Dental ceramics final

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Dental ceramics final

  1. 1. CONTENTS 1) INTRODUCTION 2) TERMINOLOGIES 3) HISTORICAL PERSPECTIVE 4) CLASSIFICATION 5) COMPOSITION 6) PROPERTIES OF DENTAL PORCELAIN 7) CONDENSATION OF DENTAL PORCELAIN 8) METHODS OF STRENGTHENING 9) AESTHETICS IN PORCELAIN 10) ALL CERAMIC SYSTEMS 11) METAL CERAMIC SYSTEMS 12) PORCELAIN INLAYS 13) PORCELAIN LAMINATE VENEERS 14) ADVANCES IN DENTAL CERAMICS 15) CONCLUSION 16) REVIEW OF LITERATURE 17) REFERENCES/BIBLIOGRAPHY 1
  2. 2. INTRODUCTION The search for excellence in restorative dentistry is a never ending endeavor.Esthetics in contemporary dentistry is partly defined by patient’s desire for naturality and beauty .The development of ceramic materials has helped the dentist to translate the patient’s wishes to reality by providing the ideal restoration. The word ‘ceramic’ is derived from the Greek word ‘keramos’ which means ‘pottery’ or ‘burnt stuff’. A ceramic is therefore an earthy material usually of a silicate nature and may be defined as “a combination of one or more metals with a non- metallic element, usually oxygen” (Gilman, 1967). All porcelains and glass ceramics are ceramics, but not all ceramics are porcelains or glass ceramics. HISTORICAL PERSPECTIVE The earliest evidence of fabrication of ceramic articles dates back to 23,000yrs B.C.Historically, three types of ceramic materials were developed: 1) Earthenware: is relatively porous, fired at low temperatures. 2) Stoneware: appeared in china in about 100 B.C, fired at higher temperature than earthenware, has higher strength and is impervious to water. 3) Porcelain: white translucent stoneware, obtained by fluxing white china clay with “chine stone”, was developed in china in 1000 A.D., stronger than earthenware and stoneware. The originator of the first porcelain paste used for denture work was a French apothecary, Alexis Duchateau. His early dentures were ill-fitting because of the uncontrolled firing shrinkage. It was not until he teamed up with the Parisian dentist, Nicolas Dubois de Chement, that the two were able to construct complete dentures from a material they referred to as ‘Mineral paste”. The first single porcelain teeth were launched in 1808 by an Italian dentist,Giuseppangelo Fonzi and were called “terrometallic teeth”. The idea of fusing porcelain to a thin platinum foil is credited to Charles.H.Land. 2
  3. 3. TERMINOLOGIES COMMONLY USED: • ALUMINOUS PORCELAIN: A ceramic composed of a glass matrix phase and 35% vol or 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/technique.. • COPY MILLING: A process of machining a structure using a device that traces the surface of a master metal, ceramic or polymer pattern and transfers the traced spatial positions to a cutting station where a blank is cut or ground in a manner similar to a key-cutting procedure. • DEVITRIFICATION: Occurs when there is excessive disruption of the glass forming SiO4 tetrahedra in dental porcelain resulting in the crystallization of glass or “devitrification”. Is often associated with high expansion glasses due to increased introduction of alkali’s (soda-Na2O) to increase the thermal expansion. Increase in sodium and potassium ions can cause too much disruption of the SiO4 tetrhedra and subject to devitrification, which appears as cloudiness in the porcelain, accentuated by repeated firings. Once devitrified, it is increasingly difficult to form a glaze surface on the porcelain. The regular or aluminous porcelain is less susceptible to devitrification due to their higher silica to alkali ratio. Consequently since the soda (Na2O) content is less, there thermal expansion is also lower. • FELDSPATHIC PORCELAIN: A ceramic composed of a glass matrix phase and one or more crystal phases of which the more important phase is leucite, which is used to create high expansion porcelain that is thermally compatible with gold-based, palladium-based and nickel-based alloys. Hence this class of dental ceramics are also called “leucite porcelain”. 3
  4. 4. • FRITTING: The process of blending, melting and quenching the glass components is termed “fritting”. The term “frit” is used to describe the finer glass product. The raw mineral powders are mixed together in a refractory crucible and heated to a temperature well above their ultimate maturing temperature. The oxides melt together producing gases, which are allowed to escape and the melt, is then quenched in water. The red-hot glass striking the cold water immediately breaks up into fragments and this is termed the “frit”. • GLASS CERAMIC: A solid consisting of a glassy matrix and one or more crystal phases produced by the controlled nucleation and growth of crystals in the glass. • INCONGRUENT MELTING: Is the process by which one material melts to form a liquid plus a different crystalline material. • SINTERING: A 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. • VITRIFICATION: Is the development of a liquid phase by reaction or melting, which on cooling provides the glassy phase. The structure is termed “vitreous”. CLASSIFICATION According to type: - • Feldspathic porcelain • Leucite-reinforced porcelain • Aluminous porcelain • Glass infiltrated alumina • Glass infiltrated spinel • Glass ceramic 4
  5. 5. According to use: • Denture teeth • Metal ceramic restorations • Veneers • Inlays/Onlays • Crowns and anterior bridges According to method of processing: • Sintering • Casting • Machining According to firing temperature: • High fusing -1300o C(2372o F) • Medium fusing-1101-1300o c(2013-2072o F) • Low fusing-850-1100o c(1562-2012o F) • Ultra low fusing-<850o c(1562o F) The medium fusing and high fusing types are used for the production of denture teeth. The low fusing and ultra low fusing porcelains are used for crown and bridge construction. According to application: • Core porcelain: is the basis of porcelain jacket crown, must have good mechanical properties. • Dentin or Body porcelain: more translucent than core porcelain, largely governs the shape and color of restoration. 5
  6. 6. • Enamel porcelain: is used in areas requiring maximum translucency, for example- at the incisal edge. According to Phillips’ science of dental materials: • Silicate ceramics - Dental porcelains with SiO2 (amorphous glass phase) as the main component and additions of crystalline Al2O3, MgO, ZrO2 and other oxides are included in this category. • Oxide ceramics - contains a principal crystalline phase of Al2O3 MgO, ThO2 or ZrO2 with either no or a small amount of glass phase. • Non-oxide ceramics - are impractical for use in dentistry due to their high processing temperatures and complex processing methods. Eg: Borides, Carbides, Nitrides • Glass ceramics- Dicor, Dicor MGC COMPOSITION The main ingredients are: • Feldspar • Silica (Quartz or Flint) • Kaolin (clay) • Coloring pigments • Opacifiers • Stains • Glass modifiers FELDSPAR: Is a precursor of common clay. Feldspar makes up the bulk of dental porcelains and gives a translucent quality. Potassium feldspar and sodium feldspar are naturally occurring materials composed of potash (K2O), soda (Na2O), alumina 6
  7. 7. (Al2O3) and silica (SiO2). Potash feldspar is used in most dental porcelains, due to its increased resistance to pyroplastic flow and increased viscosity, while sodium feldspar lacks the true translucent quality associated with potash feldspar. When potassium feldspar is mixed with various metal oxides and fired at high temperatures, it 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 to coalesce together. This is called “liquid- phase sintering”. Another important property of feldspar is its tendency to form the crystalline mineral leucite (potassium aluminium silicate) with a large co-efficient of thermal expansion (20-25x10-6 /o c) compared with feldspar glasses (10x10-6 /o c). The tendency of feldspar to form crystals of leucite in liquid glass during “Incongruent melting” when heated at temperatures between 1150o c and 1530o c is used to advantage for metal bonding in the manufacture of dental porcelains. SILICA: used in dental porcelain as a strengthner. Silica is a high fusing material, remains unchanged at temperature normally used in firing porcelain and this contributes stability to the mass during heating by providing a framework around which the other ingredients can flow. It prevents the crown from slumping during the liquid phase. The high melting temperature of fused silica is attributed to the three-dimensional network of covalent bonds between silica tetrahedra, which are the basic structural units of the glass network. The stability of the glass is dependant on the silicon -oxygen lattice and decreased reduction of covalent bonds, if otherwise, results in problems of hydrolytic stability and devitrification. KAOLIN :(Al2O3.2SiO2.2H2O) It serves as a binder. Kaolin gives porcelain its property of opaqueness. When mixed with water, it forms a sticky mass, allowing the unfired porcelain to be easily worked and molded. When subjected to high heat during firing, it adheres to the framework of quartz particles and shrinks considerably. 7
  8. 8. COLORING PIGMENTS:- The coloring pigments added to the porcelain mixture are called “color frits”. These powders are added in small quantities to obtain the delicate shades necessary to imitate the natural teeth. They are prepared by grinding together metallic oxides with fine glass and feldspar, fusing the mixture in a furnace and regrinding to a powder. These powders are blended with the unpigmented powdered frit to provide the proper hue & chroma. The color pigments used in dental porcelain consists of the following:- 1) Titanium oxide → Yellow - Brown Shade 2) Indium → Yellow / Ivory Indium or praseodymium (Lemon) is the most stable pigment for producing an ivory shade. Vanadium - Zirconium or Tin oxide plus chromium can be used, but are not so stable. 3) Iron oxide / Nickel oxide → Brown 4) Cobalt salt → Blue – particularly useful for producing some of the enamel shades. 5) Copper or Chromium Oxide → Green 6) Chromium - tin or chrome alumina → pink 7) Iron oxide (black) or platinum grey → Grey 8) Manganese oxide → Lavender OPACIFYING AGENTS:- The addition of concentrated color frits to dental porcelain is insufficient to produce a life- like tooth effect since the translucency of the porcelain is too high. 8
  9. 9. Dental porcelain materials having varying degrees of translucency can be manufactured by the addition of opaque materials. An opacifying agent generally consists of a metal oxide (between 8% & 15%) ground to a very fine particle size (<5µm) to prevent a speckled appearance in porcelain. commonest oxides used are:- a) Zirconium Oxide (Refractive Index – 2.2) b) Cerium Oxide c) Titanium Oxide (RI – 2.52) d) Tin oxide (RI – 2.0) e) Zircon Oxide STAINS AND COLOR MODIFIERS:- Stains are generally low fusing colored porcelains used as surface colorants or to provide/ imitate markings like enamel check lines, decalcification spots, fluoresced areas etc. Color modifiers are less concentrated than stains, used to obtain gingival effects or highlight body colors, and are best used at the same temperature as dental porcelain. Stains in finely powdered form are mixed with water/ glycerin or any other special liquid & the wet mix is applied with a brush either on the surface of porcelain before glazing or built into the porcelain (internal staining). GLASS MODIFIERS:- Potassium, sodium and calcium oxide are the most commonly used glass modifiers and act as fluxes by interrupting the integrity of sio4 network. The sintering temperature of crystalline silica is too high for use in veneering esthetic layers onto dental casting alloys. At such temperatures, the alloys would melt. In addition, the thermal contraction co-efficient of crystalline silica is too low for these alloys. Bonds between the silica tetrahedra can be broken by the addition of alkali metal ions such as sodium, potassium and calcium. These ions are 9
  10. 10. associated with the oxygen atoms at the corners of the tetrahedra and interrupt the oxygen - silicon bonds. As a result, the three-dimensional silica network contains many linear chains of silica tetrahedra that are able to move more easily at lower temperatures than the atoms that are locked into the three-dimensional structure of silica tetrahedra. This ease of movement is responsible for the increased fluidity (decreased viscosity), lower softening temperatures and increased thermal expansion conferred by glass modifiers. Too high a modifier concentration, reduces the chemical durability of the glass (Resistance to attack by H2o, acids and alkali). In addition, if too many tetrahedra are disrupted, the glass may crystallize (devitrify) during porcelain firing operations. Hence, a balance between a suitable melting range and good chemical durability must be maintained. GLAZES AND ADD-ON PORCELAIN:- The main purpose of glaze is to seal the open pores in the surface of fired porcelain. Dental glazes consist of colorless low-fusing porcelain, which can be applied to the surface of a fixed crown to produce a glossy surface. A glaze should normally mature at temperature below that of the restoration, and the thermal expansion of the glaze should be fractionally lower than the ceramic body to which it is applied. In this way, the glaze surface is placed under compression and crazing or peeling of the surface is avoided. Glazing can be of two types: - Self-glazing (auto glazing) and add-on glazing. In self glazing procedure, an external glaze layer is not applied, but the completed restoration itself is subjected to glazing. In add-on glazing, an external glaze layer is applied on the surface. Add-on porcelains are generally similar to glaze porcelains except for the addition of opacifiers and coloring pigments. The glazing temperature is reduced by fined grinding of the powder. Add-on porcelains are used for simple corrections of tooth contour or contact points. Disadvantage of glazes:- - Glazes are difficult to apply evenly and are often used to seal off a poorly baked restoration. 10
  11. 11. - Detailed surface characterization is almost impossible to obtain when separate glazes are used as they tend to produce too high a gloss or a rough surface. CONDENSATION OF DENTAL PORCELAIN Porcelain powder is built to shape using a liquid binder to hold the particles together. The process of packing the particles together and of removing the liquid binder is known as condensation. - Distilled water is the most commonly used liquid binder; additions of glycerine, propylene glycol or alcohol have also been tried. - Propylene glycol is used in the alumina core build up. Alcohol or formaldehyde based liquids are used for opaque or core build up, paint on liquid for stain application. - Starch can also be incorporated in the powder but is generally confined for use in the coarser air-fired powders. - The main driving force involved in condensing dental porcelain, is surface tension. The withdrawal of water from the porcelain powder will cause the powder particles to pack more closely. - Therefore during build-up, the porcelain should be kept moist. High room temperatures and dry atmospheres are to be avoided since porcelain powder can rapidly dry out and further placement of even wet porcelain powder on a dry surface will not allow the undersurface to be condensed properly due to air spaces being created in the powder bed which resists further ingress of water. METHODS OF CONDENSATION:- 1) Wet brush technique/ Brush additive technique: The consistency of the mix has a marked effect on the handling properties of porcelain. The mix should be creamy and capable of being transferred in small increments. The reasons for favoring the wet brush technique are:- 11
  12. 12. a) A wet brush maintains the moisture content in the porcelain. Metal spatulas causes more rapid drying out. b) The brush can be used to introduce enamel colors, effect masses or stains without changing instruments. c) Greater control over application of small increments of porcelain. d) Blending of enamel veneers can be achieved with greater delicacy. Metal instruments such as spatulas or Le cron carver can be used but many favor the use of high quality sable hair brushes as it is possible to rapidly transfer small increments of wet porcelain to the metal substructure using the fine point of a sable brush. 2) Brush application method:- In this method, dry powder is sprinkled over the wet porcelain surface. It enhances the risk of porcelain drying out and also the control of powder is very difficult and time consuming. Hence this method is not recommended. 2) Vibration:- Done with the serrated handle of a porcelain instrument which is lightly passed over the model or die pin. The particles of porcelain powder become suspended in the excess liquid and are so oriented that when the liquid is blotted away, the surface tension of the liquid pulls these particles into closer proximity; the irregularity of one particle thus fits into spaces between others. The more varied the particle size, the greater the number of times vibration can be employed with a visible amount of liquid removal. 4) Spatulation;- Makes use of a porcelain carver, the wet porcelain is rubbed and patted, bringing the liquid to the surface to be absorbed. But there is danger of dislodging the porcelain particles during manipulation which could cause invisible cracks, and also the sand paper like effect that the porcelain particles have on the metal, could remove traces of metal from the instrument and incorporate them into the porcelain resulting in discoloration in the final porcelain product. 5) Whipping: - 12
  13. 13. Sable hair brush is used with a light dusting action / whipping motion, producing a gentle vibration. This method works best with porcelain of fine grain but excessive manipulation could allow these fine particles to float away with liquid during blotting. 6) Mechanical :- A brush or spatula is attached to a vibrator and is used to agitate the porcelain particles gently, packing them together while forcing the liquid to surface. This device is used to transport the porcelain to the casting or matrix while condensing the particles with maximum efficiency. 7) Ultrasonic vibration :- The built up restoration is placed on the vibrator. The low amplitude (very little agitation) along with a high rate of vibrations per second pulls the liquid to the surface with almost no disturbance to the porcelain contour. This is a final condensing procedure used only after the porcelain has been well condensed and contoured. SINTERING/FIRING PROCEDURE OF DENTAL PORCELAIN:- Different media can employed for firing procedures:- 1) AIR FIRING PROCEDURE /AIR FIRING PORCELAIN:- When the porcelain enters the hot zone of furnace, each grain will be contacting its neighbor. The grains of porcelain will ‘lense’ at their contact points and weld together once the softening point of glass is reached. Surface tension will cause some of this porosity to be swept out via the grain boundaries, but if air firing techniques are used, a point is reached in all vitrified products where flow of the ceramic / glass grains around the air spaces traps the air remaining in the ceramic body. With rising temperature, the spaces containing entrapped air become sphere- shaped due to surface tension. Higher temperatures increases pressure of entrapped air and the bubbles enlarge to reach pressure equilibrium with the outside atmosphere. Cooling decreases pressure in the bubbles and their size decreases, to reach equilibrium. 13
  14. 14. The surface of air-fired porcelain is generally free of bubbles as the interstitial air nearest the actual surface cannot be trapped and escapes easily, but internally air bubbles remain entrapped and cannot escape due to the rapid firing techniques employed. A slow maturation period should be employed to allow the maximum amount of entrapped air bubbles to escape. During this heating, the porcelain must not be raised to its full maturing temperature, but kept 300 -500 c below the maximum firing temperature recommended by the manufacturer, such a temperature will mature the porcelain without causing loss of color and high densities will be achieved. 2) VACUUM FIRING PROCEDURE:- Vacuum firing porcelain was introduced primarily to give improved esthetics in enamel porcelains, also for ease of handling and production of dense surfaces. Densification of porcelain by vacuum firing has been described by vines et al (1958):- Most of the air is removed from interstitial spaces before sealing of the surface occurs and enables porcelain to shrink into a dense, pore-free mass without any hindrance. The little air that remains entrapped becomes sphere shaped under the influence of surface tension and increased furnace temperature. When air at normal atmospheric pressure is allowed to enter furnace muffle, it exerts a strong compression effect on surface skin and hydraulically compresses the low pressure internal bubbles resulting in a relatively dense, pore-free porcelain. Due to rapid action of vacuum sintering, the firing schedule for these porcelains can be reduced in comparison with the longer period recommended for air-fired porcelain. Factors to be considered while firing porcelain in vacuum are:- 1) Porcelain powder must be dried slowly to eliminate all water vapor and vacuum must be applied before placement of porcelain in hot zone of furnace to reduce internal pores before the surface skin seals off the interior too rapidly. 2) Vacuum firing should not be prolonged after porcelain maturation and sealing of surface skin, which otherwise causes surface blistering due to the rise of 14
  15. 15. residual air bubbles to surface through molten porcelain. Firing at too high a temperature can cause bloating or swelling. 3) The vacuum should be broken whilst the work is still in the hot zone of the furnace. This allows the dense surface skin of porcelain to hydraulically compress the low pressure internal air bubbles. Vacuum firing also has its limitations. If large bubbles are trapped in the porcelain by poor condensation techniques, these bubbles cannot be reduced in size to any significant degree. 3) DIFFUSIBLE GAS FIRING PROCEDURE:- High densities in dental porcelain can be produced by substitution of a diffusible gas for the ordinary furnace atmosphere. Air is driven out of the porcelain bed and replaced by diffusible gas like helium, hydrogen or steam. With these gases, the interstitial spaces do not enlarge under the influence of increasing temperature, but decreases in size or disappear as these gases diffuse outward through the porcelain or actually dissolve in porcelain ( vines et al; 1958). STAGES OF MATURITY The common terminology used for describing the surface appearance of un- glazed porcelain is ‘bisque’. a) Low bisque:- The surface of porcelain is porous and will easily absorb a water soluble die. Grains of porcelain begin to soften and ‘lense’ at their contact points. Shrinkage is minimal and the fired body is extremely weak and friable. b) Medium bisque:- Surface of porcelain still porous. Flow of glass grains increased and any entrapped furnace atmosphere that has not escaped via grain boundaries will be trapped and become sphere shaped. Definite shrinking takes place. c) High bisque:- Surface of porcelain completely sealed and presents a smoother surface. In non-feldspathic porcelain, a slight shine appears at this stage. The fired body is strong and any corrections by grinding can be made at this stage prior to final glazing. 15
  16. 16. Additions of porcelain can be made at any of the bisque firing stages, but is usually done at either medium or high bisque stage. METHODS OF STRENGTHENING DENTAL PORCELAIN Methods used to overcome the deficiencies of ceramics like brittleness, low fracture toughness and low tensile strength, fall into two general categories:- A) Strengthening the material per se. B) Methods of designing components to minimize stress concentrations and tensile stresses. Surface flaws are of particular importance in determining the strength of ceramics as the maximum tensile stresses created by bending forces in the oral environment occur at the surface of a restoration or prosthesis. The removal or reduction in the size and number of surface flaws can produce a large increase in strength. A) STRENGTHENING OF THE BRITTLE MATERIAL→ occurs through following mechanisms: 1) Development of residual compressive stresses within the surface of the material. 2) Interruption of crack propagation through the material. 1) Development of residual compressive stresses:- - Introduction of residual compressive stresses within the surface of the object is one of the most widely used methods of strengthening glasses and ceramics. - The residual stresses must first be negated by developing tensile stresses before any net tensile stress develop. Residual stresses can be introduced by following methods:- 1a) Ion exchange/ chemical tempering: Is a process involving the exchange of larger potassium ions for the smaller sodium ions, a common constituent of a variety of glasses. 16
  17. 17. - When sodium – containing glass article is placed in a bath of molten potassium nitrate, potassium ions in the bath exchange places with some of the sodium ions in the surface of glass articles. - Potassium ions are 35% larger than sodium ions and the squeezing of k+ ions into the place formerly occupied by sodium ion creates large residual compressive stresses (700 mpa) in the surface of glasses which produces pronounced strengthening effect. - This process is best used on the internal surface of a crown, veneer or inlay as it is protected from grinding and exposure to acids. - All ceramics are not amenable to ion-exchange. Porcelains highly enriched with potash feldspar (alumina core materials, dicor glass ceramic, conventional feldspathic porcelains) cannot be sufficiently ion exchanged with K+ . 1b) Thermal tempering: - is the most common method employed. - It creates residual surface compressive stresses by rapid cooling/quenching of the surface of the object while it is in molten/softened state. - This rapid cooling produces a skin of rigid glass surrounding a soft molten core and as the molten core solidifies, it tends to shrink, but the outer skin remains rigid, the pull of the solidifying molten core due to shrinkage creates residual tensile stresses in the core and residual compressive stresses within the outer surface. - Hot glass-phase ceramics are quenched in silicone oil or other special liquids to uniformly cool the surface. 1c) Thermal compatibility:- - The porcelain should be under slight compression in the final restoration which is accomplished by selecting an alloy that contracts slightly more than porcelain on cooling to room temperature. - The fabrication of glass or ceramic articles involves forming or processing at high temperature and the process of cooling to room temperature affords the opportunity to take advantage of potential mismatches in co-efficient of thermal contraction of adjacent material in the ceramic structure. 17
  18. 18. - In metal ceramic restorations, the metal and porcelain should be selected with a slight mismatch in their thermal contraction co- efficients (Metal thermal expansion co-efficient is larger), so that the metal contracts slightly more than porcelain on cooling from firing temperature to room temperature. This mismatch leaves the porcelain in residual compression and provides additional strength for the restoration. 2) Interruption of Crack propagation through the material: Another method to reinforce ceramics is by addition of different material that is capable of hindering a crack from propagating through the material. Two different methods are:- a) Dispersion of a crystalline phase :- - Disruption of crack propagation by use of a dispersed crystalline phase requires a close match between the thermal contraction co-efficient of the crystalline material and surrounding glass matrix. - In aluminous porcelains, a tough crystalline material such as alumina in particulate form is added to glass. The glass is toughened and strengthened as the crack cannot penetrate the alumina particles as easily as it can through glass. - In Dicor glass ceramics, the cast glass is subjected to heat treatment, that causes micron-sized mica crystals to grow in the glass and when restorations are subjected to high tensile stresses, these microscopic crystals will disrupt crack propagation, thus strengthening the crown. b) Transformation toughening:- This technique involves the incorporation of a crystalline material, capable of undergoing a change in crystal structure when placed under stress. - Partially stabilized zirconia (PSZ) is the crystalline material usually used and the energy required for transformation of PSZ is taken from the energy that allows crack to propagate. - Draw back of PSZ: - Its index of refraction is higher than that of the surrounding glass matrix and the PSZ particles scatter light as it passes 18
  19. 19. through the bulk of porcelain, producing an opacifying effect which may not be aesthetic in most dental restorations. METAL FREE CERAMIC SYSTEMS/ ALL CERAMIC RESTORATIONS Porcelain is the most natural appearing synthetic replacement material for the missing tooth structure, but due to its low tensile strength and brittleness, it has to be fused to a metal substrate to increase its resistance to fracture. Before 1980’s, the choice of dental ceramic was restricted to porcelain powders, which could be sintered onto metal substructures to form relatively strong metal-bonded crowns and bridges. But this metal base can affect the esthetics of porcelain by decreasing the light transmission through the porcelain and by creating metal ion discolorations. In addition, some patients have allergic reaction/sensitivity to various metals. These drawbacks have prompted the development of new all-ceramic systems that do not require metal, yet have the high strength and precision fit close to ceramometal systems. CLASSIFICATION OF ALL-CERAMIC SYSTEMS 1) Conventional powder- slurry ceramics:- * Hi ceram – Alumina reinforced porcelain. * Optec HSP – Leucite reinforced porcelain. * Duceram LFC – Hydrothermal low fusing ceramic. 2) Non-cored systems based on sintering of dental ceramics:- * Mairage * Fortess 3) Computer - aided design/computer-aided milling (CAD/CAM):- * Cerec 4) CAD/CAM with aluminium oxide coping:- * Procera 5) Castable ceramics:- * Dicor 19
  20. 20. 6) Infiltrated ceramics:- * In-Ceram 7) Pressable ceramics :- * IPS empress * Optec pressable ceramic 8) Machinable ceramics:- * Cerec vitablocks * Celay blocks * Dicor MGC The newer types of all –ceramic restorations have lower incidence of clinical fracture for three important reasons: 1) The newer all-ceramic restorations are made up of stronger materials and involve better fabricating techniques. 2) Most of the all-ceramic restorations can be etched and bonded to the underlying tooth structure with newer dentin adhesives. 3) Greater tooth reduction than what was previously used for PJC’s is carried out, that provides the lab technicians with enough room to create thicker and stronger restorations. I) CONVENTIONAL POWDER-SLURRY CERAMICS:- Sintered ceramics are normally based on potassium (K2O.Al2O3.6SiO2) and /or sodium feldspar and quartz which are produced by melting a number of minerals at high temperature (1200-1250o c).After cooling, the mass is ground to powder of various shades and translucencies .The technician adds water to the powder to produce a slurry, which is built up in layers on a model to form a restoration and heated whereby the surface of the powder particles melt and the particles sinter together. OPTEC HSP:- - Leucite reinforced porcelain - It was developed by Jeneric Inc./USA, sometime before it was established in the German market by Keppeler and Wohr. 20
  21. 21. - Optec ceramic is a feldspathic composition glass filled with crystalline leucite that is condensed and sintered like aluminous porcelain and traditional feldspathic porcelain. - The leucite concentration in optec was reported as 50.6%wt and was appreciably greater than IPS Empress ceramic (23.6% or 41.3wt %).The manufacturer disperses the leucite crystals in a glassy matrix by controlling their nucleation and crystal growth during the initial production of the porcelain powder. Due to its increased strength, optec HSPdoes not re quire a core when used to fabricate all-ceramic restorations, as is necessary with aluminous PJC’S - The leucite and glass components are fused together during the baking process at 10200 c. The build - up, condensation and contouring of the crown is accomplished using the powder slurry technique on a special semi-permeable die material. - The leucite porcelains can be used for both the body and incisal portions as the esthetics provided by leucite crystals does not necessitate the use of translucent porcelain. Surface stains or pigments can be applied to give the desired shade and translucency. - To reduce mismatch between co-efficient of thermal expansion, potassium ions in leucite have been exchanged for rubidium or cesium ions. - Sandblasting is generally recommended to achieve bonding with resin cement. Uses:- - Inlays - Onlays - Crowns for low-stress areas - Veneers Advantages:- a) It has a moderately opaque core compared with a metal or aluminous core as it is more translucent than alumina core crowns. b) Good flexural strength – 146Mpa. 21
  22. 22. c) Does not require special processing equipment beyond what is used for ceramo-metal restorations. d) Lack of metal or opaque substructure. e) Can be etched to allow optimum bonding to dentin or enamel. f) Restorations fit accurately. Disadvantages:- a) Increased leucite content contributes to the relatively high invitro wear of opposing teeth (as reported in recent study). b) Potential marginal inaccuracy caused by porcelain sintering shrinkage. c) Potential to fracture in posterior teeth. d) Requires a special die material. DUCERAM LFC:- Referred to as ‘hydrothermal low-fusing ceramic’. It is composed of an amorphous glass containing hydroxyl ions. - Properties claimed by the manufacturer for this non-crystalline material are greater density, higher flexural strength, greater fracture resistance and lower hardness than feldspathic porcelain. - Due to absence of leucite crystals, the hardness of the material and its ability to abrade the opposing natural tooth structure is reduced. - Higher flexural strength results from an ion-exchange mechanism of hydroxyl ions and also promotes healing of surface micro cracks. - The restoration from duceram LFC is made in 2 layers. The base layer is duceram metal ceramic (a leucite-containing porcelain), which is placed on a refractory die using powder- slurry technique and baked at 9300 c. the second layer of duceram LFC is applied over base layer using same technique and baked at 6600 c. - The material is supplied in a variety of shades and can be surface- characterized with compatible stains and modifiers. Uses:- - Ceramic inlays, veneers - Full contour crowns 22
  23. 23. Advantages:- a) Good flexural strength-110Mpa b) Greater density c) Greater fracture resistance d) Hardness is close to that of natural tooth owing to absence of leucite(decreases abrasion of natural tooth). Disadvantages:- a) Needs a special die material. II) CASTABLE CERAMIC SYSTEMS;- - Differs from sintered ceramics in that they are supplied as solid ceramics ingots, which are used for fabrication of cores or full contour restorations using the lost wax and centrifugal casting technique. - Generally one shade of material is available which is covered by conventional feldspathic porcelain or is stained to obtain proper shading and characterization of the final restoration. a) DICOR:- The use of glass ceramics in dentistry was first proposed by Mac Culloch in 1968. The first description of DICOR castable ceramic was given by Adair and Grossman in 1948 and was marketed by Dentsply Int. - It is a glass-ceramic material composed of SiO2, K2O, Mgo, MgF2, minor amounts of Al203 and ZrO2 incorporated for durability and a fluorescing agent for esthetics. - The fluoride acts as a nucleating agent (source of Fl ions) required in the crystalline phase and it also improves the fluidity of molten glass. - A full contour transparent glass crowns is cast at 1350o c, followed by heat- treatment at 1075o c for 10hrs. This heat-treatment causes microscopic plate like crystals of crystalline material (mica) to grow within the glass matrix. 23
  24. 24. This crystal nucleation and crystals growth process is called ‘CERAMMING’. The crystals function in 2 ways:- a) They create a relatively opaque material out of the initially transparent crown. b) They significantly increase the fracture resistance and strength of the ceramic as they interrupt the propagation of cracks in the material when forces are applied intraorally. - The cerammed casting is achromatic and shade is developed by adding external colorants .This ceramic was originally intended to be shaded with a thin surface layer (50-100µm) of colorant glass. - There has been evidence that the stain layer might be lost during occlusal adjustments, during routine dental prophylaxis or through the use of acidulated Fl gels. - Dentsply ( Trubyte division) has introduced ‘DICOR PLUS’→ is a shaded feldspathic porcelain veneer applied to the dicor substrate for fabrication of ‘ WILLI’S GLASS CROWNS’ ( crowns of veneered Dicor ceramic). - Marginal openings of 30-60 µm has been reported for dicor restoration, comparable to those of metal ceramic crown. - The internal fitting surface of the restoration is etched with 10% ammonium-bi-fluoride to improve their bonding strength to the composite resin and the tooth. Advantages:- a) The fit of cast glass restoration supersedes that of conventional porcelain. This decreases the amount of resin luting agent at the margins, thus decreasing the potential for ditching. b) Surface hardness and occlusal wear is similar to enamel. c) Greater flexural strength than conventional porcelain. 24
  25. 25. d) It produces good esthetics due to the ‘chameleon’ effect, in which the restoration acquires a part of the color from adjacent teeth and fillings as well as the underlying luting cement. e) Ease of adjustment. f) Inherent resistance to plaque accumulation (about seven times less than on the natural tooth surfaces). Disadvantages:- a) Chances of losing low- fusing feldspathic shading porcelains, applied for good color matching. Since the colorant is a surface stain, any grinding on the restoration leaves an unaesthetic opaque white area. b) Special investment and casting equipment is required. c) The whole process from the casting through the staining of the cerammed restoration is technique sensitive. Uses:- - Inlays, onlays - Full contour restorations or cores. b) CASTABLE APATITE CERAMIC (CERA PEARL):- The formation of a hydroxyapatite ceramic through reaction of glass ceramics with moisture was descrided by S.Hobo and T.Iwata. - Castable apatite ceramic was first developed by Hobo and Bioceram group as CaO-P2O5-MgO-SiO2 glass ceramic (crystal similar to hydroxyapatite of enamel). - Cerapearl is composed of CaO, P2O5, MgO, SiO2 and traces of other elements. CaO (45%) and P2O5 (15%) are the main ingredients in glass formation and are essential for hydroxylapatite crystal formation MgO (5%) along with CaO decreases the viscosity of the compound when melted and SiO2 (34%) 25
  26. 26. in combination with P2O5 forms the matrix with SiO2 regulating the thermal properties. - The lost wax technique is employed to produce the initial stage of restoration and a reheating phase to develop a crystalline microstructure. - Its casting once obtained has an amorphous structure, but when subjected to ceramming, forms crystalline oxylapatite (Ca 10 (PO4)6O). The unstable oxylapatite, when exposed to moisture forms stable crystalline hydroxylapatite. - Compared to normal enamel, crystals of cerapearl show an irregular arrangement which probably accounts for its superior mechanical properties. The similarity in hardness to enamel prevents wear of opposing enamel. - The young’s modulus, tensile and compressive strength of cera pearl are appreciably higher than conventional porcelain and most restorative materials. . cerapearl is a rugged material and hard to bend, and distortion with reasonably strong occlusal forces would be slight. - Cerapearl crowns should be thicker than metal crown because of poor flexural characteristics. The required tooth reduction for crown is 2mm on occlusal or incisal surfaces, 1.5mm on axial surfaces and 1.2mm on the margin. The finish line may be a heavy chamfer or shoulder. All sharp edges should be rounded so that stress concentration is reduced. - Since hydroxyapatite is the main constituent of cerapearl it is possible to fabricate a laminate veneer with excellent biocompatibility. It is also possible to acid-etch the surface of veneer for bonding treatment due to its similarity between its constituents and those of natural enamel. When this laminate veneer is bonded to enamel using light cured composite resin with xylane coupling adhesive, a strong bond is produced. The shade of laminate veneer is created internally by the composite resin used for bonding. - Cera pearl is indicated for crowns, laminate veneers and inlays but is yet commercially unavailable and currently in a research phase. 26
  27. 27. III) PRESSABLE CERAMICS:- This system was first described by Wohlwend et al in 1989 and become available commercially as IPS Empress and Optec OPC with the former being more popular of the two. - The ceramic is available in the form of ingots and is primarily a precerammed glass reinforced with leucite that prevents crack propagation without significantly diminishing its translucency. a) IPS EMPRESS (Injection-molded glass ceramic):- The ceramic is primarily a glass filled with crystalline leucite [23.6% wt coloured , 41.3% wt opaque ceramic]. - IPS empress is a pre-cerammed glass-ceramic that is heated in a cylinder form and injected under pressure and high temperature into a mold over a 45 minute period to produce the ceramic substructure. In this way, the only dimensional change occurs during cooling and can be controlled with an investment having appropriate expansion. - This crown form can be either stained and glazed or built-up using a conventional layering technique. - Contains a higher concentration of leucite crystals that increases the resistance to crack propagation. Advantages:- a) Lack of metal or opaque ceramic core. b) Moderate flexural strength. c) Excellent fit of restoration. d) Excellent esthetics. Disadvantages:- a) Potential to fracture in posterior areas. b) Need for special laboratory equipment (pressing oven and die material) 27
  28. 28. Uses:- - Single anterior crowns, Inlays and veneers. b) OPTEC OPC:- Leucite reinforced hot pressed ceramic - Optec OPC is also a type of feldspathic porcelain with increased leucite content. - Is supplied in the form of ingots. The restoration is waxed, invested and placed in a specialized mold with an aluminium plunger and the ceramic ingot is placed under the plunger. - The entire assembly is heated to 1150o c and plunger is released which presses the molten ceramic into the mold. - The pressed ceramic is then baked and the pressed from can be made to full contour or as a substrate on which conventional feldspathic porcelain can be built up. Advantages:- 1) Strong, translucent, dense restoration. 2) Good flexural strength. 3) Can be etched and bonded to natural tooth. Disadvantages:- 1) Causes abrasion of natural tooth due to increased leucite content. 2) Special oven, die material and molding procedure required to fabricate restoration. c) EMPRESS 2 (E-2):- This system consists of two components:- 1) First component is a compressed core material of lithium-di-silicate glass ceramic and lithium orthophosphate crystals with a flexural strength of 350Mpa. The scope of use of empress-2 for smaller bridges and posterior teeth has widened compared to empress-1 due to its increased flexural strength. 28
  29. 29. 2) The second component is a new layer/ laminated material comprised of fluorapatite-glass crystals, which ensures a natural-dental balance of opaqueness and translucence. A higher volume of crystalline phase results in increased strength of IPS Empress-2 compared to the original IPS Empress. The pressed ceramic material is translucent such that the color of the underlying tooth structure is transmitted through it. The shade of the prepared tooth is determined with a specially formulated shade guide Firing cycle → 8500 c for 2mins in vacuum. IV) INFILTRATED CERAMICS:- This system utilized alumina as the core-material. It is supplied as a two- component system- a powder (aluminum oxide/spinel) which is fabricated into a porous substrate, and a low viscosity glass which is infiltrated at high temperature into the porous substrate. The infiltrated ceramic is then veneered using the conventional feldspathic porcelain techinique. a)INCERAM:- - This system was developed by Sadoun (Paris) and was presented in 1989 for the first time by Vita company. - It is the successor system of Hi-ceram, differing from this system by having sufficiently lower grain size of aluminum oxide and thereby greater density. - In ceram ceramic consists of 2 three - dimensional interpenetrating phases: alumina (aluminum oxide) and glass. - The core is initially extremely porous and is composed of either aluminium oxide or spinel (Al2O3 and MgO), which is subsequently infiltrated with molten glass. - The spinel cores are more translucent than aluminium oxide cores due to the crystalline nature of spinel, which provides isotropic optical properties and partly due to its lower refractive index compared with alumina. 29
  30. 30. - But the spinel-based core ceramic (In-ceram spinel) was not as strong as the alumina-based material. The core is made from fine grained particles that are mixed with water to from a suspension referred to as a ‘slip’, is painted on a gypsum die (absorbent refractory die). The die draws water from the slurry under capillary pressure thereby depositing a layer of solid alumina on the surface, which is subsequently sintered/baked at 1120o c for 10hrs to produce an opaque porous core. This process is called ‘slip casting’. - At this stage, careful handling is a must as the material is very fragile. Also during baking, the slip (aluminous core) undergoes a shrinkage, which must be compensated for by the expansion in die stone. In the second phase, glass infiltration material (low viscosity glass) of appropriate shade is applied onto the core and fired at 11000 c for 3-5hrs. The molten glass infiltrates into the porous alumina core by capillary action resulting in a dense composite structure, increasing the strength of the core to about 20 times its original strength. - The aluminium oxide or spinel crystals limit crack propagation and the glass infiltration reduces porosity and both these factors explain why inceram is currently one of the strongest ceramic materials in the market. - The final infiltrated core has about 85% of the crystalline component which confers a 3 times increase in its flexural strength (450Mpa) compared to any other dental ceramic. - The core is then veneered with dentin and enamel conventional feldspathic porcelains/ vitadur N (vident) aluminous veneering porcelain using conventional powder-slurry technique to create the proper shade and contour. - A compositional analysis has revealed alumina to be 99.56 wt% and the infiltration glass to be lanthanum alumino silicate with small amounts of sodium and calcium. - Lanthanum decreases the viscosity of the glass to assist infiltration and increases its index of refraction to improve translucency. 30
  31. 31. - The core of aluminium oxide/spinel is so dense that traditional internal surface etching to improve the bond to tooth structure is not possible. - Sadoun and Asmussen (1994) describe the creation of a roughened surface by the application of fine grained silica to the inceram core. The reduction in fitting accuracy was claimed to be of the order of 10µm and could be accommodated by the use of die spacer. - Kern and Thompson (1994) have also applied silica coatings to enhance bonding by a thermal process tribochemical process. - A further development in this system is the ‘Inceram spinel’, which uses spinel instead of alumina. Spinel is a composition containing aluminium and magnesium oxide. - Due to the lower refractive index of spinel compared to alumina, the translucency of ceramic is improved but has a comparatively lower flexural strength. The fabrication process is similar to Inceram. Uses:- - Single anterior and posterior crowns. - Anterior 3 - unit bridges. - Implant supported bridges (recently). Advantages:- a) Lack of metal substructure. b) It has extremely high flexural strength - 450Mpa strongest all ceramic dental restorations presently available. c) Excellent fit, as little shrinkage occurs due to sufficient time at optimum temperature, which causes bonding between particles at small areas. Disadvantages:- Opacity of the material and hence can be used only as a core. a) Unsuitable for conventional acid - etching. b) Special die material and high temperature oven is required. c) Wear of opposing occluding enamel or dentin occurs if the In ceram restoration is a part of heavy incisal guidance or canine rise. 31
  32. 32. b) TECH CERAM:- This system (Tech Ceram Ltd, Shipley, Uk) for the production of high strength, all ceramic restoration has evolved following an 8 year development by a British team. -A thin (0.1 – 1mm) alumina core base layer is produced using a thermal spray technique, resulting in a density of 80-90%. -Optimum strength and translucency are achieved by a sintering process at 1170o c. -The range of base layer thickness makes this technique versatile and appropriate to a range of restoration types. -Subsequent reproduction of esthetics is achieved by incremental application of a range of specially developed porcelain in the traditional manner. -The inner fitting surface is rough and according to manufacturers, it does not require etching or silane bonding. GlC or dual cure resin-composite luting agents are recommended. V) MACHINABLE CERAMICS;- These products are supplied as ceramic ingots in various shades and are used either in computer aided design - computer aided manufacturing (CAD-CAM) procedures or in copy- milling techniques. Two classes of ceramics are available for machining fabrication of individual ceramic restoration and veneers:- a) Two fine – scale feldspathic porcelains (Vita MarkI& II and clay) b) Two glass ceramics (Dicor MGC light and Dicor MGC dark; Dentsply). The difference between the microstructure of mark I and mark II is determined by the particle size of the feldspar. The particles of mark I have larger particle size and a broader size range (10-50µm) than those of markII (4µm). MarkII has 32
  33. 33. significantly higher strength achieved by the higher homogeneity of the microstructure, optimization of the composition and increased densification. The basics for the feldspathic cerec vitablocs are the natural mineral feldspar. The advantage of feldspar compared with other ceramic raw materials are broad softening temperature intervals, the wide spread occurance and natural purity of the materials. Dicor MGC ( Dentsply L.D caulk division):- This is a machinable glass ceramic composed of fluorosilicate mica crystals in a glass matrix. It has greater flexural strength than cast dicor (Rosenblum and Schulman, 1997). They have been shown invitro to be softer than conventional feldspathic porcelain and produces less abrasive wear of the opposing tooth structure than cerec mark I but causes more wear than cerec markII. CELAY:- This material can be used for CAD-CAM produced restorations or used in the copy milling techniques. It is a fine-grained feldspathic porcelain. In the celay copy milling technique, a resin composite restoration is made on a master die, the restoration is then traced with a contact digitizer that transfers the shape to the celay milling device. CAD-CAM:- Computer aided design – computer aided manufacturing. This original system is replaced by a much-improved CAD-CIM - computer aided design – computer integrated manufacturing. Cerec system (developed in Zurich, Switzerland) is an application- oriented synthesis of 3-D imaging, computer aided design and numerically controlled machining. The basic philosophy of the cerec unit is to combine the scan head for the optical impression with the reconstructuion and fabrication module in a single mobile workstation. 33
  34. 34. CEREC 1:- The original cerec system introduced a revolutionary method of fabricating ceramic restoration to the dental profession. The main change or revolution in the hardware of cerec machine, was the introduction of an electrically driven milling machine with a more efficient cutting disc. The accuracy of cut was improved by the finer grit and by the increased rigidity of the disc during cutting due to increased disc thickness. The final software release for cerec [ cerec operating system ( COS ) 2.11], allowed 3D shaping of the fitting surface of the restoration against the cavity floor, improving the level of fit and allowing a wider range of shapes to be accommodated. The main limitation was, not being able to cut concave surfaces and also extending veneers into areas of missing tooth structure proved problematic. The main characteristic of the current generation machinable cerecVita porcelain include ease of machinability combined with low wear characteristic of the restoration together with minimal wear of the opposing tooth. The strongly translucent nature of ceramic allowed the use of light activated composite material as luting agent. These materials have greater density and filler loading than the traditional composites and therefore suffer less wear at the exposed marginal interface. These modern ceramics also offer long term color stability and can be polished to a high state of micro smoothness so that a glazing layer is not required. -The porous free nature of the entire block ensures that wear on the surface of the ceramic does not break through into an abrasive mass that can cause premature or accelerated wear on antagonist enamel surface. CEREC 2:- The cerec 2, a computer-aided design and integrated milling machine was introduced in the United States in 1996. The introduction of this unit has addressed virtually all of the limitations of cerec I. - It designs and fabricates porcelain inlays, onlays, crowns and veneers and allows immediate one visit esthetic restorations. 34
  35. 35. - A white glass free powder containing titanium oxide is placed on the tooth and a CCD sensor makes an infra red three-dimensional scan of the preparation in about 0.1 seconds at a resolution of 25µm. - A self-contained microprocessor displays the digitized image and the dentist designs the restoration. - Pre-fabricated /preformed porcelain ingots available in several shades (17- vita, Vivadent ; Pro CAD, Ivoclar ) are used by a digitally controlled six- axis micromilling machine to fabricate the prosthesis. Milling time is approximately 10mins for a simple restoration. - After fitting, the porcelain is custom stained, glazed, acid etched, silanated and cemented with standard adhesive composite resin luting agents. - The new-camera provides more data with greater accuracy resolving down to 25µm.The data set describing the tooth was increased from 4million voxels to 32million voxels. - The new technology employs two 32- bit processors and the user interface were improved by increasing the resolution of the monitor and adding a color screen. - Full shaping of the internal and external surfaces of restoration including detailed formation of occlusal pits and fissures is now possible. - Beyond full crowns, these is the possibility of making bridges by CAD- CAM, but the longevity of ceramic materials in this application remains questionable. Benefits of Cerec 2 system:- 1) Benefits for the patients:- -Esthetic and cosmetic restoration -Best material properties in dental ceramics. -Biocompatible. -Cost-effective. -Quick turn around time (1 day laboratory time) -Perfect occlusion. -High marginal integrity. 35
  36. 36. -No metal in mouth. 2) Benefits for the dentist:- - Economic production in the laboratory. - Increased precision. - Better interproximal integrity. - No polishing needed. - Contacts optimized in the Laboratory. CEREC 3:- The cerec technique was developed in 1984 to answer the practical need for long- lasting dental restorations with sound marginal seals. - The cerec 3 system (Sirona) simplifies and accelerates the fabrication of ceramic inlays, onlays, veneers and quarter, half and complete crowns for anterior teeth and posterior teeth. - The system has several technical improvements over cerec 2, including the three-dimensional cerec 3 intraoral camera, manipulation of the picture and the grinding unit. - Proper occlusion is established accurately and quickly as the software simplifies occlusal and functional registration. - The separate grinding device working true to morphologic detail and with fine surface quality is connected to the optical unit by radio control. Equipped with a laser scanner it can also be used for indirect application through a standard personal computer. - The cerec 3 system is network and multimedia ready and in combination with an intraoral color video camera or a digital radiography unit, can be used for patient education and for user training. Technical Characteristics:- 36
  37. 37. New technology Property Advantages a) cerec camera:- Principle: Active double Triangulation i.e → recording Of the cavity from 2 different triangulation angles provides immediate depth scale of > 20mm Eliminates adjusting procedure. Saves time, especially in the two important techniques, function and correlation. b)Image processing:- Rapid ‘twin grab board’ for the sirocam cerec intraoral measuring camera. Provides vertically oriented optical impression on the oblong format monitor. Eliminates relearning for previous users and also waiting time. c) Computer: Medically approved personal computer with shock protected hard disk. d) Double grinding unit:- 1.6mm cylindrical diamond or 1.2mm and 1.6mm c.d- 45o taper angle on top → grinding instruments. Control of grinding→ symmetric grinding by the instruments. e) Radio control communication:- Security → 1m distance to other electric devices. Incorporates greater versatility and efficiency of the windows system, network is ready. Provides an occlusal design that is true to detail. Allows tension free grinding, reducing stress on ceramic and grinding instruments. Allows cable free bi- directional data transfer to grinding unit. -Eliminates waiting time in design. -Performs grinding and construction simultaneously. -Saves time in finishing of occlusion. -Places less stress on ceramics and protects the grinding instruments. Allows placement of cerec 3unit components as desired. 37
  38. 38. ADVANTAGES OF CAD/CAM:- a) Negligible porosity levels in the CAD/CAM core ceramics. b)The freedom from making an impression. c) Reduced assistant time associated with impression procedures. d) Need for only single patient appointment. e) Good patient acceptance. DISADVANTAGES:- a) Need for costly equipment. b) Lack of computer – controlled processing support for occlusal adjustment. c) Technique sensitive nature of surface imaging required for the prepared teeth. PROCERA:- The Procera systems were produced by Nobel Biocare, Goteberg, Sweden and was first described in 1993.The original Procera system was designed to fabricate crowns and fixed partial dentures by combining a titanium sub-structure with a low fusing veneering porcelain .The machine has since been modified to use a densely sintered high-purity alumina coping combined with a compatible veneering porcelain to create all ceramic restorations. Clinical procedure: The Procera system uses a conventional impression and stone model. The die is properly ‘ditched/guttered’ below the finish line to clearly define the extent of the preparation and then is placed on the rotating platform of the Procera scanner. A sapphire stylus forms the tip of the scanner probe that contacts the surface of the die as it rotates around a vertical axis , and ‘reads’ the shape into a computer in a manner similar to a key-copying machine. Extremely light pressure of approximately 20g maintains the probe in contact with the die as it rotates .As the platform rotates, one-data point is collected at every degree around 360- degree circumference of the die .Following each complete rotation, the stylus is elevated 38
  39. 39. by 200µm and another circle of recordings are taken until the entire preparation has been digitized. The average preparation will require around 50,000 data points for accurate digitization and is extremely accurate with maximum shape-related errors of ± 10µm.The emergence angle of the coping from the tooth is selected and the relief space for the luting agent is automatically established by a computer algorithim. The digital information is sent to a Procera sandivik dental laboratory (Stockholm, Sweden) via a communication link. To account for sintering shrinkage, a model 20% larger than the original tooth is fabricated. A high strength aluminium oxide coping (600µm) is manufactured by compacting the material against the enlarged model and then milling the outer shape. The ceramic coping is returned to the local dental laboratory via express mail and an all- ceram veneering porcelain is added by the local laboratory technician. The restoration is custom stained and glazed and then returned to the dentist. Restorations produced with this system have been shown to have precise fit with marginal openings of less than 70µm and produces minimal wear of the opposing dentition, provided the material is suitably polished. Single crowns produced with this novel ceramic material can be expected to give acceptable clinical service for atleast 5 years. PROCERA ALLTITAN:- Its low thermal conductivity, low density, high strength, biocompatibility make titanium a desirable restorative material. The external contours of the individual titanium cores for Procera all titan bridges are milled and graphite electrodes create the fitting surface by a spark erosion process. Individual components of the bridge are welded by laser before the structure is finally veneered with special porcelain .In this way, the application of CAD-CAM technology, coupled with traditional spark erosion is said to enable the production of accurately fitting titanium-based restoration, while eliminating the need for costly and technically difficult casting procedures. CELAY SYSTEM:- The celay technique developed by Dr Stefan .T.Eidenberg at the University of Zurich, is a variation in the direct - indirect restoration concept, but without the 39
  40. 40. need for a laboratory technician. A direct process is used instead of a conventional impression in which a moldable precision imprint material is modeled directly inside the mouth in the cavity preparation, where it is adjusted for occlusion, contact relations and marginal integrity. The material then undergoes a light- hardening /curing process before it is removed from the tooth, to serve as a prototype model to be copied and reproduced in ceramic on a unique milling system developed by Claude of Microna technology. The milling centre has two distinct aspects. In one half, the model to be copied is centered in a holder, where it is manually scanned .A second part of the milling machine contains a rotary turbine with various cutting tools. The directly formed pattern in the vise is manually scanned with a sensor. This sensor is directly connected to the milling aspect. Any form scanned is thus simultaneously reproduced in all three dimensions in a block of ceramic by the rotary turbine .The gross form is developed with a diamond disk and refined with a diamond point .The system can also be used as a purely indirect process, in which an impression is made and a die developed in the laboratory .A resin composite restoration is made on a master die and is then traced with a contact digitizer that transfers the shape to the celay milling device. The composite resin imprint prototype material is placed in the milling unit in a bipoint metallic vise and the surface is scanned similarly manually and reproduced in ceramic on the milling unit, which carves out an exact replica of the plastic prototype in ceramic. Additional characterization or colorization of the inlay if required can be accomplished in the laboratory by refining the inlay prior to final finishing. ADVANTAGES: a. A precisely fitting ceramic restoration can be developed in one patient session. b. It can be developed without the need for laboratory technician. c. The restoration is developed in factory fired high grade porcelain. d. The processing time required is very short .A small inlay can be milled in 3 mins, a mesioocclusodistal inlay in less than 8 mins and a complete onlay in 12-13 mins. 40
  41. 41. METAL CERAMIC SYSTEMS /TECHNOLOGY Metal ceramic systems combine the strength and accuracy of cast metal with the esthetics of porcelain. The six basic principle features which distinguish a metal ceramic alloy from both crown and bridge and removable partial denture alloys are:- a. A metal ceramic alloy (MCA) must be able to produce surface oxides for chemical bonding with dental porcelains. b. A MCA should be formulated so that its co-efficient of thermal expansion is slightly greater than that of the porcelain veneer to maintain the metal- porcelain attachment. c. The alloy must have a melting range considerably higher than the fusing range of the dental porcelain fired on it. This temperature separation is needed so that porcelain build-up can be sintered to a proper level of maturity without the fear of distortion of the metal substructure. d. The alloy must have high temperature strength or sag resistance → that is the ability to withstand exposure to high temperatures without undergoing dimensional change. e. Processing should not be too technically demanding. f. A casting alloy should be biocompatible. 41
  42. 42. ADVANTAGES OF METAL CERAMICS:- a) High strength values due to prevention of crack propagation from the internal surfaces of crowns due to metal reinforcement. b) Improved fit on individual crowns provided by cast metal collar. DISADVANTAGES: a)Difficult to obtain good esthetics due to increased opacity of metal substructure. b)Porcelains used in metal ceramic techniques are more liable to devitrification which can produce cloudiness. c))Preparation for metal ceramic requires significant tooth reduction to provide sufficient space for the materials. d)More difficult to create depth of translucency due to ‘mirror’ effect of the dense opaque masking porcelain. INDICATIONS: a)Extensive tooth destruction as a result of caries, trauma, or existing previous restorations that precludes the use of a more conservative restoration. b)To recontour axial surfaces or correct minor malinclinations . c)Teeth requiring fixed splinting or being used as bridge abutments. CONTRAINDICATIONS: a)Patients with active caries or untreated periodontal disease. b)In young patients with large pulp chambers due to high risk of pulp exposure caused by substantial tooth reduction. c)Teeth where enamel wear is high and there is insufficient bulk of tooth structure to allow room for metal and porcelain. d)Anterior teeth where esthetics is of prime importance →high shades of very translucent teeth. CLASSIFICATION OF METAL CERAMIC ALLOYS:- BY NAYLOR,1986. 42
  43. 43. Based on composition: Firstly all alloys are separated into one of two major types: - Noble (precious) -Base metal (non-noble /non precious) Next the alloys are arranged by system, after which each system is further subdivided into constituent groups if present. SYSTEM GROUP A) NOBLE METAL ALLOYS: 1) Gold-platinum-palladium 2) Gold-palladium-silver High silver Low silver 3) Gold-palladium 4) Palladium-silver 5) High palladium B) BASE METAL ALLOYS: 1) Nickel-chromium Beryllium Beryllium free 2) Cobalt-chromium 3) Other systems Nature of metal ceramic bond:-Four mechanisms have been proposed to explain the bond between the ceramic veneer and metal substructure. 1) Vanderwaals forces. 2) Mechanical retention/entrapment 3) Compressive forces. 4) Direct chemical bonding. Chemical form of attachment is the predominant and most important mechanism. 1) VANDERWAAL’S FORCES:- Vanderwaals forces are secondary forces generated by a physical attraction between charged particles rather than by an actual sharing or exchange of electrons as in primary (chemical) bonding. These forces are generally weak as 43
  44. 44. only a minimal attraction exists between the electrons and nuclei of atoms in one molecule with those of adjacent molecule. The better the wetting of the metal surface, greater the vanderwaals forces .Porcelain adhesion to metal can be diminished or enhanced by alterations in the surface character of the porcelain bearing surface on the substructure. A rough contaminated metal surface will inhibit wetting and reduce the vanderwaals bond strength, where as a slightly textured surface created by finishing with uncontaminated aluminium oxide (Al2O3) abrasives followed by air abrasion with 50µm aluminium oxide, reportedly will promote wetting by the liquid porcelain. Vanderwaals forces are only minor contributions to the overall bonding a attachment process. 2) MECHANICAL RETENTION OR ENTRAPMENT :- It creates attachment by interlocking the ceramic with microabrasion in the surface of the metal coping which are produced by finishing the metal with non-contaminating stones or discs and air abrasion thus enhancing the wettability , providing mechanical interlocking and increasing the surface area for chemical bonding .Despite its presence , contribution of mechanical retention to bonding are relatively limited. 3) COMPRESSIVE FORCES:- Dental porcelain is strongest under compression and weakest under tension .Hence if the coefficient of thermal expansion of the metal substrate is greater than that of the porcelain fired over it, the porcelain is placed under compression (McLean, 1980) .When cooling a restoration with full porcelain veneer, the metal contracts faster than porcelain, but it is resisted by the porcelain’s lower coefficient of thermal expansion. This difference in contraction rates creates tensile forces on metal and corresponding compressive forces on porcelain. Without the wraparound effect created in full porcelain restoration, there is less likelihood of the ‘compression bonding’ developing completely .As a consequence, a partially veneered restoration may not encompass sufficient porcelain bearing surface to exert significant compressive bonding forces. 44
  45. 45. 4) CHEMICAL BONDING:- Two mechanisms may exist in the chemical bonding theory. According to one hypothesis, the oxide layer is permanently bonded to the metal substructure on one side while the dental porcelain remains on the other .The oxide layer is sandwiched between the metal substructure and opaque porcelain .This though is undesirable, as a thick oxide layer might exist that would weaken the attachment of metal to porcelain. The second and more likely theory suggests that the surface oxides dissolve or are dissolved by the opaque layer. The porcelain is then brought into atomic contact with the metal surface for enhanced wetting and direct chemical bonding by sharing of electrons between metal and porcelain .Both covalent and ionic bonds are thought to form but only a monomolecular layer of oxide is believed to be required for chemical bonding to occur. Chemical bonding is generally accepted as the primary mechanism in the porcelain metal attachment process .There are several ways of providing a surface oxide on the metal for porcelain bonding:- a. By introducing traces of base metals into precious metal alloys, which on heating produces thin oxide films. Eg: gold-platinum alloys for ceramic bonding. b. By direct oxide production via the constituents of the alloy. Eg: Nickel-chromium, cobalt-chromium alloys. c. By surface coating of metals with oxidizable metal films such as indium or tin. Eg: Electrodeposition of tin on platinum. Electrodeposition of metals:- Electrodeposition provided a strong bond when the tin coating was in the range of 0.2-2µm.The relevant facts emerging from this work in relation to the bonding of porcelain to metal were:- 1) A tin coating on platinum of 0.2-2µm gave optimum bond strengths for aluminous core porcelain of around 50N/mm2 .In this range; failure was always cohesive in the core porcelain. 45
  46. 46. 2) Below 0.2µm of tin, the strength falls and the incidence of interfacial failure increases at zero coating thickness. 3) Above 2µm thickness, a reduction in strength occurs .Failure is cohesive but porcelain is weakened. OXIDATION OR DEGASSING PROCESS:- It refers to a procedure recommended to clean the metal of organic debris and remove entrapped surface gases such as hydrogen .High temperature processing like this is vital for the removal of volatile contaminants that might not otherwise be eliminated either by metal finishing, steam cleaning or air abrading. This process also allows specific oxides to form on the metal surface which is responsible for the porcelain furnace to form a mature, stable oxide layer for the porcelain metal attachment. Majority of manufacturers consider this process a necessary step in the fabrication of the metal ceramic restorations although some manufacturers do not recommend the oxidation / degassing step. Instead they advocate minimizing the number of firings to which the casting is subjected to .Oxidation heat softens certain types of alloys through molecular rearrangement, which results in marginal distortion and decrease in bond strength. Post oxidation treatment:- The first oxides formed may be most desirable to form bond, but to reduce oxide layer or other contaminants, manufacturers suggest air abrading or acid treating the casting. Acid treatment (chemical method):Hydrofluoric , hydrochloric and dilute sulfuric acids are used .The potential hazards of these acids require that they be stored in clearly marked resalable plastic bottles. Use of a rubber tipped instruments to place oxidized castings into the acid is advocated .The container is then placed in the ultrasonic unit for the recommended time .After removal the casting is rinsed thoroughly under water .For final cleaning it is dipped in distilled water and cleaned in ultrasonic unit for 10-15mins. 46
  47. 47. Non-acid treatment : Castings can be air abraded with pure , 50µm aluminium oxide that is non-recycled as, if reused can cause metal contamination .It is then steam cleaned or ultrasonically cleaned in distilled water for 10-15 minutes. TYPES OF METAL CERAMIC BOND FAILURE:- Classification of ceramo metal failures has been given by O’Brien W J (1977) according to the interfaces formed at fracture. a) Metal - porcelain: Interfacial fracture occurs leaving a clean surface of metal and is generally seen when the metal surface is totally depleted of oxide prior to firing porcelain or when no oxides are available .It may also occur due to contaminated or porous metal surfaces. b) Metal oxide-porcelain: It is a common type of failure in the base metal alloy system .The porcelain fractures at the metal oxide surface, leaving the oxide firmly attached to the metal. c) Metal - metal oxide: It also commonly occurs in base metal alloy systems due to over production of chromium and nickel oxide at the interface .In this interfacial fracture, the metal oxide breaks away from the metal substrate and is left attached to the porcelain. d)Metal oxide-metal oxide: Fracture occurs through the metal oxide at the interface, resulting from an over production of oxide causing a sandwich effect between metal and porcelain. e) Cohesive within metal: Unlikely type of fracture for the individual metal ceramic crown .It occurs in cases where the joint area in bridges breaks. f)Cohesive within porcelain :In this fracture , tensile failure occurs within the porcelain when the bond strength exceeds the strength of porcelain .Most commonly occurs in the high gold content alloys as an ideal situation is created when the oxide film is only a few molecules thick and forms a solid solution with porcelain .Prolonged or repeated firing of base metal alloy and ceramic crown can cause excessive dissolution of the oxide layer I the glassy porcelain resulting in tensile failure which may not be revealed at the bond interface due to differential thermal expansion. FORMATION OF METAL COPING WITHOUT CASTING PROCESS: 47
  48. 48. PLATINUM BONDED ALUMINA CROWN:- Ceramics suffer from static fatigue believed to be due to stress-dependant chemical reaction between water vapour and the surface faults in the porcelain crown which cause flaws to grow to critical dimensions, allowing spontaneous crack propagation .One method of alleviating this problem was found to be the bonding of porcelain to foil. A method was devised whereby the surface of the platinum foil was coated with upto 2µm of tin and oxidized in a furnace to provide a continuous film of tin-oxide for porcelain bonding. The bonded alumina crown consists of an aluminous porcelain crown bonded to a 0.025mm thick platinum coping .The bonded platinum foil acts as an inner skin on the fit surface, reducing subsurface porosity and micro cracks in the porcelain and increasing the strength of the unit .In order to provide a porcelain butt fit and eliminate the dark shadow produced by a metal collar, the crown is baked onto an inner platinum foil matrix(unplated)which is removed after baking and provides space for the cement. TWIN FOIL TECHNIQUE:- (By McLean et al, 1976) This technique involves the laying down of two platinum foils in close apposition to each other. The inner foil of 0.025mm platinum provides a matrix for the baking of the porcelain and the outer foil which forms the inner skin to the crown is tin- plated and oxidized to achieve strong chemical bond with aluminous core porcelain. The outer or ‘bonded foil’ remains in position on the internal fit surface of the crown and will eliminate surface microcracks in the porcelain. The bonded alumina crown was developed with the following objectives:- a) Reduction of metal and labor costs in construction. b) Provision of a porcelain butt fit on the labial/buccal surface of the crown, eliminating the dark shadow of a metal collar. c) Reduction of stresses at the porcelain metal interface during cementation procedures. d) Improvement in strength of aluminous porcelain crown by reducing internal microcracks and subsurface porosity. 48
  49. 49. The shrinkage of porcelain makes it difficult to achieve an accurate fit of the core porcelain in one bake .Hence it is important to allow for shrinkage and prevent the fired porcelain from lifting the platinum skirt and spoiling the fit. Two ways have been advocated:- a) The cervical contact technique. b) The cervical ditching technique. The cervical contact technique relies on the application of a layer of porcelain around the shoulder area to shrink first. The second bake will then shrink towards the cervical porcelain and maintains the fit. The cervical contact technique does not always work since the bulk of the core porcelain still has to shrink during the second bake. It is for this reason that the cervical ditching technique is strongly recommended, where the porcelain is removed from the shoulder area after the initial build-up is complete, such that a thinnest ditch possible is made and just expose the cervical platinum at the shoulder. Removal of platinum foil: It is facilitated by soaking the crown in water. A fine pointed tweezer is used to lift the skirt away from the edge. Peel the platinum away from the entire circumference without damaging the fine porcelain edge (internally towards the incisal edge). Cementation: Subsequent to tin plating and oxidation of the platinum foil, a tin- oxide layer will be present on the fit surface of the crown. Hence hydrogen and metal ion bridges can be formed between the polar oxide layer and the poly anions in carboxylate or glass ionomer cement .On cementation of the crown or inlay, strong physico-chemical bonding between the platinum, cement and tooth can be obtained, thus reducing the risk of microleakage. BONDING TO GOLD FOIL:- In 1979, Rojers reported a method of bonding porcelains to electroformed, pure gold copings.UMK68 porcelain was used .More recently, the ceraplatinum or Renaissance crown has been marketed .This uses a formed laminated gold- palladium alloy coping, approximately 0.05mm thick which is swaged to the die. The coping is umbrella shaped and the corrugations allow for some expansion and reburnishing, according to the size of the tooth preparation. 49
  50. 50. All gold foil techniques require the use of metal bonding porcelains, since aluminous core porcelains with their high temperatures would cause the gold to melt. INDICATIONS FOR THE USE OF THE BONDED ALUMINA CROWN:- a) Porcelain veneer crowning of adolescent teeth where minimal tooth preparation is necessary. b) Anterior teeth, when metal reinforcement is essential. c) Complete porcelain cantilever bridges on anterior teeth(replacing lateral incisors) d) In heavily worn teeth, thin or short teeth where minimal occlusal clearance present (not less than 0.8mm), porcelain crowning of all anterior teeth is indicated. e) Repair of fractured metal- ceramic bridges, when removal of bridge or splint is undesirable. f) Coping jacket crowns on unit built bridge -work. CONTRAINDICATIONS:- a) In periodontally involved teeth, where preparations extend deeply into root- face and no shoulder preparations are possible. b) Posterior teeth where large areas of tooth are missing and uneven bulk of porcelain is inevitable. c) If lingual shoulder preparations are impossible particularly in molar region. CAPTEK SYSTEM: An alternative methodology for the elimination of the casting process from metal - bonded crowns and bridges has been developed by Davis Schottlander and Davis, UK. This technique involves the adaptation of a wax strip impregnated with gold- platinum-palladium powdered alloy to a refractory die .Firing produces a rigid porous layer which is then infilled with gold from a second wax strip by capillary action. The finalized metal coping is then veneered with porcelain. The advantages of this system include: 50
  51. 51. • Improved marginal fit(due to use of capillary cast rather than lost wax technique) • Enhanced esthetics. • Biocompatibility (since 88% of the alloy is non-oxidizing) RECENT ADVANCES IN CERAMICS:  SHRINK –FREE CERAMICS: The application of an all ceramic crown employing a unique shrink-free alumina substrate with specially formlulated porcelain veneers (cerestore system) offers a viable alternative to both the metal ceramic crown and traditional PJC. The development of the advanced alumina ceramic substrate allows the construction of highly durable all ceramic restorations with exceptional fit. COMPOSITION: The microstructure of the core material consists of a multiphased component or mixed-oxide system of aluminates. Aluminium oxide is the primary component with alpha-aluminium oxide (corundum) being the dominant phase in the microstructure. Calcium and beryllium aids in sintering process and contributes to high corrosion resistance due to absence of alkalis (Li,Na).Alpha aluminium oxide and magnesium aluminate spinel are the major crystalline phases of the core material. Direct molding /Transfer/Injection molding technique:- 51
  52. 52. The shrink free- ceramic can be formed directly on the master die, producing extreme accuracy of fit .The molding procedure is done on the master die made from a special epoxy resin die material(cerestore epoxy), which is heat stable and undergoes permanent controlled expansions during curing. The ceramic substrate which is supplied as a dense pellet of the compacted shrink free formulation is heated until it is flowable (160o C) and then transferred by pressure into a suitable mold directly on the master die. After it sets, the green substrate is removed from die and sintered/controlled firing is carried out resulting in zero shrinkage of the ceramic.  MAGNESIA CORE CERAMIC:- The co-efficient of thermal expansion of the alumina core material is 8X10-6 /o C and requires similar low expansion veneer porcelains and since the coefficient of expansion values for porcelain used with metals averages around 13.5x10-6 /o C , extensive cracking results upon bonding these materials owing to thermal stresses. The magnesia core material has a modulus of rupture strength of 19,000 psi after firing and a coefficient of thermal expansion value of 14.5x10-6 /o C. The required strength is achieved by dispersion strengthening by the magnesia crystals in a vitreous matrix and also by crystallization within the matrix. The strength can be doubled by application of a glaze which may either penetrate into surface pores to fill in the subsurface porosity or react with the core material to produce crystallization which places the surface layer in compression. The magnesia core crown is fabricated by means of a modified platinum foil matrix technique developed by Lazar, McPhee and O’Brien, which gives an excellent fit compared to the conventional technique.  HYBRID CERAMICS:- These ceramics opened simplified ways of application .Hahn (1995, 1997) proposed a new ceramic material that is a hybrid between organic and inorganic components. A precursor material consisting of 50%vol polyvinyl 52
  53. 53. siloxane, 30% active filler (titanium 1µm), 15% inert filler (aluminum oxide, 15µm) and 5% titanium boride has been formulated. This mixture can be handled like composite and cured .The precursors are stable and remain so during the heat treatment→6 hrs at 11500 C in N2 atmosphere followed by a few minutes in O2 for surface treatment .Since the resulting ceramic is yellow, only copings are made. They can be veneered with feldspathic ceramic such as Vitadur.  YTZP-CERAMIC-yttriumstabilized tetragonal polycrystals ceramic:- Zirconium oxide (Zro2) is currently the strongest white shaded ceramic available .This type of ceramic powders provides high performance .In case of stress, the tetragonal crystal phase is transformed into the hexagonal crystal phase, stress is absorbed and no crack formation occurs. This material cannot be processed by simple technologies. The machining tool must be very strong and abrasion resistant since Zro2 ceramic is an extremely abrasive product.  ORMOCER:- Ormocer is an acronym for “organically modified ceramics”. These compounds are also known in literature as “Ormosils” (organically modified silicates). Ormocers chemically are methacrylate substituted alkosilanes/organic- inorganic copolymers. The ormocer matrix consists of ceramic polysiloxane(silicon- oxygen chains).The alkyl silyl groups of the silane allow the formation of an inorganic Si-O-Si network by hydrolysis and condensation polymerization reactions to give cross- linked structures and their properties may be modified by filler particle substitution. This chemical reaction increases the molecular weight upto values between 2000 and 20,000.In comparison, conventional BisGMA only exhibits a molecular weight of approximately 500.Due to larger initial molecule polymerization, shrinkage can be reduced considerably compared to conventional hybrid composites. 53
  54. 54. This new material can be used for filling in the anterior and posterior areas, serves as an optimum and upto date replacement for amalgam, composites and compomers. It has a biocompatible polysiloxane net with low shrinkage even prior to light curing. Traditional composites and compomers polymerize organic monomers (methacrylate) only during light curing resulting in high shrinkage .In addition, residue monomers may remain and are released leading to allergic reactions, as with the polymerizations lamps used, only 60-70% of the free monomers can be converted throughout the lifetime of the restoration. Silicon oxide, a filler serves as a basic substance for ormocer. It is modified organically by adding polymerisable side chains in the form of methacrylate group. Through bonding of the methacrylate molecules to the carrier medium, the molecules can no longer be eluted during incomplete polymerization. To achieve x-ray opacity, higher than that of enamel, silicon can partly be replaced by heavier elements such as zirconium. Incorporaion of special glass fillers results in a paste like composite material which should be easy to use. A material based on the ormocer concept has recently become available-“DEFINITE”, Degussa dental, Hanau, Germany. The manufacturers claimed low shrinkage, high abrasion resistance, condensability, timeless esthetics, biocompatibility and protection against caries (No long term clinical trial results are yet available.)It is supplied in 12 tooth shades that are based on the VITA shade guide. Properties of DEFINITE:- a) ‘DEFINITE’ is an aqueous extract and is biocompatible. b) Polymerization shrinkage of Definite is only 1.88%. c) Finer structure of ormocer provides Definite with an excellent abrasion resistance. Consequently Definite can be used in the posterior area that is exposed to masticatory load and ensures outstanding long term stability of the filling in this area. d) Definite permanently releases fluoride, calcium and phosphate ions that protect the adjoining cavity margin. 54
  55. 55.  CEROMER:- Ceromer is an acronym for “ceramic optimized polymer”. This restorative material is biocompatible, metal free which exhibits the strength and potential wear resistance of metal supported restorations and can be effectively adjusted and polished chair-side. Chemistry: The new ceromer system “TETRIC CERAM “and “TETRIC FLOW” (Vivadent) offer two consistencies to accommodate clinical demands. Tetric ceram belongs to a new generation of ceromer materials with an enhanced filler composition, containing 80% filler –Ytterbium trifluoride with addition of barium alumino flurosilicate glass particles to provide high fluoride release and improve radioopacity. Spherical ceramic particles and pyrolitic silica complete the composition. This unique combination of fine silanised filler particles contribute to the wear resistance of Tetric ceram and the particle size varies from 0.04µm-3µm.A spherical rheological modifier was incorporated into the ceromer, which consists of silicate platelet agglomerates. During the application, the platelets deaggregrate and disperse in the composite increment, allowing the material to be easily contoured and modeled without significant slumping. In 1996, Ivoclar launched Targis, which contains approximately 77 wt%filler (57vol %) and 23wt% organic resin. The filler part is trimodal and consists of barium glass with a mean particle size of 1µm; spheroidal silica filler with a mean particle size of 25µm as well as colloidal silica. The resin matrix consists of conventional monomers. INDICATIONS: a) Represents alternatives to conventional crown and bridge remedies for the treatment of single and multiple unit anterior and posterior restorations in which supragingival preparation design can improve soft tissue compatibility. b) Benefit of adhesive bonding to short clinical crown preparation. c) Posterior bridges with single pontics between the abutment teeth are primary indication for TARGIS system. 55
  56. 56. d) Indicated for inlays, onlays, single and multiple unit restorations, implant superstructures and bridges with metal framework. e) Also indicated in cases in which centric holding cusps are weakened or undermined. CONTRAINDICATIONS: a) When complete isolation cannot be achieved. CONCLUSION: Ceramics have a great past in dentistry. Their unsurpassed aesthetic and biocompatible qualities have positioned them in the high -end segment of restorative dentistry and provide the stimulus to seek to overcome their limitations and continue laboratory and clinical research .The future of ceramics is even greater since they offer great potential for improvement , especially in manufacturing technology .However the co-operation between the excellent dentist and the highly - skilled dental technician is unavoidable. 56
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