mixed
with distilled water or special liquid supplied by the manufacturer to form a thick
paste. Small portions of the paste are then applied to the platinum matrix in jacket
crown preparation over the die until the desired shape of the crown has been attained.
Excess water is removed by blotting with a linen cloth or similar absorbent
material.The remaining water serves as a binder for the powder so that the crown may
be properly shaped before firing. Powder consisting of a mixture of particle sizes
compact more easily than those with particles of one size only. This reduces the size
of the spaces between the particles and thus reduces firing shrinkage.A well
compacted crown not only reduces firing shrinkage but also shows a regular
contraction over its entire surface.
Methods of condensation:
1) Vibration: Mild vibration are used to densely pack the wet powder upon the
underlying matrix. The excess water comes to the surface and its is blotted with a
tissue paper.
2) Spatulation: A small spatula is used to apply and smoothen the wet porcelain.
This action brings excess water to the surface.
3) Wet brush technique: The mix should be creamy and capable of being transferred
in small increments to the platinum matrix with hair brush.
4) Ultrasonic: A ceramosonic condenser can induce supersonic vibration in
porcelain creates intimate inter relation between metal and opaque porcelain.
5) Gravitational:
6) Whipping: Any method may be used for condensation but care is taken not to
allow the porcelain to dry out completely as the porcelain powder is held together
due to surface tension of water.
Dry the wet structure in a warm atmosphere before placing into the hot
furnace. After condensation the compacted mass supported by the matrix or metal
coping should be placed on a fire tray and inserted into the muffle of the ceramic
furnace.
Porcelain Condensation
Porcelains for ceramic and metal - ceramic prostheses, as well as for other
applications, is supplied as a fine powder that is designed to be mixed with water or
anther vehicle and condensed into the desired form (see Fig 21-2). The powder
particles are of a particular size distribution to produce the most densely packed
porcelain when they are properly condensed. If the produce the densely packed
porcelain when they are properly condensed. If the particles are of the same size, the
density of packing would not be nearly as high. Thorough condensation is also crucial
in obtaining dense packing would not be nearly as high. Thorough condensation is
also crucial in obtaining deus packing of the powder particles. Dense packing of the
powder particles. Dense packing of the powder particles dense packing of the powder
particles. Dense packing of the powder particles provides two benefits: lower firing
shrinkage and porosity in the fires porcelain. This packing, or condensation, may be
achieved by various methods, including vibration, spatulation, and brush techniques.
27

28. The first method uses mild vibration to pack the wet powder densely on the
underlying framework. The excess water is blotted or wiped away with a clean tissue
or fine brush, and condensation occurs toward the blotted or bushed area, in the
second method a small spatula is used to apply and smooth the it is removed. The
second method a small spatula is used to apply and smooth the wet porcelain. The
smoothing action brings the excess water to the surface, where it is removed. The
third method employs the addition of dry porcelain powder to the surface to absurd
the water. A brcelain powder to the surface to accord the water places the dry powder.
A brush to the side opposite from an increment of wet porcelain places the dry
powder. As the water is drawn toward the dry powder, the wet particles are pulled
together. Whichever method is used, it is important to remember that the surface
tension of the water is the driving force for condensation, and the porcelain must
never be allowed to dry out until condensation is complete.
Condensed mass is gradually heated by first placing it in front of the muffle of a
preheated furnace and later inserting into the furnace.
1) Low bisque stage: The flux begins to melt and flow in between the porcelain
particles. The mass attains some rigidity but very little cohesion. At this stage the
material is porous and undergoes minimum of shrinkage. The porcelain do not
have translucency and glaze.
2) Medium bisque stage: Here the flux flows freely in between the particles the
material is still porous, but there is complete cohesion between the particles and
most of the shrinkage is complete. In this stage also there is lack of translucencey
and glaze.
3) High bisque stage: Here with shrinkage is completed. There is very little
porosity, the mass has attained complete rigidity and smoothness, the body does
not appear to be glazed. Most of the addition and alterations are carried out after
the porcelain has attained medium bisque stage.
Less the number of firing, higher is the strength and better the esthetics. Too
many firings give a life less, over translucent porcelain.
PORCELAIN FURNACE
The ordinary air fire porcelain furnace consists of a muffle, a pyrometer, a
thermocouple and in its most simple form a rheostat or variable transformer for
control of firing temperature and sophisticated automatic and programmable time and
temperature controller for the most modern furnaces.
The muffle is the heating unit providing necessary high temperature for baking
of porcelain. The heating element is a coiled wire of platinum and is embedded into
the refractory material of the muffle. The muffle is provided with a door for easy
access and to prevent fluctuation of temperature due to heat loss.
The pyrometer is a millivoltmeter calibrated to read in degree of temperature.
The thermocouple consists of platinum wire joined at one end with another wire made
of 90% platinum and 10% rhodium. The joint is placed inside the muffle, this is
known as hot junction of the thermocouple. The free ends of the thermocouple are
attached to the pyrometer outside the muffle. When heat is generated inside the
muffle, the dissimilar metals of the thermocouple at the hot junction generates and
28

29. electromotive force which deflects the needle of the pyrometer indicating the
calibrated temperature.
As the electromotive force varies with variation in temperature inside the
muffle, such variations can be measured as temperature on the pyrometer. The
temperature controller regulates the current fed to the heating element inside the
muffle thereby inducing increase or decrease in muffle temperature.
The main problem in air fired furnace is the opacity of the porcelain due to
porosity.
Cooling:
The cooling of dental porcelain is complex matter, particularly when the
porcelain is fused to metal a metallic substrates.
Multiple firings of metal ceramic restorations can cause the co-efficient of
thermal contraction the porcelain to increase and can actually make it more likely to
craze or craze because of tensile stress development.
Cooling must be carried out slowly and uniformly. If shrinkage is not uniform
it causes cracking and loss of strength. During cooling, subsurface submicroscopic
surface cracks occur. Because of the low thermal conductivity of porcelain, the
differential between the thermal dimensional change of the outside and inside can
introduce stresses which embrittle the porcelain.
Different methods and porcelain firings are:
2) Air firing
3) Pressure firing
4) Gas firing
5) Vacuum firing
1) Air firing:
Air inside a furnace is modulated to the same atmospheric pressure during this
procedure. There is more chances of air entrapment in porcelain. We will get more
porous, less translucent porcelain.
2) Pressure firing:
The air inside the furnace is subjected to a pressure equal to 10 atmosphere as
the porcelain reaches its maturation temperatures. This compresses the air inside
the porcelain mass and reduces the size of the air bubble.
3) Gas firing:
The air in the furnace is replaced by a diffusible inert gas like argon or
hydrogen which diffuses out through the maturing porcelain.
4) Vacuum firing:
Partial vacuum firing reduces air voids, so porosity is reduced, so better
translucent effect.
The air from the furnace is evacuated and this eliminating air from porous
spaces which collapses on itself. This is the best and widely used method.
29

30. LABORATORY MANUFACTURING PROCEDURE
In the production of porcelain tooth, the powder ingredients are weighted and
mixed with water containing starch, gum tragacanth, or other organic materials to
form a putty like mass that can be handled conveniently.
The molding technique varies with different manufacturers. Generally, the
split molds are made of bronze and may be separated so that one portion contains the
negative pattern for the lingual surface of 12 teeth and the other contains the negative
pattern for the labial surface or face of the teeth.
When the two piece molds are used, a thin layer of the enamel mix is placed in
the labial mold to provide the enamel color, and the body mix, which forms the bulk
of the tooth, is placed over this. Then a thin veneer of enamel mix is placed in the
incisal portion of the lingual mold and is also covered with body mix. When
combined, the two halves with the porcelain mixes form a tooth with contours and
coloration similar to natural teeth.
The technique employs a third portion, which also fits against the labial
surface for the purpose of accurately forming the enamel colored porcelain separately
before the body portion is added.
The technique for the three piece molds involves placing the dough like
enamel mix in the labial half first, pressing the third or blender mold into it and
heating the molds until the mix stiffness. They are then opened, the excess mix is
trimmed away and the body mix is added to all the second and larger lingual half of
the mold.
Small noble metal rings are embedded in the porcelain to provide a base for
the gold plated nickel pins used for the retention of the teeth in the denture base.
These rings are made of a metal or alloy with a high melting point and usually are
split to allow for the shrinkage of the surrounding porcelain during fusion. Before the
moulds are filled, the rings are placed over the tips of tapered points that extend into
the tooth from the lingual half of the mould.
After the moulds are filled by either method, they are placed in a press and
heated until the porcelain mix develops sufficient hardness to allow handling. Each
anterior tooth at this stage is approximately one fifth over size to allow for shrinkage.
After 3 stage of firing, the teeth have been cooled slowly to prevent crazing,
all that remains and the attachment of the pins. For this operation small bits of solder
are stamped to the ends of the gold clad pins and they are inserted, solder downward,
to contact the metal ringes at the base of the tapered openings in the lingual body of
the teeth. When heated, either in a furnace or electrically, the solder melts and joins
the pin firmly to the embedded rings.
CERAMIC PROSTHESES:
Aluminous Porcelain Crowns:
Another method of bonding porcelain to metal makes use of tin oxide coatings
on platinum foil. The objective of this technique is to improve the aesthetics by a
replacement of the thicker metal coping with a thin platinum foil, thus allowing more
room for porcelain. The method consists of bonding aluminous porcelain to platinum
30

31. foil copings. Attachment of the porcelain is secured by electroplating the platinum
foil with a thin layer of tin and then oxidizing it in a furnance to provide a continuous
film of tin oxide for porcelain bonding. The rationale is that the bonded foil will act
as an inner skin on the fit surface to reduce subsurface porosity and formation of
microcracks in the porcelain, thereby increasing the fracture resistance of crowns and
bridges. The clinical performance of these crowns has been excellent for anterior
teeth, but approximately 15% of these crowns fractured within 7 years after they were
cemented to molar teeth with a glass ionomer cement.
Based on a 1994 survey, metal-ceramic crowns and bridges were used for
approximately 90% of all fixed restorations. However, recent developments in
ceramic products with improved fracture resistance and excellent aesthetic capability
have led to a significant increase in the use of all-ceramic products. Ceramic crowns
and bridges have been in widespread use since the beginning of the twentieth century.
The ceramics employed in the conventional ceramic crown were high fusing
feldspathic porcelains. The relatively low strength of this type of porcelain prompted
McLean and Hughes (1965) to develop an alumina-reinforced porcelain core material
for the fabrication of ceramic crowns.
The alumina-reinforced crowns are generally regarded as providing slightly
better aesthetics for anterior teeth than are the metal-ceramic crowns that employ a
metal coping. However, the strength of the core porcelain used for alumina-
reinforced crowns is inadequate to warrant the use of these prostheses for posterior
teeth. In fact McLean reported a fracture rate of molar aluminous porcelain crowns of
approximately 15% after 5 years.
Castable and machinable Glass-Ceramics (Dicor and Dicor MGC)
When used for posterior crowns, ceramic crowns are most susceptible to
fracture. Shown in Figure 21-6 (see also the color plate) is the stress distribution
computed by finite element analysis in a 0.5mm-thick molar Dicor crown loaded on
the occlusal surface, just within the marginal ridge area. The maximum tensile stress
is located within the internal surface directly below the point of applied force and just
above the 50 m-thick layer of resin cement (see the arrow in fig. 21.6). this site
represents the critical flaw responsible for crack initiation under an applied intraoral
force. The location of initial crack formation was consistent with the location of
maximum tensile stress predicted by the finite element calculations as shown in figure
21.6. an SEM image of a fractured clinical crown of Dicor glass-ceramic is shown in
fig 21.8. because of the smaller forces exerted on anterior crowns, the risk for
fracture of anterior crowns is significantly less than that for posterior crowns.
The first commercially available castable ceramic material for dental use,
Dicor, was developed by Corning Glass works and marketed by Dentsply
international. Dicro is a castable glass that is formed into an inlay, facial veneer, or
full-crown restoration by a lost-was casting process similar to that employed for
metals. After the glass casting core or coping is recovered, the glass is sandblasted to
remove resideual casting investment and the sprues are gently cut away. The glass is
then covered by a protective "embedment" material and subjected to a heat treatment
that causes microscopic platelike crystals of crystalline material (mica) to grow within
31

32. the glass matrix. This crystal nucleation and crystal growth process is called
ceramming. Once the glass has been cerammed, it is fit on the prepared dies, ground
as necessary, and then coated with veneering porcelain (as shown in fig. 21.8) to
match the shape and appearance of adjacent teeth. Dicor glass-ceramic is capable of
producing surprisingly good aesthetics, perhaps because of the "chameleon" effect,
where part of the color of the restoration is picked up from the adjacent teeth as well
as from the tinted cements used for luting the restorations.
Dicor glass-ceramic contains about 55 vol% of tetrasilicic fluormica crystals.
The ceramming process results in increased strength and toughness, increased
resistance to abrasion, thermal shock resistance, chemical durability, and decreased
transluency. Dicor MGC is a higher quality product that is crystallized by the
manufacturer and provided as CAD-CAM blanks or ingots. The CAD-CAM ceramic
Dicor MGC contains 70 vol% of tetrasilicic fluormica platelets, which are
approximately 2m in diameter. The mechanical properties of Dicor MGC are similar
to those of Dicor glass-ceramaic, although it has less translucency (contrast ratio of
0.41 -0.44 versus 0.56, respectively).
Dicor has recently been discontinued presumably because of low tensile
strength and the need to color the prosthesis on the exterior region rather that within
the core region, which would more closely resemble a natural tooth. Although Dicor
is no longer sold, the principles for selection are useful when products of similar
mechanical and physical properties are being considered. The advantages of Dicor
glass-ceramic were ease of fabrication, improved aesthetics, minimal processing
shrinkage, good marginal fit, moderately high flexural strength, low thermal
expansion equal to that of tooth structure, and minimal abrasiveness to tooth enamel.
The disadvantages of Dicor glass-ceramic were its limited use in low-stress
areas and its inability to be colored internally. As designed, it was colored with a thin
outer layer of shading porcelain and surface stain to ieve acceptable aesthetics.
However, Dicor MGC ingots have been supplied in light and dark shades, making it
possible for technicians to build depth of color into the fabrication process.
Although both of the Dicor products were based on a glass-ceramic core that
was minimally abrasive to opposing tooth enamel, the required shaduing or veneering
porcelains were more abrasive. Aesthetically, Dicor crowns were more lifelike than
metal-ceramic crowns, which often exhibit a metal collar, a gray shadow
subginigivally, or poor translucency. The life expectancy of Dicor crowns in high-
stress areas is not as good as that of PEM crowns. Two veneering materials were
used to improve the color of Dicor crowns: Dicor Plus, which consisted a pigmented
feldspathic porcelain veneer, and Willi's Glass, a veneer of Vitadur N aluminous
porcelain.
Tooth preparation for glass-ceramic of this type is the same as that required
for metal-ceramic prostheses except that, for first and second molars a reduction of
2mm is recommended. Occlusal surfaces and incisal edges must be reduced a
minimum of 1.5mm. Axial surfaces should be reduced a minimum of 1.0mm. The
preparation should be either a shoulder with a rounded gingivoaxial line angle or a
heavy chamfer.
32

33. Pressable Glass-Ceramics:
A glass-ceramic is a material that is formed into the desired shape as a glass,
then subjected to a heat treatment to induce partial devitrification (i.e., loss of glassy
structure by crystallization of the glass). The crystalline particles, needles, or plates
formed during this ceramming process serve to interrupt the propagation of cracks in
the material when an intraoral force is applied, thereby causing increased strength and
toughness. The use of glass-ceramics in dentistry was first proposed by MacCulloch
in 1968. He used a continuous glass-molding process to produce denture teeth. He
also suggested that it should be possible to fabricate crowns and inlays by centrifugal
casting of molten glass.
Pressure molding is used to make small, intricate objects. This method uses a
piston to force a heated ceramic ingot through a heated tube into a molk, where the
ceramic form cools and hardens to the shape of the mold. When the object has
solidified, the refractory mold (investment) is broken apart and the ceramic piece is
removed. It is then debrided and either stained and glazed (certain inlays) or veneered
with one or more layers of a thermally compatible ceramic.
IPS Empress is a glass-ceramic provided as core ingots that are heated and
pressed until the ingot flows into a mold. It contains a higher concentration of leucite
crystals that increase the resistance to crack propagation (fracture). The hot-pressing
process occurs over a 45 min period at a high temperature to produce the ceramic
substructure. This crown form can be either stained and glazed or build up using a
conventional layering technique.
The advantages of this ceramic are its lack of metal, a translucent ceramic
core, a moderately high flexural strength (similar to that of Optimal Pressable
ceramic), excellent fit, and excellent aesthetics. The disadvantages are its potential to
fracture in posterior areas and the need to use a resin cement to bond the crown
micromechanically to tooth structure.
IPS Empress and IPS Empress2 are typical products representative of several
other leucite-reinforced and lithia disilicate-reinforced glass-ceramics, respectively.
Some properties of IPS Empress and IPS Empress2 glass-ceramic core materials are
listed in table. 21.6. IPS Empress is a leucite-containing glass-ceramic that contains
about 35 vol% of leucite (KAISI2O6) crystals, which increases the resistance to crack
propagation (fracture). The veneering ceramic also contains leucite crystals in a glass
matrix. After hot pressing, divesting, and separation of the ceramic units the sprue
segments, they are veneered with porcelain containing leucite crystals in a glass
matrix.
A cross-sectional illustration of an IPS Empress crown is illustrated in fig.
21.9. The IPS Empress2 is similar except that the core consists of lithia disilicate
crystals in a glass matrix and the veneering ceramic contains apatite crystals. The
very small apatite crystals cause light scattering in a way that resemble by the
structure and components of tooth enamel. The coefficient of expansion of the apatite
glass-ceramic veneering ceramic is 9.7 ppm/0
C, which is similar to that of IPS
Empress2 core ceramic (10.6 ppm/0
C). Obviously, this veneering ceramic should not
33

34. be used with the IPS Empress core ceramic that has a much higher expansion
coefficient (150 ppm/0
C).
The core microstructure of IPS Empress2 glass ceramic is quite different from
that of IPS Empress, as evidenced by the 70 vol% of elongated lithia disilicate
crystals in IPS Empress2. The primary crystal particles in IPS Empress2 are 0.5 to
4m in length. A smaller concentration of lithium orthophosphate crystals (Li2 Si2 O5)
approximately 0.1 to 0.3µm in diameter has also been reported (Holand et al., 2000).
The microstructural difference between IPS Empress and IPS empress2 results in a
slight decrease in translucency for IPS Cmpress2 (0.55) (Holland et al., 2000). As is
the case for most pressable glass-ceramics, the advantages of IPS empress and IPS
Empress2 glass-ceramic core materials are their potential for accurate fit, excellent
transluency and overall aesthetics, and a metal-free structure. Disadvantages are their
low to moderately high flexural strength and fracture toughness. These properties
limit their use to conservative designs in low to moderate stress environments. Shown
in fig. 21-10, 21-11 and 21-12 are three-unit glass-ceramic FPDs made from a lithia-
disilicate-based core material. The FPD shown in fig. 21-12 was made without a
veneering ceramic to enhance the fracture resistnce. A summary of important
properties is presented in Table 21-7 for a variety of dental ceramics. A list of
pressable ceramics and their veneering ceramics is summarized.
OPC and OPC 3G are two pressable ceramics that are similar in nature to IPS
Empress and IPS Empress2, respectively. OPC is a leucite-containing ceramic and
OPC 3G contains lithia disilicate crystals. The ultralow-fusing temperature of the
veneering porcelain suggests a low level of wear of opposing enamel. However,
insufficient clinical data are available to support this hypothesis.
In-Ceram Alumina, In-Ceram Spinell, and In-Ceram Zirconia
In-Ceram is supplied as one of three core ceramics: (1) In-Ceram spinell (2)
in-Ceram Alumina, and (3) in-Ceram Zirconia. A slurry of one of these materials is
slip-cast on a porous refractory die and heated in a furnace to produce a partially
sintered coping or framework. The partially sintered core is infiltrated with glass at
11000
C for 4 hr to eliminate porosity and to strengthen the slip-cast core. The initial
sintering process for the alumina core produces a minimal shrinkage because the
temperature and time are sufficient only to cause bonding between particles and to
produce a desired level of sintering. Thus the marginal adaptation and fit of this core
material should be adequate because little shrinkage occurs. The flexural strength
(modulus or rupture) values of the glass-infiltrated core materials are approximately
350 Mpa for in-Ceram spinell (ICS), 500 Mpa for In-Ceram Alumina (ICA)and 700
Mpa for In-Ceram Zirconia (ICZ) compared with strengths of 100 to 400 Mpa for
Dicor, Optec Pressable Ceramic, IPS Empress and IPS Empress2. Despite the
relatively high strength of these materials, failures can still occur in single crowns as
well as FPDs.
Because of the variation in strength, the primary indications for these core
ceramics vary as shown in Table 21-9. For example, ICS is indicated for use as
anterior single-unit inlays, onlays, crowns, and veneers, ICA is indicated for anterior
and posterior crowns and anterior three-unit FPDs. Because of its high level of
34

35. opacity, ICZ is not recommended for anterior prostheses. However, because of its
extremely high strength and fracture toughness, it can be used for posterior crowns
and posterior FPDs. As suggested in chapter 4, it is essential that the gingival
embrasure areas of ceramic FPD connectors be designed with a large radius of
curvature to minimize the stress-raiser effect in areas of moderate to high tensile
stress. The connectors also should be sufficiently thick to minimize stresses during
loading. For Empress and Empress2 ceramics used in molar areas, the connector
height should be at least 4mm.
3 tables: Page No. 687 to 689
Until in-Ceram was introduced, aluminous porcelain had not been used
successfully to produce FPDs because of low flexural strength and high sintering
shrinkage. Thus the principal indications for aluminous porcelain crowns were the
restoration of maxillary anterior crowns when aesthetics was important and their use
in patients with allergies to metals. Its advantages and disadvantages are summerized
in the following.
A schematic drawing of an In-Ceram crown is shown in Fig 21.13 The same
diagram can be used to illustrate crowns made with In-Ceram Spinell (ICS) and In-
Ceram Zirconia (ICZ), which will be discussed below. The three In-Ceram ceramics
are glass-infiltrated core materials used for single anterior crowns (all three products),
posterior crowns (In-Ceram Alumina and in-Ceram Zirconia), anterior three-unit
FPDs (In-Ceram Alumina), and three-unit posteriro bridges (In-Ceram Zirconia).
The most translucent of the three ceramics- In-Ceramics, In-Ceram Spinell,
was introduced as an alternative to in-Ceram Alumina. This ceramic has a lower
flexural strength, but its increased translucency provides improved aesthetics in
clinical situations in which the adjacent teeth or restorations are quite translucent.
The core of ICS is MgAl1O4 and that for ICZ is a mixture of Al2O3 and ZrO2. These
core ceramics are also infiltrated with glass, and they are fabricated in a manner
similar to that for ICA, although the firing temperatures and times may be different.
The final ICA core consists pf 70 wt% alumina infiltrated with 30 wt%
sodium lanthanum glass. The final ICS core consists of glass-infiltrated magnesium
spindl (MgAl2O4). ICZ contains approximately 30wt% zirconia and 70 wt% alumina.
The power-liquid slurry is slip cast onto a porous die that absorbs water from the
slurry, thereby densifying the agglomeration of particles onto the die. Steps for
fabricating in-Ceram prostheses are as follows: (1) prepare teeth with an occlusal
reduction of 1.5 to 2.0mm and a heavy circumferential chamfer (1.2mm), (2) make an
impression and pour two dies, (3) apply Al2O3 on a porous duplicate die, (4) heat at
1200
C for 2 hours to dry Al2O3 , (5) sinter the coping for 10 hours at 11200
C, (6)
apply a sodium lanthanum glass slurry mixture on the coping, (7) fire for 4 hours at
11200
C to allow infiltration of glass, (8) trim excess glass from the coping with
diamond burs, (9) build up the core with dentin and enamel porcelain, (10) fire in
the oven, grind in the anatomy and occlusion, finish, and glaze.
The advantages of ICA include a moderately high flexural strength and
fracture toughness, a metal-free structure, and an ability to be used successfully with
conventional luting agents (Type 1 cements). The collective advantages of the three
35

36. glass-infiltrated core materials are their lack of metal, relatively high flexural strength
and toughness, and ability to be successfully cemented using any cement.
In spite of this high flexural strength (429 Mpa), the Weibull modulus of ICS
is quite low (5.7), which is indicative of a large scatter in the distribution of strength
values relative to the probability of fracture. (Tinschert et al., 2000). Its marginal
adaptation may not be as good as that achieved with other ceramic products. In one
study the mean marginal discrepancies were 83 m for Procera All Ceram, 63 m for
IPS Empress, and 161m for In-Ceram Alumina. Other drawbacks of ICA include its
relatively high degree of opacity, inability to be etched, technique sensitivity, and the
relatively great amount of skilled labor required. These disadvantages apply also to
In-Ceram Zirconia. Compared with ICM, the opacities of ICA and ICZ core ceramics
are much greater.
Although these newer core ceramics have excellent fracture resistance
inproper design of the connector area of a FPD can significantly reduce the fracture
resistance and clinical survivability of the prosthesis. Shown in fig. 21-14 is the stress
distribution in a three-unit FPD, which shows relatively high principal tensile stress
(red area) at the tissue side of the interproximal connector when an occlusal load of
250 N is applied to the occlusal surface of the pontic.
In summary, In-Ceram Spinell (ICS) is a glass-infiltrated core ceramic that
offers greater translucency for crowns than either the ICA or ICZ core ceramics.
However, ICA has lower strength and toughness compared with ICA and ICZ. Thus
the use of ICS is limited to anterior inlays, onlyas, veneers, and anterior crowns.
Although ICZ is the strongest and toughest of the three core ceramics, its use is
limited to posterior crowns and FPDs because of its high level of core opacity. ICZ is
a much stronger and tougher material and has greater opacity than ICA.
Procera AllCeram:
The Procera AllCeram crown is composed of densely sintered, high-purity
aluminum oxide core combined with a compatible allCeram veneering porcelain.
This ceramic material contains 99.9% alumina, and its hardness is one of the highest
among the ceramics used in dentistry. Procera AllCeram can be used for anterior and
posterior crowns, veneers, onlays, and inlyas.
A unique feature of the Procera system is the ability of the Procera Scanner to
scan the surface of the prepared tooth and transmit the data to the milling unit to
produce an enlarged die through a CAD-CAM process. The core ceramic form is dry
pressed onto the die, and the core ceramic is then sintered and veneered. Thus the
usual 15%-20% shrinkage of the core ceramic during sintering will be compensated
by constructing an oversized ceramic pattern, which will shrink during sintering to the
desired size to accurately fit the prepared tooth.
CAD-CAM Ceramics:
As shown in the ceramic classification chart in all-ceramic cores can be
produced by processes of condensation and sintering, casting and ceramming,
hotpressing and sintering, sintering and glass infiltration, and CAD-CAM processing
for the Cerec DAB-CAM system the internal surface of inlays, onlays, or crowns is
ground with diamond disks or other instruments to the dimensions obtained from a
36

37. scanned inage of the preparation. for some systems, the external surface must be
ground manually, although some recent CAD-CAM systems are capable of forming
the external surface as well.
A milling operation within a Cerec CAD-CAM unit (Siemens
Aktiengesellschaft, Bensheim, Germany ). The ceramic lock is being ground by a
diamond-coated disk whose translational movements are guided by computer-
controlled input. A cerec CAD-CAM ceramic block is shown in Figure 21-16 before
milling, at an intermediate milling stage, and after completion of the milling operation
for an inlay. These ceramics are supplied as small blocks that can be gound into inlays
and veneers in a computer-driven CAN-CAM system. Vitablocs MK II are feldspathic
procelains that are used in the same way as is Dicor HGC (machinable glass-ceramic).
The disadvantages of CAD-CAM restorations include the need for costly equipment,
the lack of computer-controlled processing support for occlusal adjustment, and the
technique-sensitive nature of surface imaging required for the prepared teeth.
Advantages include negligible porosity levels in the CAD-CAM core ceramics, the
freedom from making an impression, reduced assistant time associated with
impression procedures, the need for only a single patient appointment (with the Cerec
system), and good patient acceptance. A list of CAD-CAM and copy-milled
ceramics is given in Table 21-10.
An advantage of CAD-CAM ceramics is that one can select a core ceramic
either for strength and fracture resistance, for low abrasiveness, or for translucency.
for example, the extensive wear of opposing enamel that occurs when it is opposed by
a feldspathic porcelain surface in the absence of posterior occlusion can be
minimized by selscting a core ceramic that is minimally abrasive to enamel.
Cercon and Lava Zirconia Core Ceramics:
The cercon Zirconia system (Destsply Ceramco, Burlington, NJ) consists of
the following procedures for production of zirconia-based prostheses. After preparing
the teeth (2.0mm incisal or occlusal reduction and 1.5 mm axial reduction), an
impression is made and sent tot he laboratory, where it is poured with a model
material. A wax pattern approximately 0.8 mm in thickness is made for each coping
on the holding appliance on the left side of the scanning and milling unit (Cercon
Brain). A presintered Zirconia blank is attached to the right side of the Brain unit.
(Cercon Brain). A presintered zirconia blank is attached to the right side of the Brain
unit. The blank has an attached barcode, which contains the enlargement factor and
other milling parameters for computer control or the milling procedure After the unit
is activated, parameters for computer control of the milling procedure After the unit is
activated, the pattern in scanned and the blank is rough-milled and fine-milled on
occlusal.
37

38. Ceramic Block Ceramic type
Ceramic
veneer
Indications Manufacturer
CerAdapt Highly sintered Al2O3 All cream Implant
superstructure
Nobel Biocare
Cercon Base Presintered ZrO2;
postsintered after
milling
Coercion
Cream S
Crowns and
FPDs
Dentsply
Ceramco
DC- Kristen Leucite-base Triceram Crowns DCS Dental
AG/Esprident
DC- Zirkon Presintered ZrO2 hot
isostatic
postcompaction
Vitadur D
Triceram
Crowns and
FPDs
DCS Dental
AG/Vita/Espride
nt
Denzir Presintered ZrO2; hot
isostatic
postcompaction
Empress2 Crowns and
FPDs
Decim, Ivoclar
LAVA Frame ZrO2; presintered and
postsntered
LAVA
Ceram
Crowns and
FPDs
3M ESPE
ProCad Leucite-based Malthechnik Veneers, inlays,
onlays, and
crowns
Ivoclar
Procera
AllCeram
Al2O3; presintered and
postsintered
AllCeram Crowns and
FPDs
Nobel Biocare
Synthoceram Al2O3; reinforced;
pressed and
postsintered
Sintagon Crowns Elephant
VitaBlocs Mark
II
Feldspathic porcelain
block
Maltechnik Veneers, inlays,
onlays, and
crowns
Vident
VitaBlocks
Alumina
Sintered Al2O3;
followed by glass
infiltration
Vitadur
Alpha
Crowns and
FPDs
Crowns and
FPDs Vident
VitaBlocs
Spinell
Sintered
MgO-Al2O3 spinel
followed by glass
infiltration
Vitadur
Alpha
Crowns Vident
VitaBlocs
Zirconia
Sintered Al2O3/ZrO2
followed by glass
infiltration
Vitadur
Alpha
Crowns and
FPDs
Vident
38

39. Zircogon ZrO2; presintered and
postsintered
Zircogon Crowns Elephant
And gingival aspects in an enlarged size to compensate for the 20% shrinkage
that will occur during subsequent sintering at 13600
C. the processing times for
milling are approximately 35 for a crown and 80 min for a four – unit fixed FPD. The
milled are approximately 35 min for a crown and 80 min for a four- unit fixed FPD.
The milled prosthesis is removed from the and the remaining extraneous extension are
removed. The zircon coping or framework is then placed in the Cercon furnace and
fired at 13500
C for approximately 6 hours to fully sinter the yttria-stabilized zirconia
core coping or framework. The sintering shrinkage is achieved uniformly and linearly
in three – dimensional space by the in targeted process of scanning, enlarging the
pattern design, controlled milling and sintering.
After any subsequent trimming with a water – cooled, hinge- speed diamond
bur the finished ceramic core framework is then veneered with a veneering ceramic
(Cercon Ceram S) and stain ceramic
All-ceramic prostheses represent the most aesthetically pleasing, but also the
most fracture-prone prostheses. However, with adequate tooth reduction, an excellent
quality impression, a skilled technician, and a ceramic with reasonably high flexure
strength (≥250 MPa) and fracture toughness (≥ 2.5Mpa.m1/2
), reasonably high success
rates can be achieved. The material that has the greatest potential fracture toughness
(9 MPa.m½
) and flexural strength that has the greatest potential fracture toughness (9
MPa.m½
) and flexural strength (>900 MPa) is pure tetragonal stabilized zirconia
(ZrO2). Tinschert et al (200lb) reported that the fracture resistance of three-unit
ceramic FPDs (1278 N) made of Cercon zirconia core ceramic (Dentspy Ceramco)
was more than twice as great as the values reported for In-Ceram Alumina (514 N)
and Empress2 (621 N). Shown in Figure is a comparison of the force required to
fracture three-unit FPDs cemented to dies with zinc phosphate cement. The zirconia
product (Cercon) would be expected to exhibit less fracture resistance in this case, but
clinical data are needed to confirm this hypotehsis.
To ensure maximum survival times, adequate occlusal tooth reduction is
essential for posterior teeth. Optimal clinical performance of some ceramic products
require a minimal occlusal reduction of 2 mm for molar tooth preparations. If the
ceramic will be supported by a material with high elastic modulus such as a ceramic
or metal post or an amalgam build-up, less occlusal reduction (1.5 mm) may be
possible without compromising the survivability of the crowns. For patients
exhibiting extreme brusism, either metal or metal-ceramic prostheses should be used.
METHODS OF STRENGTHENING CERAMICS:
Minimize the effect of stress Raisers
Why do dental ceramic prostheses fail to exhibit the strengths that we would
expect from the high bond forces between atoms? The answer is that numerous
minute scratches and other defects are present on the surfaces of these materials.
These surface flaws behave as sharp notches whose tips may be as narrow as the
spacing between several atoms in the material. These stress concentration areas at the
39

40. tip of each surface flaw can increase the localized stress to the theoretical strength of
the material even though a relatively low average stress exists throughout the bulk of
the structure. When the induced mechanical stress exceeds the actual strength of the
material, the bonds at the notch tip break, forming a crack. This stress concentration
phenomenon explains how materials fail at stresses far below their theoretical
strength.
Stress raisers are discontinuities the ceramic and metal-ceramic structures and
in other brittle materials that cause a stress concentration in these areas. The design of
ceramic dental restorations should also avoid stress raisers in the ceramic. Abrupt
changes in shape or thickness in the ceramic contour can act as stress raisers and
make the restoration more prone to failure. Thus the incisal line angles on an anterior
tooth prepared for a ceramic crown should be well rounded.
In ceramic crowns, several conditions can cause stress concentration. Creases
or folds of the platinum foil or gold foil substrate that become embedded in the
porcelain leave notches that act as stress raisers. Sharp line angles in the preparation
also create areas of stress concentration in the restoration. Large changes in porcelain
thickness, a factor also determined by the tooth preparation, can create areas of stress
concentration.
A small particle of porcelain along the internal porcelain margin of a crown
also induces locally high tensile stresses. A stray particle that is fused within the
inner surface of a shoulder porcelain margin of a metal-ceramic crown can cause
localized tensile stress concentrations in porcelain when an occlusal force is applied to
the crown.
Even though a metal-ceramic restoration is generally stronger than most
ceramic crowns of the same size and shape, care must be taken to avoid subjecting the
porcelain in a PFM to loading the produces large localized stresses. If the occlusion is
not adjusted properly on a porcelain surface, contact points rather than contact areas
will greatly increase the localized stresses in the porcelain surface as well as within
the internal surface of the crown.
Fracture mechanics is a science that allows scientists to analyze the influence
of flaw/stress interactions on the probability of crack propagation through an elastic
brittle solid. The principles of linear elastic fracture mechanics were developed in the
1950s by Irwin (1957). This pioneering research on fracture phenomena was based
on earlier investigations by Griffth (1921) and Orowan (1944, 1949, 1955). Irwin
found that when a brittle material was subjected to tensile stresses, specific crack
shapes in certain locations were associated with greatly increased stress levels. He
also recognized the importance of determining the fracture toughness of these
materials as a measure of their ability to resist fracture. The fracture toughness (KIC)
of a material represents the resistance of a material to rapid crack propagation. In
contrast, the strength of a material depends primarily on the size of the initiating crack
that is present. The strength of dental ceramics and other restorative materials is
controlled by the size of the cracks or defects that are introduced during processing,
production and handling. In this chapter a description is given of the processing
40

41. methods used to produce ceramic prostheses and the potential of these methods to
introduce flaws or cracks that may limit their clinical survival.
The brittle fracture behavior of ceramics and their low tensile strengths
compared with those predicted from bonds between atoms can be understood by
considering stress concentrations around surface flaws. As ceramics tends to have no
mechanism for plastically deforming without fracture as do metals, cracks may
propagate through a ceramic material at low average stress levels. As a result,
ceramics and glasses have tensile strengths that are much lower than their
compressive strengths in the oral environment, tensile stresses are usually created by
bending forces, and the maximum tensile stress created by the bending forces occurs
at the surface of a prosthesis. It is for this reason that surface flaws are of particular
importance in determining the strength of ceramics.
As the crack propagates through the material, the stress concentration is
maintained at the crack, a pore, or a crystalline particle, which reduces the localized
stress. The removal of surface flaws or the reduction of their size and number can
produce a very large increase in strength. Reducing the depth of surface flaws in the
surface of a ceramic is one of the reasons that polishing and glazing of dental
porcelain is so important. The fracture resistance of ceramic prostheses can be
increased through one or more of the following six options: (1) select stronger and
tougher ceramics; (2) develop residual compressive stresses within the surface of the
material by thermal tempering, (3) develop residual compressive stress within
interfacial regions of weaker, less tough ceramic layers by properly matching thermal
expansion coefficients, (4) reduce the tensile stress in the ceramic by appropriate
selection of stiffer supporting materials, (5) minimize the number of porcelain firing
cycles, (6) design the ceramic FPD prosthesis with greater bulk and broader radii of
curvature to minimize the magnitude of tensile stresses and stress concentrations of
curvature to minimize the magnitude of tensile stresses and stress concentrations
during function, and (7) adhesively bond ceramic crowns to tooth structure.
Develop Residual Compressive Stresses:
One method of strengthening glasses and ceramics is the introduction of
residual compressive stresses within the veneering ceramic. Consider three layers of
porcelain: the outer two of the same composition and thermal contraction co-efficient
and the middle layer of a different composition and a higher thermal contraction
coefficient. Suppose that the layers are bonded together and the bonded structure is
allowed to cool to room temperature. The inner layer has a higher coefficient of
thermal contraction and thus contracts more as it cools. Hence, on cooling to room
temperature, the inner layer produces compressive stresses in the outer layers as
previously described for thermal tempering. This three-layer laminate technique is
used by Corning Glass works to manufacture dinnerware.
A similar condition can develop in a veneering porcelain bonding to an alloy
coping used for metal-ceramic crowns and FPDs and adjacent ceramic layers in all-
ceramic prostheses. The metal and porcelain should be selected with a slight
mismatch in their thermal contraction coefficients (the metal thermal contraction
41

42. coefficients (the metal thermal contraction coefficient being slightly larger) so that the
metal contracts slightly more than the porcelain on cooling from the firing
temperature to room temperature. This mismatch leaves the porcelain in residual
compression and provides additional strength for the prosthesis. Examples of how
residual tensile stresses can weaken a metal-ceramic crown or FPD and how residual
compressive stresses can weaken a metal-ceramic crown or FPD and how residual
compressive stresses can increase fracture resistance are shown in Figure.
The same principle applies to ceramic prostheses in which the thermal
contraction coefficient of the core ceramic is slightly greater than that of the
veneering ceramic (such as opaceous dentin or body/gingival porcelain).
The fabrication of metal-ceramic and all-ceramic prostheses usually involves
processing at high temperature, and the process of cooling to room temperature
affords the opportunity to take advantage of mismatches in coefficients of thermal
contraction of adjacent materials in the ceramic structure. Ideally, the porcelain
should sustain slight compression in the final restoration. This objective is
accomplished by selecting an alloy that contracts slightly more than the porcelain on
cooling to room temperature.
A further, yet fundamentally different, method of strengthening glasses and
ceramics is to reinforce them with a dispersed phase of a different material that is
capable of hindering a crack from propagating through the material. There are two
different types of dispersions used to interrupt crack propagation. One type relies on
the toughness of the particle to absorb energy from the crack and deplete its driving
force for propagation. The other relies on a crystal structural change under stress to
absorb energy from the crack. These methods of strengthening are described below.
Minimize the Number of firing cycles:
The purpose of porcelain firing procedures is to densely sinter the particles of
powder together and to produce a relatively smooth, glassy layer (glaze) on the
surface. In some cases, a stain layer is applied for shade adjustment or for
characterization such as stain lines or fine cracks. Several chemical reactions occur
over time at porcelain firing temperatures and of particular importance are increases
in the concentration of crystalline leucite in the porcelain designed for fabrication of
metal-ceramic restorations. Leucite, K2O-Al2O34SiO2, is a highe-expansion crystal
phase, which can greatly affect the thermal contraction coefficient of the porcelain.
Changes in the leucite content caused by multiple firings can alter the thermal
contraction coefficient of the porcelain. Some porcelains undergo an increase in
leucite crystals after multiple firings that will increase their thermal expansion
coefficients. If the expansion coefficient increases above the value for the metal, the
expansion mismatch between the porcelain and the metal can produce stresses during
cooling that are sufficient to cause immediate or delayed crack formation in the
porcelain.
Minimize Tensile Stress Through Optimal Design of Ceramic Prostheses:
42

43. Tougher and stronger ceramics can sustain higher tensile stresses before
cracks develops in areas of tensile stress. Conventional feldspathic porcelains should
not be used as the core of ceramic crowns, especially in posterior areas, because
occlusal forces can easily subject them to tensile stresses that exceed the tensile
strength of the core ceramic. Of major concern are tensile stresses that are
concentrated within the inner surface of posterior ceramic crowns. Sharp line angles
in the preparation also will create areas of stress concentration in the restoration,
primarily where a tensile component of bending stress develops. A small particle of
ceramic along the internal porcelain margin of a crown will also induce locally high
tensile stresses. Thus the ceramic surface that will be cemented to the prepared tooth
or foundation material should be examined carefully when it is delivered from the
laboratory. Furthermore, when grinding of this surface is required for adjustment of
fit, one should use the finest grit abrasive that will accomplish the task.
Because the forces on anterior teeth are relatively small, the low to moderate
tensile stresses produced can be supported by ceramic crowns more safely. However,
if there is a great amount of vertical overlap (overbite) with only a moderate amount
of horizontal overlap (overjet), high tensile stresses can be produced. Metal-ceramic
crowns use a metal coping as the foundation of the restoration to which the porcelain
is fused. The stiff, metal coping minimizes flexure of the porcelain structure of the
crown that is associated with tensile stresses.
Most dental restorations containing ceramics should be designed in such a way
as to overcome their weaknesses, that is their relatively low tensile strength, their
brittleness, and their susceptibility to flaws in the presence of surface flaws. The
design should avoid exposure of the ceramic to high tensile stresses. It should also
avoid stress concentration at sharp angles or marked changes in thickness. One
way to reduce tensile stresses on the cemented surface in the occlusal region of
ceramic inlays or crowns is to use the maximum occlusal thickness possible.
However, within practical limits of tooth reduction, this thickness is typically 2.0
mm.
Aluminous porcelain crowns are contraindicated for restoring posterior teeth
because occlusal forces can induce tensile stresses, which are often concentrated
near the internal surface of the crown. Metal-ceramic crowns use a metal coping as
the foundation of the restoration to which the porcelain is fused. In an attempt to
overcome these stresses, the strong, stiff, yet ductile metal coping minimizes
flexure of the porcelain structure of the crown that is associated with tensile
stresses. Both the bonded platinum foil aluminous porcelain corwn technique and
the swaged gold alloy foil technique are also based on this same concept.
The tensile stresses in a ceramic FPD can be reduced by using a greater
connector height and by broadening the radius of curvature of the gingival
embrasure portion of the interproximal connector. However, a connector height
greater than 4 mm makes the anatomic form in the buccal area of a posterior FPD
too bulky and unaesthetic.
Ion Exchange:
43

44. The technique of ion exchange is one of the more sophisticated and
effective methods of introducing residual compressive stresses into the surface of a
ceramic. The ion-exchange process is sometimes called chemical tempering
(Anusavice et al, 1992) and can involve the sodium ion since sodium is a common
constituent of a variety of glasses and has a relatively small ionic diameter. If a
sodium-containing glass article is placed in a bath of molten potassium nitrate,
potassium ions in the bath exchange places with some of the sodium ions in the
surface of the glass article and remain in place after cooling. Since the potassium
ion is about 35% larger than the sodium ion, the squeezing of the potasium ion into
the place formerly occupied by the sodium ion creates very large residual
compressive stresses.
The product GC Tuf-Coat (GC Corp., Tokyo, Japan) was a potassium-rich
slurry that could be easily applied to a ceramic surface and, when heated to 4500
C
for 30 min (in any standard porcelain furnace) caused a sufficient. Increases of
100% or more in flexural strength have been achieved with several porcelain
products that contained a significant concentration of small sodium ions. However,
the depth of the compression zone is less than 100 µm (anusavic et al. 1994).
Therefore this strengthening effect could be lost if the porcelain or glass - ceramic
surface is ground, worn, or eroded by long - term exposure to certain inorganic
acids.
Thermal Tempering:
Perhaps the most common method for strengthening glass is by thermal
tempering thermal tempering creates residual surface compressive stresses. By
rapidly cooling ( quenching) the surface of the object while it is hot and in the
softened (molten) state. This rapid cooling produces a skin of rigid glass
surrounding a soft (molten) core As the molten core solidifies, it tends to shrink,
but the outer skin remains rigid. The pull of the solidifying molten core, as it
shrinks, creates residual tensile stresses in the core and residual compressive
stresses within the outer surface.
Thermal tempering is used to strengthen glass for uses such as automobile
windows and windshields, sliding glass doors, and diving masks. Often, jets of air
directed at the molten glass surface accomplish the rapid cooling of the outer skin.
If one observes the rear window of an automobile through polarized sunglasses, it
is usually possible to discern a regular pattern of spots over the entire window.
This pattern of spots corresponds to the arrangement of the air jets employed by the
manufacturer in the tempering process. For dental applications, it is more effective
to quench hot glass phase ceramics in silicone oil or other special liquids rather
than using air jest that may not uniformly cool the surface. This thermal tempering
treatment induces a protective region of compressive stress within the surface
( DeHoff and anusavic,1992)
Dispersion Strengthening
44

45. A further, yet fundamentally different, method of strengthening glasses and
ceramics is to reinforce them with a dispersed phase of a different material that is
capable of hindering a crack from propagating through the material. This process is
referred to as dispersion strengthening. Almost all of the newer higher- strength
ceramics derive there improved fracture resistance from the crack- blocking ability
of the crystalline particles. Increasing the crystal containing primarily can
strengthen dental ceramics containing primarily a glass phase can be strengthened
by increasing the crystal content of leucite (K2O.Al2O3.4SiO2) lithia dislocate
(Li2O.2SiO2), alumina ( Al2O3) magnesia - alumina spinel (MgO. Al2O3), zirconia
(ZrO2), and other types of crystals. Some crystal phase additions are not as
effective as others in toughening the ceramics. Toughening depends on the crystal
type, its size, its volume fraction, the interpartics. Toughening, and its relative
thermal expansion coefficient relative to the glass matrix. For example, the fracture
toughness (KIC) of soda lime silica glass is 0.75 Mpa . m1/2
If one disperses 70 vol
% of terasilicic flourmill crystals in the glass.(Dicor MGC glass - ceramic), the
toughness increases only to 1.5 Mpa. m1/2
. However, by dispersing 70 vol% if lithia
dislocate crystals in the glass matrix ( IPS Empress2), KIC increased to 3.3 Mpa. M
1/2
( Holand and Beall, 2002).
When a tough, crystalline material such as alumina ( Al2O3) is added to a
glass the is toughened and strengthened because the crack cannot pass through the
alumina [particles as easily as it can pass through the glass matrix. This technique
has found application in dentistry in the development of alimonies porcelains
( Al2O3 particles in a glassy porcelain matrix) for porcelain jacket crowns. Most
dental ceramics that heave a glassy matrix utilize reinforcement of the glass by a
disposed crystalline substance.
Tinschert et al (200lb) evaluated the mean strength and standard deviation
values for several ceramics. This mean strength values were as follows; (Mpa ±
SD) were Cerec Mark II, 86.3±12.2; In Ceram Alumina, 429.3 ± 87.2; IPS
Empress. 83.9 ± 11.3; Vitadur Alpha Core, 131.0 ± 9.5; Vitadur Alpha Dentin, 60.7
± 6.8; Vita VMK 68, 82.7± 10.0: and Zirconia-TZP, 913.0 ± 50.2. There was no
statistically significant difference among the flexure strength of Cerec Mark II,
Dicro, IPS Empress, Vitadur Alpha Dentin, and Vita VMK 68 ceramics (P>0.05).
The highest Weibull moduli were associated with Cerec Mark II and Zirconia-TZP
ceramics (23.6 and 18.4). Dicor glass-ceramic and In-Ceram Alumina had the
lowest values of Weibull modules (m) (5.5 and 5.7), whereas intermediate values
were observed for IPS Empress, Vita VMK 68, Vitadur Alpha Dentin, and Vitadur
Alpha Core ceramics (8.6, 8.9, 10.0 and 13.0 respectively).
Except for In-Ceram Alumina, Vitadur Alpha, and Zirconia-TZP core
ceramics, the investigated ceramic materials fabricated under the condition of a
dental laboratory were not stronger or more structurally reliable than Vita VMK 68
veneering porcelain. Only Cerec Mark II and Zirconia-TZP specimens, which were
prepared from an industrially optimized ceramic material, exhibited m values
greater than 18. Hence, we conclude that industrially prepared ceramics are more
45

46. structurally reliable materials for dental applications, although CAD-CAM
procedures may induce surface and subsurface flaws that may offset this benefit.
Transformation Toughening:
When small, tough crystals are homogeneously distributed in a glass, the
ceramic structure is toughened and strengthened because cracks cannot penetrate
the fine particles as easily as they can penetrate the glass. Dental ceramics are
strengthened and toughened by a variety of dispersed crystalline phases including
alumina (Vitadur Alpha, Procera AllCeram, In-Ceram alumina), leucite (Optec
HSP, IPS Empress, OPC), tetrasilicic fluormica (Dicor, Dicor MGC), lithia
disilicate (OPC 3G, IPS Empress2), and magnesia-alumina spinel (In-Ceram
spinell). In contrast, dental ceramics based primarily on zirconia crystals (Cercon
and Lava) undergo transformation toughening that involves a transformation of
ZrO2 from a tetragonal crystal phase to a monoclinic phase at the tips of cracks that
are in regions of tensile stress. The unit cells for tetragonal and monoclinic lattices
are shown.
When pure ZrO2 is heated to a temperature between 14700
and 20100
C and
it is cooled, its crystal structure begins to change from a tetragonal to a monoclinic
phase at approximately 11500
C. During cooling to room temperature, a volume
increase of several percentage points occurs when it transforms from the tetragonal
to monoclinic crystal structure. This polymorphic transformation can be prevented
with certain additives such as 3 mol% yttrium oxide (yttria or Y2O3). This material
is designated as ZrO2. TZP (tetragonal zirconia polycrystals). The volume increase
in this case is constrained if the zirconia crystals are sufficiently small and the
microstructure is strong enough to resist the resulting stresses. This material is
extremely strong (flexural strength of approximately 900 MPa) and tough (fracture
tougheness, KIC, of approximately 9 MPa) and tough (fracture tougheness, KIC, of
approximately 9 Mpam1/2
).
The toughening mechanism of crack shielding results from the controlled
transformation of the metastable tetragonal phase to the stable monoclinic phase.
Several types of crack shielding processes are possible, including microcracking,
ductile zone formation, and transformation zone formation. By controlling the
composition, particle size, and the temperature versus time cycle, zirconia can be
densified by sintering at a high temperature and the tetragonal structure can be
maintained as individual grains or precipitates as it is cooled to room temperature.
The tetragonal phase is not stable at room temperature, and it can transform to the
monoclinic phase with a corresponding volume increase under certain conditions.
When sufficient stress develops in the tetragonal structure and a crack in the area
begins to propagate, the metastable tetragonal crystals (grains) or precipitates next
to the crack tip can transform to the stable monoclinic form. In this process a 3 vol
% expansion of the ZrO2 crystals or precipitates occurs that place the crack under a
state of compressive stress and crack progression is arrested. For this crack to
advance further, additional tensile stress would be required. Because of this
46

47. strengthening and toughening mechanism, the yttria-stablilized zirconia ceramic is
sometimes referred to as ceramic steel.
Abrasiveness of Dental Ceramics:
A review of the factors and material characteristics that cause excessive
wear of enamel by ceramic prostheses is extremely important to optimize the
performance of ceramic-based prostheses. Ceramics are generally considered the
most biocompatible, durable, and aesthetic materials available for rehabilitation of
teeth, occlusal function, and facial appearance. Currently available products
exhibit variable mechanical properties (hardeness, flexure strength, fracture
toughness, and elastic modulus), physical properties (index of refraction, color
parameters, translucency, chemical durability, and thermally compatible expansion
coefficients for the substrates. In spite of their overall excellence in meeting the
ideal requirements of a prosthetic material, dental ceramics have one major
drawback. These materials can cause catastrophic wear of opposing tooth structure
under certain conditions. The most extreme damage occurs when a roughened
surface contacts tooth enamel or dentin under high occlusal forces, which may
occur because of bruxing, premaguided disclusion is ensured, the wear of opposing
enamel and dentin will be greatly reduced. In addition, if the occluding ceramic
surface area is periodically refinished after occlusal adjustment or frequent
exposure to carbonated beverages and/or acidulated phosphate fluoride, the
abrasive wear of opposing tooth structure is further reduced.
Abrasive wear mechanisms for dental restorative materials and tooth enamel
include (1) adhesion (metal and composites), in which localized bonding of two
surfaces occurs, resulting in pullout and transfer of matter from one surface to the
other, and (2) microfracture (ceramics and enamel), which results from gouging,
asperties, impact, and contact stresses that cause cracks or localized fracture. For
ceramic and enamel, two-phase brittle strucutres are involved. The ceramic onsists
of a glass matrix that contains variable levels and sizes of crystals. Tooth enamel
consists of a small volume fraction of organic phase matrix and a high volume
fraction of hydroxyapatite crystals. The wear of either material depends on the ease
with which cracks can propagate through the structure. If microscopic cracks are
forced to pass around the crystal particles rather than through them, the material
will usually be more fracture and cracks through the glass phase, the particles are
less fracture resistant than the glass matrix, or excessive voids or other defects exist
along the pathway. The relative strengthening effect is dependent on several
factors, including the strength of the glass and crystal phases, the size and spacing
of crystalline paricles, the inferfacial bond strength of the crystal-glass interphase
region, and the type and magnitude of residual stresses in the structure. These
factors are beyond the control of the dentist, although the dentist and laboratory
technician can select ceramics that are highly fracture-resistant.
The microfracture mechanism is the dominant mechanism responsible for
surface breakdown of ceramics and the subsequent damage that a roughened
ceramic surface can cause to tooth enamel surfaces. eNamel is also susceptible to
47

48. this kind of microfracture through four specific mechanisms: (1) asperities
extending from the ceramic surface that produce high localized stresses and
microfracture; (2) gouging that results from high stresses and large hardness
differences between two surfaces or particles extending from these surfaces; (3)
impact or erosion that occurs through the action of abrasive particles carried in a
following liquid such as saliva; and (4) contact stress microfracture that increases
localized tensile stress and also enhances the damage caused by asperties, gouging,
and impact or erosion. Because of microfracture mechanisms, it may be necessary
to polish the ceramic surface periodically to reduce the height of asperities and to
minimize enamel wear rates. Of major concern is the potential catastrophic
damage that can be incurred by enamel in contact with polycrystalline asperities
having high fracture toughness (KIC) values such as alumina (3.5-4.0 Mpa.m1/2
) or
cerium-stabilized zirconia (10-16 Mpam1/2
), yttrium-stabilized zirconia (6-9-
Mpa.m1/2
), or cerium--stabilized zirconia (10-6 Mpa.m1/2
). In contrast glass has a
fracture toughness of only 0.75 Mpa.m1/2
and should cause less gouging, contact
stress, and impact damage within contacting enamel surfaces.
The abrasiveness of ceramics against enamel is affected by numerous
factors and properties of the crystal phase particle and the glass matrix (if present).
These include hardness, tensile strength, fracture toughness, fatigue resistance,
particle glass bonding, particle-glass interface integrity, chemical durability,
exposure frequency to corrosive chemical agents (acidulated phosphate fluoride,
carbonated beverages), abrasiveness of foods, residual stress, subsurface quality
(voids or other imperfections), magnitude and orientation of applied forces,
chewing and bruxing frequency, contacting area, lubrication by saliva, and wear
frequency. Thus it is understandable why the hardness of the ceramic is not a good
predictor of the potential wear of enamel surfaces by a ceramic. However, the
larger the hardness difference between two sliding surfaces, the greater is the
degree of gouging.
Wear of ceramics Compared with other Materials
To minimize enamel abrasion by a contacting ceramic structure, we should
use a ceramic that exhibits uniform surface microfracture at the same rate as tooth
enamel under the same conditions of loading, antagonist structure, food substance
abrasiveness, applied forces, and degree of lubrication. The breakdown of the
ceramic surface should be uniform so that asperities such as large crystalline
inclusions do not project out from the surface.
These asperities produce high stress concentration areas within the opposing
enamel surface that lead to gouging, troughing, and greater localized microfracture
of the enamel structure. If such nonumiform surface wear of ceramics occurs
duraing oral function, the only solutions available to reduce enamel wear are to
reduce the occlusal load by occlusal adjustement or to polish the ceramic surface
periodically to reduce stress concentrations and the height of these asperities.
Some of the new ultralow-fusing ceramics have a wide range of thermal
expansion coefficients as listed. These are approximate values estimated for
48

49. several low-fusing ceramic products produced by the Ducera company: Duceragold
(7800
C) and Duceram LFC (Dentsply Ceramco) were introduced between 1991 and
1992. Duceram LFC is classified as a hydrothermal ceramic that was claimed to
develop a hydrothermal layer approximately 1µm thick vitro. In theory, this
property allows a protective layer to seal microscopic surface cracks. Its veneer is
a low-fusing ceramic that minimizes shrinkage of the core ceramic during
subsequent firings. Duceram LFC and Duceragold do not contain large leucite
crystals and therby retain a stable thermal expansion coefficient over several
firings. The opalescence the fluorescence are also easier to achieve than for
conventional low-fusing fieldspathic porcelains because of the ability to maintain
very small crystal particles (400-500nm). Because of its high expansion
coefficient, Duceragold is intended as a veneer for high-expansion, low-fusing
porcelains, were subsequently developed as veneering ceramics for titanium metals
and Procera AllCeram (Nobel Biocare, Goteborg, Sewden) core ceramics,
respectively. TiCeram is another ultralow-fusing ceramic and has a firing
temperature of approximately 7400
C. The initial veneering porcelain for Procera
AllCeram was Vitadur N (Vita Zahnfabrik, Bad Sackingen, Germany), a large-
particle aluminous porcelain. Currently, AllCeram porcelain (Degussa Dental) is
used. Finesse, an ultralow-fusing ceramic (~7600
C) that contains larger leucite
crystals, was introduced by Dentsply Ceramco, Inc. (E. Windsor, NJ). Vita Omega
900 (Vita Zahnfabrik) is another ultralow-fusing ceramic. For CAD-CAM
processing, dicor MGC glass-ceramic (Caulk/Dentsply, Milford, DE), Vita MK I
ceramic (Vita Zahnfabrik), and Vita MK II ceramic (Vita Zahnfabrik) blocks are
available, which also offer a small-particle distribution of crystals that may reduce
wear of opposing enamel surfaces.
It is not known what effect, if any, thermal mismatch differences will
produce on the surface quality of these ceramics. Microcracking can lead to
surface flaws, loss of surface material, and increased wear of enamel. However,
this effect should only occur when a gross mismatch occurs between a core ceramic
and its veneering ceramic.
Based on a study of abrasion by a 500-g slurry of glass (Derand and Vereby,
1999), 100g alumina (100 µm), and 120g water, the mean wear depths (in microns)
after a specified time period for several ceramics and tooth enamel were as follows:
enamel (24.3), Finesse (20.3), Vitadur Alpha (16.3), Procera (15.8), Dentsply
Ceramco II (13.6), Vita Omega (13.1), Ti-Ceram (12.1), IPS Empress (11.8),
Duceragold (11.5), and Creation (10.8). The enamel wear was significantly greater
than that of all ceramics tested. The wear depth of Vitadur Alpha was significantly
than that of all ceramics tested. The wear depth of Vitadur Aplha was significantly
greater than that for IPS Empress, Duceragodl, and Creation ceramics. It is clear
that the relative wear rate of enamel by a highly abrasive medium is greater than
that of most porcelains.
Wear of Enamel by Ceramic Products and other restorative materials
49

50. Another factor that can increase wear of ceramics against enamel is the non-
uniform distribution or clustering of crystals. IPS Empress after hot-pressing at
1800
C exhibits clusters of relatively large (5-10 µm) leucite crystals (KAISi2O6)
with cracks between the crystal agglomerates. This non-interlocking arrangement
of elucite crystals also occurs in the veneering ceramic after it is sintered at 9100
C.
in contrast, IPS Empress2 core ceramic exhibits a uniform dispersion of smaller
lithia disilicate (LiSi2O4) crystals after hot pressing at 9200
C and veneering at 8000
C.
One should expect greater wear of enamel by IPS Empress compared with
IPS Empress2. IPS Empress ceramic contains 35 ± 5 vol% of leucite crystals that
are formed in a noninterlocking particle cluster pattern (Holand et al, 2000). The
core microstructrue of IPS Empress2 is quite different from that of IPS Empress,
evidenced by elongated lithia disilicate crystals 0.5 to 4 µm in length and a smaller
concentration of lithium orthophosphate crystals (Li2Si2O5) approximately 0.1 to
0.3 µm in diameter (Holand et al, 2000).
Studies of ultralow-fusing ceramics have generally revealed significantly
lower enamel wear rates than those produced by conventional low-fusing
porcelains. However, the results of a recent study suggest that one of these
ceramics, Duceram LFC, caused significantly more enamel wear (0.197 mm3
),
presumably because of the higher void volume within the surface layer of Duceram
(Magne et al, 1999). In this study, the combined enamel/ceramic wear rates were
significantly greater for Duceram LFC (0.363 mm3
). Veneering ceramics contain
either large crystalline filler particles or a glass structure with no crystals or very
small crystals.
Similarly, the results from another in vitro study (Al-Hiyasat et al, 1999) of
enamel wear (after 25,000 simulated chewing cycles) by ceramics using a corn
meal slurry (three-body condition) revealed greater relative wear depth in enamel
by Duceram LFC (0.74mm) and Vitadur Alpha (0.80 mm) compared with Vita
Cerec Mk II (0.48 mm). The explanation given for the higher wear rate of enamel
by Duceram LFC was the presence of porosities within the surface of the ceramic.
This result for Duceram LFC is in contrast with two-body wear data (al-Hiyasat et
al, 1998a) that indicated significantly less enamel wear by Duceram LFC also Vita
Mk II (0.65 mm) and a gold alloy (0.09 mm) and Vita Omega porcelain (0.96 mm).
The wear Vitadur Alpha porcelain (0.93 mm) and in distilled water without an
abrasive food medium (0.54 mm after 25,000 simulated chewing cycles) compared
with Vitadur Alpha porcelain (0.93 mm) and Vita Omega porcelain (0.96 mm).
The wear rate of enamel by gold alloy was significantly less than by the four
ceramics.
One would expect the latter types of ceramics to cause minimal wear of
enamel. Metzler et al (1999) reported that the relative enamel loss was less for two
lower fusing ceramics, Finesse (0.56) and Vita Omega 900 (0.60), compared with
Dentsply Ceramco II porcelain (0.85), large-particle leucite-based porcelain.
50

51. Krejci et al (1994) reported significantly lower estimated 5-year enamel
wear rates for amalgam (50 µm) and a new Cerec CAD-CAM ceramic (95 µm),
Vita Cerec Mk II V7K, compared with the original Vita Cerec Mk I ceramic (225
µm). In comparison, the wear of enamel by enamel was 107 µm. Hacker et al
(1996) found considerably lower enamel wear rates for a gold-palladium alloy (9
µm) compared with AllCeram veneer ceramic (60 µm) and Dentsply Ceramco
porcelain (230 µm).
Jagger et al (1995) reported the following wear depths of dentin after
exposure to wear by restorative materials: amalgam (0 µm), microfilled composite
(7 µm), gold alloy (16.7 µm), conventional composite (31.7 µm), and Vitadur N
aluminous porcelain (100 µm). these results indicate that direct filling materials are
less abrasive to dentin than aluminous porcelain. This result is not surprising.
What is of importance of importance is the significant potential benefit of amalgam
and microfilled composite as the least abrasive restorative materials fr situations in
which dentin is exposed.
Al-Hiyasat et al (1998b) investigated the effect of a carbonated beverage
(Coca Cola) on the wear of human enamel and three dental ceramics: Vitadur
Alpha (feldspathic porcelain), Duceram LFC (ultralow-fusing porcelain), and Vita
Mark II, a machinable ceramic. Tooth and ceramic specimens were tested in a
wear machine under a load of 40 N, at 80 cycles per minute, for a total of 25,000
cycles. The test was performed in distilled water or with intermittent exposure to a
carbonated beverage (Coca Cola). When tested in water Alpha porcelain caused
significantly more enamel wear and also exhibited greater wear than Duceram LFC
and Vita Mark II. However, after exposure to the carbonated beverage, the enamel
wear produced by Duceram LFC did not differ significantly from that produced by
Alpha porcelain. Vita Mark II produced the least amount of enamel wear.
Exposure to the carbonated beverage significantly increased the enamel wear. The
wear of Duceram LFC and vita Mark II increased with exposure to the carbonated
beverage. It was produced by Duceram LFC and Vita Mark II ceramics. Overall,
Vita Mark II was the most resistant to wear and also significantly less abrasive than
Vitadur Alpha porcelain.
Reducing Abrasiveness of Ceramics by Polishing and Glazing:
In theory, the smoothest surface should cause the least wear damage to
opposing surfaces. Depending on the initial surface roughness of the ceramic
surface, glazing the surface may not adequately decrease the surface roughness
since the glassy layer may be of insufficient thickness to fill in scratches and
grooves within the ground surface. Thus, under certain conditions, polishing or
polishing followed by glazing may be required.
Jagger and Harrison (1994) reported that the amount of enamel wear
produced by both glazed (28.8 µm) and unglazed Vitadur N aluminous porcelain
(29 µm) was similar; however; the wear produced by polished porcelain (12µm)
51

52. was substantially less. Polished or glazed porcelain caused significantly less wear
than unglazed porcelain. Polishing was accomplished with 3M Soflex disks and
Shofurubber points.
After 25,000 cycles of abrasion testing of various porcelain surface on
human enamel in vitro, Al-Hiyasat et al (1997) reported no significant difference
between the enamel wear of glazed and polished groups, but wear produced by the
unglazed groups was significantly higher (P<0.05). Sixty pairs of tooth-porcelain
specimens were tested under load in distilled water with and without intermittent
exposure to a carbonated beverage. Wear of enamel and Vitadur Alpha porcelain
specimens was determined after 5,000, 50,000 and 25,000 cycles. Exposure to
carbonated Coca Coal and schweppes beverages significantly increased the amount
of enamel wear produced by all porcelain surfaces (P <0.001). The finish of the
porcelain surface did not influence its wear resistance under these conditions.
Guidelines for Minimizing Excessive Wear of Enamel by Dental ceramics:
To minimize the wear of enamel by dental ceramics, the following steps
should be taken: (1) ensure cuspid-guided disclusion: (2) eliminate occlusal
prematurities: (3) use metal in functional bruxing areas; (4) if occlusion in ceramic,
use ultralow-fusing ceramics (5) polish functional ceramic surfaces; (6) repolish
ceramic surfaces periodically; and (7) readjust occlusion periodically if needed.
Some ultralow-fusing ceramics are less abrasive than traditional low-fusing
ceramics, but few clinical studies have been reported on any of these materials to
validate the in virto findings. Caution should be exercised in selecting these new
ceramics for use since they exhibit widely variable expansion co-efficients and may
not be thermally compatible with certain ceramic core materials or metal substrates.
Malocclusion is likely the major wear-causing factor that must be avoided to
achieve acceptable wear performance with any ceramic product.
Polishing is preferred over glazing as a procedure to reduce abrasion
damage of enamel. Ceramic surfaces should be refinished periodically after acid
exposure especially acidulated phosphate fluoride. The Shofu porcelain polishing
kit followed with diamond paste or SofLex discs (3M) without a diamond paste
follow up are useful as effective finishing products.
Ceramic and opposing surfaces should be examined periodically for
evidence of excessive wear. Occlusal adjustment and polishing of the ceramic
surfaces should be performed to reduce the risk for further surface degradation.
Noble metal surfaces are especially indicated for individuals who exhibit evidence
of severe bruxing since the wear rates of gold alloys are very low compared with
the wear damage caused by either traditional ceramics or recent lower-fusing
ceramics.
A rough ceramic surface that is in hyperocclusion with opposing enamel is
very likely to cause great abrasive wear of tooth surfaces. to minimize the risk of
such wear damage to tooth enamel or other surfaces, the smoothest possible
ceramic surface should be produced. This can be accomplished by (1) polishing
only, (2) polishing followed by glazing, or (3) glazing only. The second choice is
52

53. preferred. Glazing is recommended whenever possible before cementation of a
prosthesis. if this is not possible, polishing along is acceptable. However, glazing
of a very rough ceramic surface may not sufficiently reduce the surface roughness
to minimize wear damage. It is clear that a glazed rough, nonglazed surface
because the more-abrasive crystalline particles tend to be covered by the
lessabrasive glass phase.
it is not always possible to polish a ceramic surface in the clinic. Because of
the heat generated during the polishing of ceramic-based prostheses that require
extensive polishing, the temperature increase of pulpal tissue may lead to
irreversible pulpitis. This is especially true when the tooth has been greatly
reduced in size and the pulp chamber is within 0.5 mm of the external surface of
the prepared tooth.
There are several clear indications for polished ceramic surfaces. Polishing
of ceramic prostheses should be performed when they cannot be autoglazed.
Polishing of ceramic restorations that have functional occlusal pathways or
subgingival extensions will ensure optimal smoothness. All CAD-CAM inlays or
other ceramic prostheses that will not receive veneering ceramic should also be
polished.
Intraoral instrumentation can produce a smoother surface than an
autoglazing procedure. Highly polished porcelain may also be naturally glazed or
overglazed without significantly increasing the surface roughness. Increased time
or cycles of glazing with decrease surface roughness.
Polishing instruments should be selected according to type of ceramic, type
of restoration, and level of smoothness desired. If the crown was ground with a
100 µm grit diamond, the first polishing abrasive should be 75 µm or less. If the
abrasive is too fine, more time will be needed to polish the surface.
Hulterstrom and Bergman (1993) found that two of the best polishing
systems are Sof-Lex disks (3M Dental) and Shofu Porcelain Laminate Polishing
Kit followed by diamond paste. For the Sof-Lex disks, one should start with a disk
that is the most effective at removing the initial grinding patterns. If the abrasive
grit size on the disk is too small, it will take too long to decrease the roughness. If
the grit size on the disk is too small, it will take too long to decrease the roughness.
If the grit size is too large (e.g., extra-coarse or coarse), the surface will become
rougher. Also, if the polishing procedure is performed in the mouth, care should be
taken to avoid heat build-up. Sof-Lex Sof-Lex disk are made to be used in a wet
environment, so water coolant should be used whenever possible.
LUTING AGENTS
The development of new materials such as all-ceramic restorations has
brought about a substantially different attitude concerning luting agents.While zinc
phosphate cement has been the standard for nearly a century, its overall properties are
insufficient for certain types of restorative systems. For example, this traditional
cement, which has been quite successful for metal-based restorations, falls far short
for porcelain restorations. When zinc phosphate cement is used to lute all-ceramic
53

54. restorations, bulk fractures can be expected. Specifically, during mastication energy is
transmit. A series of cores fabricated by means of a pressing process. A low-fusing
porcelain (Finesse, Dentsply Ceramco) fused to a gold metal substrate —a typical
example of this material’s potential. In the case of zinc phosphate cement, the energy
is distributed in localized regions at the restoration-preparation interface. The
application of high levels of stress in localized zones then predisposes the restoration
to fracture. The fractures associated with the early all-ceramic crowns could be
attributed to this phenomenon. The ability to bond not only to the surface of the tooth
but also to the internal surface of the ceramic restoration reduces this potential
appreciably. When all contact surfaces are bonded as a single structure, the
masticatory energies are distributed more uniformly. Consequently, fractures within
the ceramic restoration are reduced significantly. It is important, therefore, to select a
luting agent that exhibits the appropriate properties. Such a cement should not only
bond to the surfaces of the preparation through the hybridizing process but also to the
internal surfaces of the ceramic restoration by means of silanation and acid etching.
Finally, the cement should exhibit sufficient flexural strength, flexural modulus and
fracture toughness. Other desirable properties include color stability and sufficient
release of fluoride to offer protection against secondary caries. This luting agent
should possess dual-cure potential as well as multiple viscosities for use with different
types of restorations. Newer composite-based resins such as Calabra (Dentsply Caulk)
show promise in fulfilling these requirements.
PORCELAIN INLAYS/ONLAYS
Porcelain is indicated primarily for restoring gingival cavities in the buccal or
labial surfaces of the teeth, where the restoration is free from direct force applications.
Cavities are prepared with uniform thickness to the inlay, with the outer walls tapered
outward only slightly.
Cavities on the proximal surfaces of the anterior teeth are occasionallyl
restored by means of porcelain inlays. For access for impression and placement of
inlay labial and lingual walls and the cavity should be removed. Etching the finished
inlay with hydrofluoric acid and undercutting the cavity slightly after the impression
is taken aid in developing a cement bond for adequate inlay retention.
Incisal angles and edges of the anterior teeth may also be restored with
porcelain inlays. Usually a step is prepared in the remaining portion of the tooth and
plantium pins are cemented in the dentin to provide adequate retention.
Porcelain is rarely indicated for either occlusal or two surface inlays in the
posterior teeth because of its tendency to fracture under direct force application.
For inlay fabrication, the procedure that permits the fusion of the porcelain
directly on the ceramic investment die material. Three types of investment materials
have been developed for this purpose, including a silicate, a phosphate, and a calcium
sulfate bonded type, having differential physical properties.
Although the process eliminates the platinum matrix, it is time consuming and
does not effectively shorten procedure. Results on one study did not yield on inlay
54

55. with significantly improved margin accuracy over that obtained with the conventional
platinum matrix technique.
The conventional technique for porcelain inlays, with the platinum matrix, has
been modified to what has recently been described as the “rapid inlay technique”. The
use of this modification has increased in popularity. Essential to the procedure in the
proper cavity design and the formation of the accurate impression on good dies, as
well as careful adaptation of the platinum matrix. In this technique, time is varied in
the three firing operation by insertion of the porcelain to be fused directly into the
furnace at 12050
C on a special quick heating tray after it has been dried briefly by
being placed in front of the furnace door. The firing time at this temperature is about
25 seconds, after which it is air cooled to room temperature. The second and third
applications of porcelain are made with comparable firing time for each. This method
has significantly shortened the inlay firing operation.
The cementing medium in an important factor in the final appearance of the
restoration, since shrinkage tends to make porcelain inlays undersize and a fine
cement line may become visible at the margins. The more translucent silico phosphate
rather than the opaque zinc cements is sometimes selected as a cementing medium for
porcelain restorations.
PORCELAIN VENEERS
Indications:
1. Discoloration – Tetracycline, revitalization, fluorosis and even teeth darken with
age.
2. Enamel defects like hypoplasia and malformations
3. Diastemata
4. Malposition and malocclusion
5. Poor restorations
6. Aging
7. Wear pattern
8. Agenesis of lateral incisor
Contraindications:
1. Non-availability of enamel
2. Oral habits like bruxism, tooth to foreign body object habits.
Advantages:
1. Color and esthetics
2. Bond strength
3. Periodontal health
4. Resistance to abrasion
5. Inherent porcelain strength
6. Resistance to fluid absorption
Disadvantages:
1. Time involved
2. Difficulty in repair
3. Technique sensitive
4. Tooth preparation
55

56. 5. Fragility of material
6. Cost
Tooth reduction:
1. Labial reduction 0.3 to 0.5mm
2. Incisal or lingual finishing line should be modified chamfer
3. Sulcular marginal placement exactly at gingival margin with rounded shoulder
preparation.
Methods of fabrication:
1. Refractory investing technique
2. Platinum foil technique
3. Cost ceramic laminate technique
a. Castable ceramic (Dicor)
b. Castable appetite (cerapearl)
Etching of enamel:
1. Neighbouring teeth are protected by mylar strips or a dead soft metal matrix band
2. Etched with 30-37% phosphoric acid solution for 15 seconds.
3. Washed with copious amount of water for 30 seconds.
Etching of porcelain veneer:
1. Etch the inner surface using a etching gel 7.5% hydrofluoric acid constantly
brushed upwards to the periphery of the laminate and allow to stand for 7-10
minutes.
2. Completely submerge the veneer in a 10% solution of baking soda and water until
the acid is neutralized
Bonding and porcelain veneers forms the following layers (from tooth surface
outside)
1. Etched enamel
2. Combined E-D bonding agent (Light activated)
3. Composite resin luting agent layer
4. Unfilled resin layer
5. Silane coupling agent to enhance adhesive properly of resin
6. Etched porcelain veneer
Expanded use of porcelains:
1. Conventional – lingual etched porcelain retainers
2. For posteriors – buccal etched porcelain retainers
3. Combination of buccal and lingual etched retainers
4. Interproximal retainers
5. Etched porcelain pieces.
56

57. PORCELAIN JACKET CROWN AND PLATINUM REINFORCED CROWNS
The original porcelain jacket crown, made of feldspathic porcelain, possessed
excellent esthetic but was very prone to fracture. With the development of aluminous
reinforcement, the restoration again generated interest amongst dentists. It is still an
inherently weak restoration, however its use should be restricted to incisors, where a
maximum cosmetic result is necessary.
More than any other restoration, the porcelain jacket crown depends on tooth
support which is more critical for the fracture resistance of the restoration than the
bulk of porcelain. The “Crescent moon fracture” often seen in this type of restoration
is a direct result of inadequate preparation length.
The recommended incisal reduction range from 1 to 1.4 to 2mm. To produce
adequate esthetic result, it is best to reduce the incisal edge by 2mm. Any greater
reduction will increase the stress in the facial surface which can result in the facial
half moon fracture.
Axial reduction if facial and lingual surfaces should about 1mm.
Recommendations for reduction of the lingual surface range from 0.5 to 1mm. All the
line angles to be round off.
A shoulder 1mm wide is generally preferring shoulders narrower than 1mm
should be reserved for those teeth too small to safely permit more than minimal
destruction of tooth structure.
Finally sufficient dentin must remain to provide a strong core.
Maximum accuracy requires dies that is hard enough to with stand the
pressures of fabrication, an acrylic resin backed, silver plated die should always be
used. Undercuts below the shoulder must be blocked out by acrylic resin prior to foil
adaptation. If undercut is not blocked it makes difficult to remove and causes
distortion on removal.
The finished jacket preparation therefore should have the appearance of a
miniature tooth, with no undercuts and shoulder about 0.5 to 1mm wide placed
slightly beneath the gingival tissues around the entire tooth.
The separating medium is applied to all parts of the die that will be contacted
by block out material. The block out material and excess foil will be removed later for
final finishing and evaluation.
Foil adaptation:
To fabricate the porcelain jacket crown a piece of 0.025mm or 0.001 inch
platinum foil of correct shape and size is selected. The foil is annealed in a Bunsen
burner flame, quenched and carefully wrapped around the die to give the basic matrix
form. The shoulder area can be formed with the thumbnail. A pair of fine scissors are
used to remove excess foil. A tinner’s joint approximately 1.5mm wide is formed on
the mesial or distal aspect and along the incisal edge.
The uppermost piece of foil is reduced by half, and a wedge shaped section
axio-incisal corner. A wedge may also be removed at the axiiogingival corner. The
longer edge of the foil is folded over the shorter one and the tinner’s joint closed.
57

58. The foil is burnished gently but firmly with a beaver foil burnisher. Burnish
from the incisal edge to the shoulder, beginning at the joint and then around the
preparation from the joint. once the basic shape is established, the foil is removed
from the die and excess foil removed from the skirt leaving a 1-2mm extension. The
skirt adds strength and stability to the matrix. Reannel the foil, replace it on the die
and complete the burnishing.
If a wedge shaped section was not removed from the tinner’s joint at the
axiogingival corner of the shoulder, then four thickness of foil will remain. If so, the
area can be carefully reduced with a rubber wheel with care not to perforate the foil.
Remove and reanneal the foil to burn of any remaining debris. The procedure of
annealing is to remove gaseous impurities from the platinum. The matrix is degassed
in an oven at 21000
C F for 6 minutes under vacuum and replace the matrix on the die.
Platinum reinforced crowns:
For the fabrication of a platinum reinforced crown either – a twin foil
technique.
Single foil technique maybe used
A twin foil technique requires the fabrication of the second foil with the
Tinner’s joint on the opposite side from the first to be adapted on the die. The skirt of
the second foil is removed. The first foil replaced on the die, and a second foil pushed
into place over it. Since it is the second foil that will later remain in the crown. It is
removed, abraded tin-plated, oxidized and replaced.
In the single foil technique, atleast one day prior to foil fabrication, the die,
except for the shoulder portion is spaced with die relief material equal to one
thickness of foil. After the foil is adapted, the should skirt portions are protected prior
to tin plating by covering them with either inlay wax or cyanocrylate gel. This burns
of during oxidation, leaving an area of clean foil to which the porcelain will not bond.
After the finished crown has been glazed, this area of foil is removed with a sharp
scalpel and the die spacer removed from the die to produce an accurate fit.
Aluminous Core Porcelain Build Up:
Although the core material is different, the technique for core fabrication is the
same for both the standard porcelain jacket crown and the platinum reinforced crown.
Platinum reinforced core material is slightly more opaque to mask out the gray colour
of the oxidized foil.
The core material is mixed to a stiff consistency with the recommended liquid,
blotted to remove excess liquid applied to the foil. An even thickness of 0.5mm
should cover all of the foil with the exception of the skirt. The core material should be
well condensed and excess moisture removed by blotting. After the basic core
buildup. The die is replaced on the cast and the occlusion adjusted as necessary. More
porcelain may be added to reinforce high stress areas, such as the cingulum of
maxillary anteriors or the marginal ridge area of premolars. The lingual area should
also be thickened, as with a metal coping, for extra strength.
58

59. In core design, the high stress cervical areas should be taken into
consideration. If these are areas of excess tooth reduction, the core thickness is
increased to compensate and allow even and optimum veneer porcelain thickness of
approximately 1mm. After completing the core build up, excess material is removed
from the skirt and the porcelain slightly ditched with the tip of the brush. A ditch is
made in the porcelain all around the shoulder to prevent contraction of the shoulder
portion of the matrix during firing. The porcelain is condensed well by vibrating and
blotting to avoid porosity. The core is then fired. Aluminous core porcelain is fired
under vacuum to 11200
F with the first firing.
In other method of firing system, the porcelain is fired from 15000
F to 19000
F
for six minutes. The vacuum is broken and the porcelain is raised to 20000
F and held
for 15 minutes. With the first firing, there will be some shrinkage away from the
shoulder and perhaps some fissuring. The surface of the porcelain should be ground
with a clean diamond stone and any fissures opened slightly. The foil is checked to
ensure adaptation and reburnished if necessary, it is then placed in an ultrasonic
cleaner.
A more fluid porcelain mixture is used for the second core bake to allow the
material to flow into any small discrepancies and ditched area. The porcelain is
condensed and fired. The second core firing is better done in air because subsequent
core firings in vacuum may cause ‘bloating’ or bubbling of the porcelain. The core
porcelain is allowed to mature in air at 11200
C for 15 minutes to obtain higher density
and therefore higher strength.
Firing requirements of core porcelain:
1. Raise firing temperature slowly 1500
C per minute, rapid drying and firing
increase the risk of fissuring in the core.
2. Break vacuum immediately on reaching maximum maturing temperature.
Continues firing in vacuum will cause hone comb porosity.
3. Air fire for at least 15 minutes at maximum maturing temperature or even
higher for extra strength.
Alumina crystals improves density and bonding of crystals to the porcelain
matrix and strength can be greatly increased.
4. After cooling check degree of fusion by immersing in ink surface in dense with
no honey comb porosity, the dye wash off clearly.
Dentin porcelain:
Mix dentin porcelain with distilled water on a glass slab to a creamy
consistency. Moisten the core porcelain with distilled water and apply the weak
porcelain in small increments to the gingival area using the moist brush, smooth the
porcelain towards the incisal area so that it is chamfered to a fine edge about 1/3rd
the
distance from the cervical margin. Make certain that the porcelain has a gradual
chamfer since a sharp angle can create demarcation lines. Condense the porcelain by
vibrating and drying with a paper tissue.
59

60. Gingival or cervical dentin porcelain and body porcelain are different in
colour bleeding.
Apply the body dentin porcelain to the remaining core porcelain using the
incremental brush technique work the porcelain from the incisal edge first and carry it
down the labial surface towards the gingival porcelain. In this way the body porcelain
will blend easily with the slightly moist gingival porcelain.
Continue to build around the approximal areas but do not fill in approximal
space of the gingival. Always maintain the correct contour. Extend the porcelain
incisally to a length of 1mm to allow for shrinkage. Finally build up the lingual
portion of porcelain by dentine porcelain to the correct occlusal relationship.
Porcelain will always shrink towards its greatest bulk and these areas are to be
found at the incisal, the proximal and at the supra-bulge, line and point angles.
Enamel porcelain:
Before mixing the enamel and translucent porcelains make sure that the dentin
build up is kept moist. Check that atleast 0.5mm of dentin porcelain is covering the
core at the incisal with a periodontal probe. Remove the other half of the incisal
dentin and smooth the surface using brush. Apply enamel porcelain to the surface,
making sure that each increment is positioned very gently.
Smooth the enamel porcelain towards the dentin blend line and extend it
proximally, do not alter the original contour of the tooth. The finished enamel should
be extended by 1 to 1.5mm beyond the adjoining central incisor. Thicker enamel
reduce value and increase translucency.
The restoration is dried and then placed in the oven to 15000
CF to 19500
F
under total vacuum. It is held at that temperature for one minute and then removed.
The restoration is adjusted, tried in the mouth and final morphological features
are added, it is glazed by placing it in the muffle at 19500
F is held at that temperature
for two to four minutes without vacuum before removal from the furnace.
The matrix is then removed by prying it away from the shoulder with a sharp
instrument. The loose edge is grasped firmly with cotton pliers and the platinum is
twisted out.
Aesthetic Potential of Metal - Ceramic crowns Versus All - Ceramic Crowns:
Although metal -ceramic prostheses account for about 70 % of all fixed
restorations a metal - ceramic (MC) crown is not the best aesthetic choice for
restoring a single maxillary anterior tooth. A ceramic crown offers a greater potential
for success in matching the appearance of the adjacent natural tooth, but ceramic
crowns are more susceptible to fracture, especially in posterior sites. A dark line at
the facial margin of a MC crown occasionally associated with a metal collar or
minimized margin is of great concern when gingival recession occurs. This effect can
be minimized by using a ceramic margin or by using a very thin knife - edge margin
of Mattel coated with opaque shoulder porcelain. The technician should polish and
glaze this margin to or with very thin knife - edge metal margins on the facial surface
are successful procedures for improving the aesthetics.
60

61. Sintering of porcelain
The thermocemical reactions between the porcelain powers components are
virtually completed during the original manufacturing process. Therefore the purpose
of firing is simply to sinter the particles of powder together properly to form the
prosthesis. Some chemical reactions occur during prolonged firing times or multiple
firings. Of particular importance are the observed changes in the Lucite content of the
porcelains designed for fabrication of metal - ceramic restorations. Locution is a high
- expansion ( and hinge- contraction) crystal phase whose volume fraction in the glass
matrix can greatly affect the thimble contraction coefficient of the porcelain. Changes
in the Lucite content can cause the development and the metal, which can produce
tensile stresses during cooling that are sufficient to cause crack formation in the
porcelain.
The condensed porcelain mass is placed in front of or below the muffle of a
preheated furnace at approximately 6500
C (12000
F) for low- fusing porcelain. This
preheating procedure permits the remaining water vapor water vapor to dissipate.
Placement of the conduced mass directly into even a moderately warm furnace results
in a rapid production of steam, there by introducing voids fracturing large sections of
the veneer. After preheating for approximately 5min, the porcelain is placed into the
furnace, and the firing cycle is initiated.
The size of the powder particles influences not only the degree of
condensation of the porcelain but also the soundness 0r apparent density of the final
product. At the initial firing temperature, the voids are occupied by the atmosphere of
the furnace. As sintering ;;of ;the particles begins, the porcelain particles bond at their
points of contact. As the temperature is raised, the sintered glass gradually flows to
fill up the air spaces. However, air becomes trapped in the form of voids because the
fused mass is too viscous to allow all the air to escape. An aid in the reduction of
porosity in dental porcelain is vacuum firing.
Vacuum firing reduces porosity in the following way. When the porcelain is
plac4d in the furnace, the power particles are packed together with air channels
around them. As the air pressure inside the furnace muffle is reduced to about one-
tenth of atmospheric pressure by the vacuum pump, the air around the particles is also
reduced to this pressure. As the temperature rises, the particles sinter together, and
closed voids are formed within the porcelain mass. The air inside these closed voids is
isolated from the furnace atmosphere. At a temperature about 55o
C (99o
F) below the
upper firing temperature, the vacuum is released and the pressure inside the furnace
increases by a factor of 10, from 0.1 to 1 atm. Because the pressure is increased by a
factor of 10, the voids are compressed to one-tenth of their original size, and the total
volume of porosity is accordingly reduced. Not all the air can be evacuated from the
furnace. Therefore a few bubbles are present in vacuum-sintered porcelains, but they
are markedly smaller thank the ones obtained by air-firing. A finished metal-ceramic
multiple unit bridge.
Overglazing and shading ceramics As shown in Table 21-2, natural glazed
(autoglazed or self-glazed),medium-fusing feldspathic porcelain is much stronger than
ground, rough, nonglazed porcelain.If the glaze is removed by grinding, the transverse
61

62. strength may be 40 to 46% less than that of the porcelain with the glaze layer intact.
The glaze is effective in reducing crack propagaion within the outer surface because
the surface flaws may bebridged and the surface will be under a state of compressive
stressive stress. However, the results from one study indicate that porcelains with
highly polished surfaces (1-umabrasive paste) have comparable strength to that of
specimens that were polished and glazed (Fairhurst et al, 1992). This observation is of
clinical importance because after the porcelain prosthesis is cemented kin the mouth,
it is common practice for the dentist to adjust the occlusion by grinding the surface of
the porcelain with a diamond bur, unfortunately, this procedure weakens the porcelain
markedly if the glaze is removed and the surface is left in a rough condition.
If the porcelain surface is rough, a natural glaze treatment is recommended
since the fracture resistance of the surface is greater than that of unglazed porcelains.
Porcelains for metal-ceramic and ceramic prostheses, porcelain veneers, or denture
teeth may be characterized with stains and glazes to provide a more lifelike
appearance. The fusing temperatures of glazes are reduced by the addition of glass
modifiers that lower the chemical durability of glazes somewhat. Stains are simply
glazes and are subject to the same chemical durability problem However, most of the
currently available glazes have adequate durability if they are as thick as 50 um or
more.One method for ensuring that the applied characterizing stains will be
permanent is to use them internally. Internal staining and characterization can produce
a lifelike result, particularly when simulated enamel craze lines and other features are
built into the porcelain rather than merely applied to the surface. The disadvantage of
internal staining and characterization is unsuitable.
It is logical to assume that fine polishing of a roughened surface followed by
glazing produces smoother surface than polishing alone, sandblasting followed by
glazing, or diamond grinding followed by glazing. A highly polished and glazed
surface is smoother than the surfaces of glazed specimens that have been sandblasted
or roughened with a diamond followed by glazing.
Cooling of Metal-Ceramic Prostheses
The proper cooling of a porcelain prosthesis from its firing temperature to
room temperature is the subject of considerable controversy. The catastrophic fracture
of glass that has been subjected to sudden changes in temperature is a familiare
experience. The cooling of dental porcelain is a complex matter, particularly when
the porcelain is fused to a metallic substrate. Multiple firings of a metal-ceramic
restoration can cause the coefficient of thermal contraction of the porcelain to
increase and can actually make it more likely to crack because of tensile stress
development.
The chief limitation to the use of an all-porcelain crown in fixed
prosthodontics is its lack of tensile strength. A method for minimizing this
disadvantage is to fuse the porcelain directly to a metal coping that fits the prepared
tooth. Such a metal-ceramic prosthesis is shown schematically in figure 21-1. The
metal on the facial side is approximately 0.3 to 0.5mm thick. It is veneered with
opaque porcelain approximately 0.3mm in thickness. The body porcelain is about
1mm thick.
62

63. If a stronger material is used as an inner core of a ceramic crown, cracks can
develop only when the stronger material is deformed or broken, assuming that the
veneering porcelain is firmly bonded to the stronger substrate. With proper design and
physical properties of the porcelainand metal, the porcelain is rein-forced so that
brittle fracture can be abided or at least minimized when these crowns are restricted
to anterior teeth. Although most metal-ceramic prostheses involve cast melt copings,
several novel noncast approaches (sintering, machining, swaging, and burnishing) to
coping fabrication have been developed in recent years
Creep or Sag Resistance
Unfortunately, high-temperature creep or sag of some high noble and noble
alloys occurs when the temperature approaches 9800
C(18000
F). The creep can be
reduced if the metal has the proper composition so that a dispersion strengthening
effect occurs at the high temperature. When such gold alloy is heated to 9800
C or
higher, a second phase is precipitated that can harden or strengthen the alloy such
creep has been reduced in some of the commercial alloys, but it apparently cannot be
eliminated. The solidus temperature (the lower end of the melting range) of base
metall alloys, such as nickel-chromium, is higher than that ofgold alloys; hence, base
metal alloys are les susceptible to sag than are gold-based alloys.
However, the newer ultralow-fusing veneering ceramics that were introduced
in the early 1990s are fired at lower temperatures(sintering temperatures below
8500
C) than traditional low-fusing porcelains. For alloys with solidus temperatures of
10000
C or higher, creep (sag) deformation should be negligible.
Copings for Metal-Ceramic Prostheses:
Four types of process for producing a metal coping for metal-ceramic
prostheses are available: (1) electrodeposition of gold or other metal on a duplicate
die, (2) burnishing and heat-treating metal foils on a die, (3) CAD-CAM processing
of a metal ingot, and (4) casting of a pure metal (CP Ti) or an alloy (high noble,
noble, or predominantly base metal) through the lost-wax process. This discussion
focuses primarily on the third option since it is the most widely used process.
The traditional development of the metal-ceramic prosthesis was the result of
advances in the formulation of both alloys and porcelains. To bond a ceramic veneer
to cast alloy copings, a ceramic must have a fusion temperature well above its
sintering temperature and, it also must have a co-efficient of thermal contraction that
is closely matched to that of the alloys. A metal oxide is necessary to promote
chemical bonding of the ceramic veneer to the metal substrate. Foil copings may or
may not require a metal oxide or a bonding agent to ensure retention of the veneering
ceramic. The use of one type of foil coping is discussed in the following section.
The gold alloys developed for porcelain bonding have higher melting ranges
than typical gold alloys for cast metal prostheses; the higher melting ranges are
necessary to prevent sag, creep, or melting of the coping during porcelain firing.
These gold alloys contain small amounts (about 1%) of base metals such as iron,
indium, and tin, as discussed in Chapter 19. The base metals form a surface oxide
layer during the so-called "degassing" treatment, and this surface oxide is responsible
63

64. for development of a bond with porcelain. This porcelain-metal bond is primarily
chemical in nature and is capable of forming even when the metal surface is smooth
and little opportunity exists for mechanical interlocking.
The alloys and porcelains used for the construction of such restorations have a
number of rather stringent requirements. For example, if undesirable residual tensile
stresses in the porcelain are to be avoided, both the metal and the ceramic must have
thermal contraction profiles on cooling that are closely matched or with the metal
exhibiting a slightly higher contraction. If the contraction differences are large,
stresses may occur that weaken both the porcelain and the bond. For example a
difference in the coefficients of thermal contraction of 1.7 ppm/0
C can produce a
shear stress of 280 mPa (39,800psi) in porcelain next to the gold-porcelain interface
when the porcelain is cooled from 9540
C (17500
F) to room temperature. Because the
shear resistance to failure is far less than 280 mPa, these thermal stresses could cause
spontaneous bond failure.
High tensile stresses are known to develop in porcelain veneers from a
contraction coefficient mismatch between alloy and porcelain. The tensile stresses
induced within the porcelain by occlusal forces would, of course, be added to residual
thermal tensile stresses. However, when the metal and its porcelain veneer exhkibit
similar contraction curves and an average contraction coefficient difference of
0.5ppm/0
C or less (between the porcelain"s glass transition temperature and room
temperature), fracture is unlikely to occur except in cases of extreme stress
concentration or extremely high intraoral forces. These metal-ceramic combinations
are know as thermally compatible systems. Many prostheses made from metal and
porcelain materials having contraction coefficient differences between 0.5 and
1.0ppm/0
C are known to survive for many years. These results are explainable by
survival probability analyses that assume that the maximukm biting forcxes on
anterior crowns rarely exceeds 2224 N (500lb). In fact, the Guinness book of records
(1993) cites the maximum clenching force ever recorded for posterior teeth as 4337 N
(975lb) between molar teeth and much lower forces between premolars and between
anterior teeth. Thus a rather small number of patients have bite force capabilities that
are likely to cause fracture of metal ceramic crowns or bridges even when residual
thermal incompatibility stresses are present. As general rule , lower forces are
generated by younger children versus older children, female patients versus male
patients, a more closed bite versus a raised bite table, occlusion between natural teeth
and denture teeth compared with the force generated between natural teeth against
natural teeth.
Another equally important property of metal-ceramic systems is that the alloy
should have a high proportional limit and, particularly, a high modulus of elasticity.
Alloys with a high modulus of elasticity also share a greater proprtion of stress
compared with the adjacent porcelain. The metal framework must not melt during
porcelain firing and also must resist high -temperature"sag" deformation sag or
flexural creep can occur only at high temperatures. It does not occur at oral
temperatures.
64

65. The metal-ceramic prosthesis is generally fabricated by a dental technician.
The casting procedures are similar to those described for the casting of inlays and
crowns. Because of the high melting temperature of the alloys, a phosphate-bonded
investment must be used.
The casting should be carefully cleaned to ensure a strong bond to the
porcelain. For example, an alloy such as Olympia (heraeus Kulzer), a gold-palladium,
silver-free alloy, is heated in the porcelain furnace to a temperature of 10380
C (19000
F) to burn off any remaining impurities and to form a thin oxide layer. In many alloy
systems, this so-called degassing treatment does not actuallyk degas the interior
structure of the alloy, but it does produce an oxide layer on the alloy surface that is
essential for the formation of the porcelain-metal bond.
The need for a clean metal surface cannot be overemphasized. The surface
may be cleansed adequately by finishing with clean ceramic-bonded stones or sintered
diamonds, which are used exclusively for finishing. Final sandblasting with high
purity alumina abrasive ensures that the porcelain is bonded to a clean and
mechanically retentive surface.
Opaque porcelain is condensed with a thickness of approximately 0.3mm and
is then fired to its maturing temperature. Translucent porcelain is then applied, and the
tooth form is built. Porcelain powder is applied by the condensation methods
previously described. The unit is again fired. Several cycles of porcelain application
and firing may be necessary to complete the prosthesis. A final glaze is then obtained.
Metal-Ceramic Crowns Based on Burnished Foil Copings:
Captek (Precious Chemicals Company Inc.) is a technology that is based on
the principle of capillary attraction to produce a gold composite metal. The Captek P
and G metals can produce thin metal copings for single crowns or frameworks for
metalceramic fixed partial dentures (FPDs) with a maximum span length of 18mm
(that allows space for up to two pontics). Captek is an acronym for "capillary casting
technology". The finished metal coping may be described as a composite material
consisting of a gold matric reinforced with small particles of a Pt-Pd-Au alloy. The
inner and outer surfaces contain approximately 97% Au. The grain size of the foil is
15 to 20 µm.
Malleable Captek metal strips are burnished on a refractory die to fabricate the
metal coping of a metal-ceramic crown without the use of a melting and casting
process. Examples of Captek metal-ceramic crowns are shown in Figure. The
procedural steps are described below.
Starting with a master model of the prepared tooth or teeth, a Captek
refractory die is produced. The Capsil relief liquid is sprayed onto the stone die(s) to
reduce surface tension, allowing Capsil impression material or Capvest refractory die
material to flow freely and to reduce bubbles and imperfections. After die spacer is
applied to the master die, undercuts are blocked out. The master die is placed in the
proper size-duplication flask, and Capsil silicone material is poured into the flask
around the master die(s). The master die is removed from the hardened silicone, and
an accurate high-temperature refractory is puoured into the new silicone mold.
65

66. The refractory die is heat-treated, the margins are marked with a red pencil,
and an adhesive (Captek Adhesive) is applied to the die. The adhesive aids the
adhesion of Captek material to the die and also enhances capillary attraction.
The next step requires the application of Captek p, a highly malleable gold,
platinum, and palladium alloy. This internal reinforcing skeleton provides a three
dimensional network of capillaries that will be eventually filled by Captek G material
forming a composite high gold metal alloy. This layer is burnished on the die, and
after the margins are trimmed, it is sintered in a porcelain furnace on the
recommended Captek thermal processing cycle.
This capillary structure is next infused with molten gold that is supplied by the
Captek P material. The Captek P is pressed in place using uniform firm pressure so it
conforms to the shape of die. Excessive pressure or pulling of the material will cause
it to tear or break. After trimming away any excess material, any voids are filled with
trimmings of Captek P segments.
The crown and/or bridge units are now ready for firing. The pieces are heated
at a rate of 550
to 800
C/minute to the recommended firing temperature. Next, Captek
G strips are applied. The Captek G metal strip contains 97.5 wt% gold and 2.5 wt%
silver. The copings and/or bridge components are fired again in the furnace according
to the recommended Captek firing cycle. After processing, Captek G forms a high
gold metal alloy composite through capillary attraction. The capillary action is used
as the joining method of the hard particles and the resilient particles of metal present n
the final coping. The gold melts, yet the Captek P structure remains stable. The
Captek composite metal coping is now complete and ready for refractory die removal.
The coping is now complete and ready for refractory die removal. The coping is
divested, and the margins are finished. For the production of FPDs (bridges), Captek
pontics are used. These are specially designed pontics that are precast using a metal-
ceramic alloy and plated with pure gold.
The copings and/or pontics are coated with a mixture of a powder and liquid
(Capbond), which will provide a thin covering of gold material to enhance areas of
Captek P that have been ground during adjustment. The Capbond will also provide a
gold color that is identical to that of areas that have not been ground.
Capcon liquid and powder are applied to areas between pontics and abutments,
as well as pontic surface areas. The powder contains an alloy of gold, platinum, and
palladium to provide the same base material structure as Captek P. Carcon absorbs,
through capillary attraction, the Capfil material in the same way that Captek G is
absorbed into Captek p, producing a gold connecting area that enhances the color of
the pontic surface. Capcon is also used to enhance the buccal or labial pontic areas of
Captek preformed pontics.
The units are veneered with tow thin layers of opaque porcelain and other
veneering porcelain layers. Before the opaque layer is applied, the finished Captek
coping has a thickness of approximately 0.25 mm. Thus this technique provides a
much thinner coping thickness than traditional cast metals (0.5mm) and will provide
additional space for veneering porcelain. The marginal adaptation is dependent on the
skill of the technician in trimming the burnished materials. Unlike traditional cast
66

67. metals that provide atomic bonding to opaque porcelain through an external oxide
layer, Captek metal achieves bonding to opaque porcelain through as external oxide
layer, Captek metal achieves bonding through a combination of surface interlocking
and residual stresses produced by slight differences in thermal expansion coefficients.
Although Captek metal is indicated for crowns and FPDs, no long-term clinical data
are available to determine the survivability of Captek ceramic prostheses.
Bonding Porcelain to Metal:
The primary requirements for the success of a metal-ceramic prosthesis is the
development of a durable bond between the porcelain and the alloy. Once such a
bond is achieved, there is an opportunity to introduce stresses in the prosthesis during
the porcelain firing procedures. An unfavorable stress distribution during the cooling
process can result in cracking of the porcelain, and delayed fracture can also occur.
Thus, for a successful metal-ceramic prosthesis to be realized, both a strong interface
bond and thermal compatibility are required.
Theories of metal-ceramic bonding have historically fallen into two groups:
(1) mechanical interlocking between porcelain and metal and (2) chemical bonding
across the metal-porcelain interface. Although chemical bonding is generally
regarded to be responsible for metal-porcelain adherence, evidence exists that, for a
few systems, mechanical interlocking may provide the principal bond. The oxidation
behavior of these alloys largely determines their potential for bonding with porcelain.
Research into the nature of metal-porcelain adherence has indicated that those alloys
that form adherent oxides during the degassing cycle also form a good bond to
porcelain, whereas those alloys form no external oxide at all but rather oxidize
internally. It is for these alloys that mechanical bonding is needed.
A variety of tests have been advocated for measuring the bond strength. None
can be regarded as an exact measure of the adhesion of porcelain to metal except in
cases in which the metal-porcelain couple is matched thermally so that porcelain
adjacent to the interface is essentially stress-free. This is a situation virtually
impossible to attain because the metal exhibits a linear contraction behavior as a
function of temperature and the porcelain exhibits a nonlinear contraction plot.
Clinical fractures of metal-ceramic restorations, although rare, still occur,
especially when a new alloy or porcelain is being used or when a new coping
technology has been adopted. As is generally true for all dental materials, there is a
learning curve associated with the initial use of new products. When fractures occur,
it is a good idea to make a vinyl polysiloxane impression of the fracture site for future
fractographic analysis. All information on the crown or bridge should be recorded.
Including the visual appearance of the fracture site. Although there are in infinite
number of fracture paths that may occur, three types are of particular importance in
diagnosis the cause of fracture. Shown in figure are fracture paths that have occurred
primarily at three sites: (1) along the interfacial region between opaque porcelain (P)
and the interaction zone (1) between opaque porcelain and the metal substrate (top);
(2) within the interaction zone (center); and (3) along the interfacial region between
the metal and the interaction zone (bottom). For conventional metal-ceramic crowns
67

68. made from cast copings, the interaction zone is usually synonymous with the metal
oxide layer.
For coping made using atypical methods such as the technologies associated
with the Captek system, and electroforming processes, bonding to porcelain is
achieved either through a combination of mechanical interlocking and residual
stresses that occur because of a metal-ceramic contraction mismatch or through the
application of a bonding agent. To characterize the principal site of fracture,
magnification of 3 to 100 times is required because a thin layer of retained porcelain
may not be visible without magnification. Each of the three principal fracture paths in
figure may be caused by excessive stress development, a material deficiency, of a
processing deficiency.
Bonding of Porcelain to Metal Using Electrodeposited Substrates:
Ceramic bonding to metals in certain cases requires the electrodeposition of
metal coatings and heating to form suitable metal oxides. Deposition of a layer of
pure gold onto the cast metal and a subsequent short “flashing” deposition of tin have
been shown to improve the wetting of porcelain interface. In addition, the
electrodeposited layer acts as a barrier between the metal casting and the porcelain to
inhibit diffusion of atoms from the metal into the porcelain, within the normal limits
of porcelain firing cycles.
A light color of the oxide film enhances the vitality of the porcelain when
compared with the dark oxides that require heavy opaque layers to mask dark
unaesthetic oxides. Because the activated surface can be controlled from golden
reddish-brown to gray, an additional dimension is available for color control of the
porcelain.
Alloys and metals such as cobalt-chromium, stainless steel, palladium-silver,
high-and low-gold-content alloys, and titanium all have been successful electroplated
and tin-coated to achieve satisfactory ceramic bonding. Various proprietar agents are
also available that are intended for application to the metal surface before
condensation of the opaque porcelain layer. These are applied as a thin liquid to the
metal surface and are fired in a manner similar to that of opaque porcelain.
The function of these agents is twofold: (1) they are intended to improve
metal ceramic bonding by limiting the build-up of an oxide layer on the base metal
surface during firing, and (2) they can improve aesthetics by helping to block the
color of the dark metal oxide.
Benefits and Drawbacks of Metal-Ceramics:
The properly made crown is stronger and more durable than the ordinary
aluminous porcelain crown. However, a long-span bridge of this type may be subject
to bending strains, and the porcelain ma crack or fracture because of its low ductility.
These difficulties can be partly overcome with proper prosthesis design, as discussed
earlier. Proper occlusal relationships are also particularl important for this type of
prosthesis.
The most outstanding advantages of metal-ceramic prostheses are the
permantnt aesthetic quality of the properly designed reinforced ceramic unit and their
resistance to fracture. Unlike similar acrylic resin veneered structures, almost no wear
68

69. of the porcelain occurs by abrasion and there is no staining along the interface
between the veneer and the metal. Furthermore, as shown in a clinical stud, the
fracture rate of metal-ceramic crowns and bridges is as low as 2.3% after 7.5 years
(Coornaert et al, 1984).
A slight advantage of metal-ceramic prostheses over ceramic prostheses is that
less tooth structure needs to be removed to provide the proper bulk for the crown,
especially if metal only is used on occlusal and lingual surfaces. As previously noted,
high rigidity of the structure is needed to prevent fracture of the porcelain. Ver little
flexibility can be sustained by dental porcelains because of their moderately high
modulus of elasticity e.g., 69 Gpa compared with values of 99.3 Gpa for a Type IV
gold alloy, 22.4 Gpa for amalgam, and 16.6 Gpa for a resin-based composite) and
their relatively low tensile strength. As a result, only limited elastic deformation of
the porcelain approximately (less than 0.1% strain) can be tolerated before fracture
occurs. It follows, therefore, that a sufficient bulk metal is necessary to provide the
proper rigidity. The minimal metal coping thickness necessary in the occlusal region
is approximately 0.3mm. the shape of the crown cannot be conspicuously out of line
with the anatomic form of adjacent teeth. Therefore the bulk of the natural tooth may
need to be sacrificed to provide adequate space to ensure adequate fracture resistance
and aesthetics.
REVIEW OF MATERIALS
1) Optek HSP (Leucite-Reinforced porcelain):
Optek HSP (Jeneric Pentron Inc. Walingord CT) is a high leucite porcelain
designed to make an all ceramic crown without a core. A very esthetic system, it can
be built on foil or on a refractory die. Although the strength as improved over a
normal feldspathic porcelain, it is not expected to be as strong as those built with a
core.
Advantages: Offers a very esthetic crown system.
Disadvantage: A slightly higher rate of fracture exposed.
2) The aluminous porcelain jacket crown:
Porcelain jacket restoration has been in use since Land introduced it in 1903.
Fracture of the restoration was a problem and, in 1965 McLean and Hughes
introduced the aluminous porcelain jacket crown. Porcelain was strengthened by
dispersion of alumina particles in a “core” porcelain which could be built over
platinum foil to a thickness of 0.5 to 1.0mm. Traditional (feldspathic) porcelain could
then be build over this core. The core contained 40% to 50% alumina, which
strengthened the porcelain jacket crown by about 50%, while still allowing good light
penetration and esthetics.
The composition of aluminous porcelain is similar to that of conventional
porcelain except for the increased alumina content (40-50%). The alumina particles
are much stronger than quartz and have a higher modulus of elasticity. Since the
fracture of porcelain is essentially caused by propagation of cracks through the
structure, the alumina crystals tend to obstruct the path of the crack.
69

70. Aluminous porcelain is an opaque porcelain used to construct the core layer of
the porcelain jacket crown as an initial layer to mask the color of the tooth dentin or
the color of the metal. If it is metal ceramic restoration, the opaque agent is usually
the ziroconium oxide.
Advantage: The aluminous porcelain jacket crown offers excellent esthetics,
improved strength.
Disadvantages: This material is not indicated for posterior teeth, fixed partial
dentures, or in cases of heavy bruxism. The core is sometimes bright at the neck.
3) Hi-Cream:
The authors Southan and Jorgensen found that a refractory die could get better
marginal adaptation by wetting it directly than by using the platinum foil technique.
Subsequently, Hi-Cream system was developed to make an aluminous porcelain
jacket crown on a refractory die.
Advantages: The crowns are designed to be built on a refractory die.
Disadvantages: Material not indicated for posterior teeth. FPD and heavy bruxism.
4) Al-Cream (Cerestore, Cast core):
In 1983 Sozio and Riley and Coors Biomedical Company (Lakewood CO)
introduced the Cerestore system. It employs a heat stable epoxy die and an aluminous
core porcelain which is injection moulded by controlling time and temperature of the
firing cycle, it was believed that a shrink free ceramic system resulted.
Injection moulded technique:
The coping wax pattern is fabricated on a heat stable epoxy resin die. The die
and wax pattern are invested in a special flask by plaster, and the wax can be removed
with boiling water and the flask is oven heated at 1800
C for 40 minutes.
In this system aluminous magnesium core material containing about 70%
alumina with a thermosetting binder having a transition temperature of 1600
C is used,
which is heat softened and injection moulded under air pressure in to the preheated
mould cavity, formed by the lost wax process on a specially formulated epoxy die
material compatible with this technique.
This special epoxy die is heat stable at the moulding temperature and
undergoes uncontrolled expansion during curing of the epoxy.
On cooling the thermoset material sets and is disinvested from the mould. The
green state ceramic core still on the epoxy die and is then adjusted by core is then
removed from the die and fired in a furnace under precise time and temperature
control for several hours upto a maximum of 13000
C. After firing the alumina
magnesium core is then veneered, with a special veneer porcelain similar to
aluminous porcelain following a conventional technique.
Thus expanded epoxy die in combination with magnesium core, provide for
compensation of firing shrinkage. Because the first layer of ceramic in this technique
is molded directly against a die of the preparation, an excellent marginal fit is
70

71. possible. Its compressive strength does not differ significantly from that of
conventional aluminous porcelain crowns. At present time, this system is used for
single unit restorations.
Advantages: Offers improved strength, excellent esthetics and marginal adaptation,
shrinkage compensation mechanism.
Disadvantages: Cost, time of manufacture, unavailable for purchase today.
5) In cream:
In cream composed of fine sized particles of aluminium oxide core with
significantly improved strength, developed by Dr. Sadoun in Paris and recently tested
by Seghi et al. The core is fired for 10 hours period in a special furnace on a
refractory die. Conventional veneer porcelain can then be build to contour.
Advantages: System offer strength (4 times greater than regular) and excellent
marginal adaptation.
Indications: May be used for anterior esthetic areas, possible posterior use and FPD.
6) Dicor (Cast Glass Ceramic/Cast Ceramic Crown):
Dicor (Dentsply International, York, PA) is a castable glass system composed
of SiO2, K2O, MgO, MgF2 which is manufactured by Corning Glass Works. Hence
the full contour of the crown is built with Dicor.
Technique of fabrication:
Crowns are formed by full contoured anatomic wax pattern which are invested
in a phosphate bonded investment. Once the wax has been eliminated in a burnout
furnace, a castable glass ceramic material is heated to a molten state (A special glass
is cast at about 14000
C) and cast in the mold on a centrifigual casting machine. After
cooling to room temperature, the casting of transparent amorphous glass is removed
from the investment, cleaned and trimmed then subjected to heat treatment.
For the heat treatment reinvest in a refracgtory material and fire it in a special
furnace. This converts transparent week restoration into a semicrystalline translucent
restoration. The maximum temperature required for this firing is 10750
C for several
hours. This increase the strength and promotes the development of a crystalline phase
of tetrasilica (Mica) within a glass matrix.
When the colour is deep within the tooth, this method fall short. An alternative
technique is to cast the glass as a core and bake veneer porcelain over it (Dicor plus,
Will’s glass).
In other system the castable ceramic material developed in Japan, the
ceramming process produce hydroxyapatite crystal in the glass matrix.
Advantages: Ease of fabrication, chameleon effect, conservation of tooth structure
(1.2 to 1.3mm reduction is adequate), flexibility of fabrication, adequate strength for
anterior teeth, good marginal adaptation, excellent biocompatibility, less plaque
accumulation (7 times less according to Sevit et al).
7) Sunrise:
71

72. Another gold foil supported ceramic system that uses ceramico II porcelain. If
either sunrise or renaissance system could be used with a core porcelain, strength
would be appreciably high, but no such research has been found in the literature.
8) Procera (Noble Pharma, Chicago, USA IL):
Promotes the use of commercially pure titanium coping which is fabricated
through milling and spark erosion process. The process takes the stone die and
through the use of a copy milling machine, makes three graphite copies and one
plastic *Ureol) copy die. On the plastic die the contours of a routine metal coping are
waxed. This die with the newly added external coping contour, is placed in the copy
milling machine and a copy of the external contours is milled in 99.6% commercially
pure titanium. At this time, only the external contours have been duplicated, leaving
the titanium copy still solid. Next, the solid titanium coping is inverted into low fusing
metal which holds it in place for the spark erosion process. The graphite die is placed
in the spark erosion unit and used as template for the internal contours of the coping.
Two to four graphite dies are needed to ensure accurate replication of the contours of
the preparation. This is because of the degradation of the die from the heat generated
by the spark erosion process. The coping is now hand finished on the die and Ti-
Cream is applied as specified.
A low fusing porcelain Ti-cream is developed specially for this process
applied in a conventional manner. The advantages proclaimed by the manufacturer
are:
2) Consistency – none of the problems associated with casting of alloys.
3) Biocompatibility of pure titanium
4) A porcelain kinder to opposing natural dentition.
5) Accurate unit relationships through laser welding on the master cast
Disadvantages:
2) No esthetic advantage over other metal ceramic system
3) Limited number of laboratories offers this system
Indication: Procera might be used in metal sensitive patients.
IPS empress (Ivoclar USA, Amherst, NY):
A new addition for metal free ceramic restoration, uses a wax up, lost wax
technique. The ceramic is leucite reinforced and comes in pre cerammed cylinders in
various shades. The wax is invested in a special flask with a newly developed
investment and placed into the base of the compress press-furnace. The correct shade
cylinder is heated to 11000
C at which the cylinder becomes plasticized. The cylinder
is then heat pressed under a vacuum into the mould where it is hold under pressure to
allow complete and accurate fill of the investment cavity. The system uses a special
light cure die material and a new Chromas Cop shade guide.
Advantages: High flexure strength, no shrinkage after pressing procedure,
dimensionally stable after several firings. Used for inlays, onlays, crowns and
veneers.
Disadvantages: System is expensive.
72

73. Chan and Weber studied plaque retention with different crown systems and
found ceramic crowns exhibited little plaque retention, whereas ceramic metal crowns
retained less plaque than natural teeth.
Wear Rate: Monasky and Taylor found that the rougher the porcelain surface, the
faster the wear rate. Porcelains should be highly polished before glazing to minimize
wear of opposing tooth structure.
RECENT ADVANCES IN PORCELAIN
1. Heat pressed ceramics (IPS empress):
IPS empress ceramic system used to fabricate crowns, inlays, onlays and
veneers. In this system a ceramic ingot using high temperature (11000
C) pressing
fabrication procedure is converted to a ceramic restoration. The technique consists of
slowly forcing a softened ceramic into a mould made by traditional lost wax process.
Esthetics can be enhanced by applying an enamel layers of feldspathic porcelain or
surface stains on the facial surface of anterior crowns and veneers.
By heat pressing, thermal expansion of glass and ceramic are said to induce
compressive stresses at the interface and strengthens the restoration.
Heat pressing significantly improves the flexure strength, whereas heat
treatment alone does not affect the strength.
Procedure:
1) Wax the restoration to final contour, sprue and invest as with conventional gold
castings.
If the veneering technique is used, only the body porcelain shape is waxed.
2) Heat the investment to 8000
C to burn out the wax pattern.
3) Insert a ceramic ingot and alumina plunger in the sprue and place the refractory in
the special pressing furnace.
4) After heating to 11500
C slowly press the softened ceramic into the mold under
vacuum.
5) After pressing recover the restoration from the investment by air-abrasion, remove
the spure and refit it to the die.
2. Slip cost alumina crowns and FPD’s (Inceram):
The inceram system uses a fine grain alumina core material.
Technique:
1) Inceram coping produced directly on special plaster die by brush technique
2) Heat pressed coping prepared by injection moulded technique.
Moulded technique:
The inceram system is used in the slip cost alumina technique. Here a fine
grain alumina core prepared on the porous plaster die, duplicated from the working
cost. The alumina slip is ultrasonically mixed and applied to the plaster die and
subsequently fired in a special porcelain furnace. The resultant porous coping is
infused with a special glass to give a high strength core material. Later this core is
veneered with an esthetic feldspathic porcelain.
73

74. Procedure:
1) Duplicate the working die with an elastomeric impression material and pour it
with the special plaster. Any undercuts must be blocked out first and two coats of
die spacer applied. When the plaster has fully set (2 hours) remove the die, mark
the margins, and apply the wetting agent.
2) Mix the appropriate shade of alumina slip with ultrasonic agitation, place the
mixture under a vacuum, brush apply it to the plaster die and shape with a blade,
trimming back to the margins carefully.
3) The slip is fired in a special furnace initially through a prolonged drying cycle to
1200
C that dries the die material, which shrinks away from the core. Then the
alumina is fired at 11200
C (20480
F).
The resulting core is porous and weak at this stage but can be carefully transferred
to the master die after the die spacer is removed. The relatively low sintering
shrinkage (about 0.3%) is compensated for by an expansion of the special plaster.
4) Paint a thick coat of the approapriate shade of glass mixture onto the surface of
the core and fire at 11000
C (20120
F).
5) Remove excess glass from the core by grinding and air abrasion.
6) Apply body and incisal porcelain to the core in the same manner as described for
traditional porcelain jacket crowns.
CAD-CAM restorations (Computer Assisted Design/Computer Assisted
Manufacture):
All ceramic restorations:
Since 1985, 4 major CAD-CAM systems for fabricating prosthesis have
emerged. The Duret (French) system, the Minnesota system (Denticad), Cicero
(Dritch) system are the only three capable of producing complete crowns and FPD’s.
The cerac system (Swiss) is limited to inlays.
Cerac system is a CADCAM ceramic restoration with a laser imaging camera
that constructs the tooth preparation in 3 dimensions. Here a special video camera is
used to scan the prepared tooth data from the image is manipulated by the dentist to
design a restoration that is machined from a ceramic block using computerized
milling machine. Operator can also program by design of inlay/onlay in the same
manner. Presently layers of colors and thus esthetics cannot be built into the
restoration. The surface colouration concept similar to Dicor’s seems the best
available esthetic choice today. Dicor restorations are made of a castable glass
ceramic having an advantage (1) low abrasion of opposing enamel, (2) Low plaque
accumulation.
Advantages: CAD-CAM eliminates the problems that arise from direct application
techniques currently employed.
Disadvantage: High cost, inaccuracy of marginal adaptation, and inability to built
layers of color and translucency.
CADCAM (Ceramic fused to metal restorations):
The Cicero system for the production of ceramic fused to meal restoration
used the optical scanning, nearly, net shaped metal and ceramic sintering and
computer aided crown fabrication techniques.
74

75. This system produces crowns, FPD’s with different layers such as metal,
dentin and incisal porcelain for proximal strength and esthetics
.
DEFECTS IN CERAMICS
Fabrication defects are created during processing and consist of voids or
inclusions generated during sintering. Microcracks develop upon cooling in
feldspathic porcelains and are due to thermal contraction mismatch between either the
crystals and the glassy matrix (Mackert et al, 1994) or between the porcelain and the
metal substrate. Condensation of a ceramic slurry by hand prior to sintering may
introduce porosity. Sintering under vacuum has been shown to reduce the amount of
porosity in dental porcelains from 5.6 to 0.56% (lones and Wilson, 1975). Porosity on
the internal side of clinically failed Dicor glass-ceramic restorations has been shown
to constitute a fracture initiation site (Kelly et al 1990). Surface cracks are induced by
machining or grinding.
The average natural flaw size varies from 20 to 50 micrometers (Anusavice et al,
1991). Usually, failure of the ceramic originates from the most severe flaw. The size
and spatial distribution of the flaws justify the necessity of a statistical approach to
failure analysis (Weibull, 1939). Surface crystallization of leucite can be induced by
seeding the surface of a feldspathic glass with leucite particles (Holand et al, 1995).
Ceramic materials for all ceramic restorations are in contact with refractory die
materials during firing or pressing at high temperatures. Surface reactions have also
been reported between glass-ceramics and the refractory embedment used during the
crystallization process, thereby modifying the mechanical properties of the final
product (Campbell and Kelly, 1989; Denry and Rosenstiel, 1993). Diffusion processes
are temperature-dependent, and surface reactions are likely to occur between the
porcelain and the refractory die material.
Degradability of Dental Ceramics:
The degradation of dental ceramics generally occurs because of mechanical
forces or chemical attack. The possible physiological side-effects of ceramics are their
tendency to abrade opposing dental structures, the emission of radiation from
radioactive components, the roughening of their surfaces by chemical attack with a
corresponding increase in plaque retention, and the release of potentially unsafe
concentrations of elements as a result of abrasion and dissolution. The chemical
durability of dental ceramics is excellent. With the exception of the excessive
exposure to acidulated fluoride, ammonium bifluoride, or hydrofluoric acid, there is
little risk of surface degradation of virtually all current dental ceramics. Extensive
exposure to acidulated fluoride is a possible problem for individuals with head and/or
neck cancer who have received large doses of radiation. Such fluoride treatment is
necessary to minimize tooth demineralization when saliva flow rates have been
reduced because of radiation exposure to salivary glands. Porcelain surface stains are
also lost occasionally when abraded by prophylaxis pastes and/or acidulated fluoride.
In each case, the solutes are usually not ingested. Further research that uses
standardized testing procedures is needed on the chemical durability of dental
75

76. ceramics. Accelerated durability tests are desirable to minimize the time required for
such measurements. The influence of chemical durability on surface roughness and
the subsequent effect of roughness on wear of the ceramic restorations as well as of
opposing structures should also be explored on a standardized basis
76