Ceramic 2

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Ceramic 2

  1. 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 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. 1
  2. 2. 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 developed with a coefficient of thermal expansion similar to that of existing dental casting alloys. 2
  3. 3. 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. 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. 3
  4. 4. 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 %. 3. Quartz:  It imparts more strength, firmness and translucency. 4
  5. 5.  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  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. 5
  6. 6.  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 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: 6
  7. 7. 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 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. 7
  8. 8. 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 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. 8
  9. 9. 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: Medium fusing dental porcelain: Oxide Weight % SiO2 69.36 B2O3 7.53 CaO 1.85 K2O 8.33 Na2O 4.81 Al2O3 8.11 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 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) 9 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
  10. 10. 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. 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: 10
  11. 11.  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 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 11
  12. 12. 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. 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 12
  13. 13. 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 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. 13
  14. 14. 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. 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. 14
  15. 15. 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). 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 15
  16. 16. 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 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 16
  17. 17. 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 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 17
  18. 18. 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 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. 18
  19. 19. 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 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 19
  20. 20. 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. 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. 20
  21. 21. 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 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. 21
  22. 22. 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 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 22
  23. 23. 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 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, 23
  24. 24. 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. 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 24
  25. 25. 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 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 25
  26. 26. 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 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 26
  27. 27. 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 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 27
  28. 28. internal surface of inlays, onlays, or crowns is ground with diamond disks or other instruments to the dimensions obtained from a 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. 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 28
  29. 29. 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/Esprident 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 Malthechni k 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 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 29
  30. 30. 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 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 30

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