INTRODUCTION:

Ceramics is derived from the Greek word keramos, which means pottery or burnt stuff. Ceramics were probably the 1st

material to be artificially made by humans and porcelain was among the 1st material to be the subject of early laboratory research by scientists.

Examples of the early fabrication of ceramic articles have been found and dated as far back as 23,000 years B.C.

Historically, three basic types of ceramic materials were developed.1. Earthern ware (fired at low temperature and is relatively

porous).2. Stone ware (app. in China in 100 B.C. It is fired at higher

temperature than 1st, which results in both higher st & less porous).

3. Chine stone (king etching in China 1000 A.D. much stronger and more tranclucent).Pierre Fauchard (The father of modern dentistry was the 1st

to suggest the use of porcelain in dentistry in 1728).De Chateau a French person was the 1st to make a ceramic

denture in 1778.The idea of making porcelain jacket crowns is credited to

Beers of California in 1873. The 1st successful PJC is said to be made by Land of Detroit in 1882.

Porcelain inlays were said to have been produced at the Philadelphia Dental College in 1890.

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In the early 1960’s the 1st successful porcelain fused to metal system was introduced.History:

Many attempts at imitating Chinese porcelain were made in Europe in the 17th century. John Dwight of England was granted a British Patent No.164 in 1671 by King Charles-II, which claimed that he had been able to simulate transparent Chinese porcelain. However, no examples are available to indicate that he infact had succeeded.

A method of fluxing white clay was discovered by lab experiments in Meissen in Germany in 1708. However this so called white porcelain more closely resembled the northern Chinese white stoneware rather than the transluent true porcelain of souther china.

A Jesuit father named D’entrecolles was able to gain the confidence of Chinese potters and learn the secret in 1717 that had eluded manufacturers in Europe for so long.

The majority of the early Chinese porcelain was called hard paste porcelain. This hard paste product is often referred to as “true” porcelain and was highly transluent. The “green” composition of traditional hard porcelain is approximately 50% Kaolinite (Al2O3 SiO2 2H2O) 25% feldspar (K2O Al2O3 6SiO2) and 25% quartz (SiO2).

The 1st porcelain used in dentistry in the late 18th century was originally based upon the tri-axial porcelain composition.

Whereas the dental porcelain materials have evolved from the traditional tri-axial formula the actual composition of the white ware parian chain bodies used has varied greatly over the

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years ranging from those consisting of more than 90% feldspar plus 3-5% kaolin to bodies containing less than 75% feldspar plus a wide variety frits and other auxillary fluxes.

The fact that in the dental technique only small simple shapes are required has meant that the plasticity of the unfired porcelain is relatively unimportant.

This allowed a reduction in kaolin content and an inc. in feldspar, which enabled the fired material to have a much higher translucency in the absence of mullite formation. This resulted in the composition moving away from the mullite zone and into the leucite zone.

In 1774 a French apothecary named Alexis Duchateau was very dissatisfied with the way that his carved ivory dentures became badly stained.

He noticed that the glazed ceramic utensils that he used everyday for mixing and grinding his various chemicals resisted staining with the relatively non-porous surface and even also resistant to abrasion. These circumstances gave birth to the idea of using porcelain as a dental restoration material. He enlisted the help of french porcelain manufacturers at the Guerhard factory in St.German en-laye and succeeded in making himself the 1st set of all mineral dentures.

Dechateau had little success and later collaborated with a dentist named Nicholas Dubois de Chemant of Paris who considerably improved upon the method of fabrication. De Chemant was granted an invention patent by Lewis XVI. The publicity that followed upset other Parisian Dentists who accused him of stealing Duchateau’s invention.

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The Wedgewood factory supplied the porcelain powder from which he continued to manufacture his dentures.

The foundation for the modern mass production of artificial teeth was laid by the Italian dentist Giuseppangelo Fonzi when he produced the 1st individual porcelain terro-metallic teeth in 1808.Classification:

There are several categories of dental ceramics: Conventional leucite containing porcelain. Leucite enriched porcelain. Ultra-low fusing porcelain (may contain leucite). Glass ceramic. Specialized core ceramics (alumina, glass infiltrated alumina,

magnesia, spinel). CAD-CAM ceramics.Dental ceramics can be classified by type as:

Feldspathic porcelain. Leucite-reinforced porcelain. Aluminous porcelain. Alumina. Glass-infiltrated spinel. Glass ceramic.By use:

Denture teeth. PJC. Metal ceramics crowns.

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Castable glass ceramic crown. Veneers. Inlays/Onlays. Crowns. Anterior bridges. Ceramic brackets.According to processing method:

High fusing 1300o C.Medium fusing 1101-1300 o C.Low fusing 850-1100 o C.Ultra low fusing < 850 o C.According to method of firing:

Air fired i.e. at atmospheric pressure.Vacuum fired i.e. at reduced pressure gas fusing.According to substructure method:

Cast metal.Swaged metal.Glass ceramic.CAD-CAM porcelain.Sintered ceramic core.According to method of fabrication:

Condensing and sintering.Pressure moulding and sintering.Casting and ceramming.

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Slip casting.Sintering and glass infiltration.Milling by computer control.According to application:

Core porcelain.Dentin or body porcelain.Enamel porcelain.Definition: (Gilman 1967)

It is defined as a combination of one or more metals with a non-metallic element usually O2 serve as a matrix with smaller metal atoms tucked into spaces between the oxygen.According to Skinners:

It is a compound of metals and non-metals that may be used as single structural component, such as when used in a CAD-CAM inlay or as one of the several layers that are used in the fabrication of a ceramic based prosthesis.Composition:

Ceramics is a very broad term.Definition: A ceramic material may be defined as a compound of metallic and non-metallic elements, the formation of which requires high temperature.

Dental ceramics contain a glassy matrix reinforced by various dispersed phases consisting of crystalline structures such as leucite, alumina and mica.

Porcelain is a specific type of ceramic characterized by it being white and transparent.

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Early formulations:

Kerl’s handbook of 1907 gives the following mineral composition for the early dental porcelain developed by STOCKTON.Feldspar 78.0%Kaolin 15.3%Potash silicate 04.7%Dehydrated Borax 02.0%The recent composition:

Feldspar 60-80% basic glass former.Kaolin 3-5% Binder.Quartz 15-25% filler.Alumina 8-20% glass former.Boric oxide 2-7% glass former and flux.Oxides of Na, K & Ca 9-15% fluxes (glass modifiers).Metallic pigments <1% colour matching.Silica:

Silica can exist in four different forms.Crystalline quartz.Crystalline cristobalite.Crystalline Tridamite.Non-crystalline fused silica.

Silica acts as a refactory skeleton and provides strength and hardness to porcelain during fusing. It remains unchanged during firing due to the 3 dimensional network of the covalent

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bonds between silica tetra hydra. Addition of metals like Na, K or Ca can break the bonds between silica tretrahydra. These ions associate with the O2 atoms at the corners of the tetra hydra, interrupt the O2 silica bonds, as a result 3D silica network containing many linear chains of Silica tetrahydra which are amenable to move more easily at lower temperature than are the atoms locked into the 3 dimensional structure of silica tetra hydra.Leucite:

Leucite is the crystal phase, which is used to create a high expansion porcelain that is thermally compatible with gold base, palladium based and nickel based alloys. When subjected to heat treatments for 1150o to 1350o glasses in the N2O, K2O, Al2O3-SiO2 system containing not less than 11% K2O produce high exp. glasses suitable for bonding to metal. The higher thermal exp. results from the crystallization of leucite. The proportion of leucite is governed by the K2O content as well as the temperature and duration of heat Rx. The leucite crystals contract more than the surrounding glass matrix during sintering. The result is formation of compressive stress in the glass phase, which may reduce the stress at the tip of a propagating crack.Disadvantages:

The high content of leucite seems to contribute to a relatively high in vitro wear of opposing teeth (Seghi, Rosenstial, Bauer 1991).Glass modifier:

Oxides of Na, K & Ca act a glass modifiers.

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These1. Lower the fusion temperature2. Increases the flow of porcelain during firing and3. Increase thermal expansion.4. They also absorb or remove impurities. However, if the

concentration of flux is too high.It reduces chemical durability of the glass.It may cause the glass to crystallize or devitrify.Boric acid can behave as a glass modifier. Another

important glass modifier is water although it is not an intentional addition to dental porcelain. The hydronium ion H3O can replace sodium or other metal ions in an Alumina ceramic that contains glass modifier. This fact accounting for the phenomenon of slow crack growth of ceramics that are exposed to tensile strength and moist porcelain restoration.Alumina:

It replaces some silica in glass matrix. It gives strength and opacity to the porcelain. It alters the softening point and increases the viscosity of porcelain during firing. The form is used in ceramics and generally ball milled as powder below 10-20 in size.Kaolin:

It is a white clay like material. It is hydrated aluminium silicate. It acts as a binder, gives opacity to the mass (some materials without kaolin use sugar or starch for the same purpose).Feldspar:

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Potassium and sodium feldspar are naturally occurring minerals composed of potash (K2O), soda (Na2O), alumina (Al2O3) and silica (siO2).

It is the basic glass former. During firing feldspar fuses and acts as a matrix and binds silica and kaolin. It acts as a flux, matrix and surface glaze. When it is mixed with metal oxides and fired at high temperatures it can form a glass phase that is able to soften and flow slightly. Because of this the porcelain powder particles coalesce together. The process by which the particles coalesce is called liquid phase sintering (firing of particles together without complete melting) a process controlled by diffusion between particles at a temperature sufficiently high to form a dense solid.It has two important properties:1. When fused at high temperature it retains its form without

rounding (due to high content of potash which is more viscous).

2. Between 1150o C and 1530o C it undergoes incongruent melting and forms crystals of leucite in a liquid glass (Leucite is a potassium aluminium silicate material with a large co-efficient of thermal expansion compared with feldspar glasses (20 to 25 x 10-6/oC comp to 10 x 10-6/oC).

Colouring frits:

Pigmenting oxides are added to obtain various shades needed to simulate natural teeth. These colouring pigments are produced by fusing metallic oxides together with fine glass and feldspar and then regrinding to a powder. These powders are

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blended with the unpigmented powdered frit to provide the proper hue and chroma.

Examples of metallic oxides and their respective colour contribution includes:Iron or nickel oxide (brown).Titanium oxide (yellowish brown).Manganese oxide (lavender).Copper oxide (green).Cobalt oxide (blue).

Opacity may be achieved by addition of cerium oxide, zirconium oxide, titanium oxide and tin oxide. Phosphorous dentoxide (P2O5) is sometimes added to induce opalascence and is also a glass forming oxide.Methods of strengthening ceramics:

Ceramics are brittle materials and have low tensile strength. But their compressive strength is higher. In order to overcome their principle deficiencies of brittleness and low tensile strength different methods have been used:a. Methods of strengthening.b. Methods of designing components.Strengthening of ceramic materials is done by:

a. Introduction of residual compressive stresses into the surface of the material.

b. Interruption of crack propagation through the material.Introduction of residual compressive stresses:

Concept:11

If a surface is treated to introduce a residual compressive stress which is higher than the tensile stress, the resultant stress on the surface would still be a compressive stress. Thus the restoration will not yield and fracture due to the tensile stress. Some of the technique for introducing residual compressive stress are: Ion exchange (also called chemical tempering)

Residual compressive stresses can be introduced if sodium ion having a small ionic diameter which is a constituent of glass is exchanged by potassium ion (which is 35% larger). Thus there is squeezing of the potassium ion into smaller space termed as “stuffing”. This squeezing creates large residual compressive stress (700 Mpa) in the surfaces of the glasses subjected to this treatment. These residual compressive stresses produce a pronounced strengthening effect. However, this procedure is best used on the internal surface of a crown, veneer or inlay because this surface is protected from grinding and exposure to acids. Not all ceramics are amenable to ion exchange.For example:

Alumina core materials.Dicor glass-ceramic core.

Conventional feldspathic porcelain that are highly enriched with potash feldspar cannot be sufficiently exchanged with potassium to warrant this treatment.Thermal tempering:

This is the most common method for strengthening glasses. Thermal tempering creates residual surface compressive stresses by rapidly cooling the surface of the object while it is hot and in

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the softened state. This rapid cooling produces a skin of rigid glass surrounding a soft (molten) core. As the molten core solidifies, it tends to shrink but the outer skin remains rigid. The pull of the solidifying molten core as it shrinks creates residual tensile stresses in the core and residual compressive stresses within the outer surface.

For dental application it is more effective to quench hot glass-phase ceramic in silicon oil or other special liquids rather than using air jets that may not uniformly cool the surface.Thermal compatibility (thermal co-efficient mismatch):

In fabrication of ceramic in combination with metal this theory is employed. Ceramic in combination with metal are heated and cooled together. The metal which is veneered with ceramic has a higher co-efficient of thermal expansion than the ceramic. Hence on cooling the metal contracts more than the ceramic thus leaving the outer layer of ceramic in residual compressive stress.Disruption of crack propagation:

Another method of strengthening glass ceramics is to reinforce it with a dispersed phase of a different material that is capable of hindering a crack from propagating through the material.Two different types of dispersions used to interrupt crack propagation are:1. By absorption of energy by the dispersed tough particle from

the crack and thus depleting its driving force for propagation.

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2. By change of crystal structure under stress to absorb energy from the crack.

Dispersion of a crystalline phase:

When a tough crystalline material such as alumina (Al2O3) in particulate form is added to a glass, the glass is toughened and strengthened because the crack cannot penetrate the alumina particles as easily as it can the glass.

Thus aluminous porcelain was developed for PJC.Another ceramic material that uses reinforcement of a glass

by a dispersed crystalline substance is Dicor glass ceramic. The cast glass crown is subjected to a heat treatment that causes silicon sized mica crystals to grow in the glass.Transformation toughening:

A newer technology for strengthening glasses involves the incorporation of a crystalline material that is capable of undergoing a change in crystal structure when placed under stress. The crystalline material usually used is termed.

Partially stabilized zirconia (PSZ). The energy required for the transformation of PSZ is taken from the energy that allows the crack to propagate (one drawback of PSZ is that its index of refraction is much higher than that of the surrounding glass matrix). As a result the particles of PSZ scatter light as it passes through the bulk of the porcelain and this scattering produces an opacifying effect that may not be esthetic is most dental restorations.Design of dental restorations involving ceramics:

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Dental restoration containing ceramics should be designed in such a way as to overcome their weakness. The design should avoid exposure of the ceramic to high tensile stresses. It should also avoid stress concentration at sharp angles or marked changes in thickness.Minimizing tensile stress:

Conventional PJC’s are contra indicated for restoring posterior teeth (because occlusal forces can subject them to large tensile stresses which are usually concentrated near the internal surface of the crown).Reducing stress raisers:

Stress raisers are discontinuities in ceramic structures and in other brittle materials that cause stress conc. The design of ceramic dental restoration should also avoid stress raisers in the ceramic. Abrupt changes in shape or thickness in the ceramic contour can act as stress raisers and make the restoration more prone to failure.Fabricating of a ceramic restoration:

The porcelain powder is mixed with the liquid to form a plastic mass, which is condensed to form the porcelain restoration. It is then fired in the furnace for sintering. When fired the mass shrinks and flows so the built up mass has to be supported on a matrix.

The matrix should have a higher fusion temperature than the porcelains.1. Platinum foil 0.001” is adapted on the die to form a wrinkle

free matrix.

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2. Metal coping of suitable design and alloy.The matrix supports the unfired porcelain. Before

condensing porcelain the matrix is degassed in the furnace to remove the gasseous impurities and anneal the platinum foil.Building up of porcelain:

A plastics mass is prepared of porcelain powders mixed with liquid. With a brush the plastic mass is applied over the matrix. It is built up in series of layers of core, dentin and enamel.Manipulation:

Condensation:

The porcelain powder particles within the mass are closely packed in order to reduce the shrinkage of porcelain and minimise porosity in the fired porcelain. The process of packing the powder particles together and removing the excess water is known as condensation. Proper condensation gives a dense packing.

Dense packing provides 2 benefits. Lower firing shrinkage. Less porosity.Methods of condensation are:Vibration: Mild vibrations are used to densely pack the wet powder upon the underlying matrix. The excess water comes to the surface and it is blotted with a tissue.Spatulation: A small spatula is used to apply and smoothen the wet porcelain. This action brings excess water to the surface.

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Brush Technique: Dry powder is placed by a brush water is drawn towards the dry powder and the wet particles are pulled together.Ultrasonic: Mild vibrations are transmitted electrically.Gravitational:Whipping:

Any method may be used for condensation but care is taken not to allow the porcelain to dry out as the porcelain powder is held together due to surface tension of water.

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Firing:

Firing is carried out for fusing the porcelain. The process is known as sintering. The thermochemical reaction between the porcelain powder components are virtually completed during the original manufacturing process. Some chemical reaction do occur during prolonged firing times or multiple firings.

Changes occur in the leucite content of the porcelain (Leucite is a high-expansion crystal phase whose volume fraction in the glass matrix can greatly affect the thermal contraction co-efficient of the porcelain). Changes in the leucite content can cause the development of a thermal contraction coeff mismatch between porcelain and the metal and thus can produce stresses during cooling.

The porcelain should never be allowed to come directly in contact with the walls and floor of the muffle.

The condensed mass is gradually heated by first placing it in front of the muffle of a preheated muffle 650o C and later inserting into the furnace.

During firing the porcelain shrinks 30-40% by volume.During firing there is partial fusion of the particles at their

point of contact. As the temperature is raised the fused glass gradually flows to fill up air spaces. As the fused mass is viscous all the air cannot escape and some get trapped giving rise to voids or porosity vaccum offset firing is done to reduce porosity in porcelain.

Vaccum firing of dental porcelain was introduced in the late 1940’s.

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Helberger (Germany) has been given the credit of developing and using the 1st vaccum furnace in 1913.

The vaccum firing porcelain frit has a large average particle size compared to air firing porcelain frit.Gas firing:

An alternative for producing high densities in by subsitution of diffensable gas for the ordinary furnace atmosphere.

Air is driven out of the porcelain powder bed and diffusable gas is substituted. The gases used are heluim, hydrogen or steam.Manufacture Fritting:

Frit is the final glass that is produced. The raw mineral powder oxides or carbonates are mixed together in a refractory crucible heated to a temperature well above the ultimate maturing temperature in the laboratory. The oxides melt together to form a molten glass. Gases are allowed to escape and the mix is quenched in H2O. The red hot glass on sticking with cold H2O immediately breaks up into fragments and is termed as the frit. The process of blending melting and quenching the glas components is termed fritting.Devitrification:

When too many of the glass forming (S1O4) are disrupted in dental porcelain the glass may devitrify.

Vitrification in ceramic terms is the development of a liquid phase by reaction or melting which on cooling provides the glass phase. This structure is termed vitreous.

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Stages in firing:

1. Low Bisque stage: In this stage the material becomes rigid and is very porous. There is very little shrinkage.

2. Medium Bisque stage: There is complete cohesion of the powder particles. It is still porous, lacks transluency and high glaze. There is definite shrinkage.

3. High Bisque stage: The shrinkage is complete and the mass exhibits smooth surface. Slight porosity may be seen. The body does not appear to be glazed.

At any stage the work can be removed, cooled and additions can be made. Less the number of firings higher is the st. and better the esthetics. Too many firings give a lifeless, over transluent porcelain.Cooling:

Must be carried out slowly and uniformly. If shrinkage is not uniform it causes cracking and loss of strength. During cooling subsurface submicroscopic surface cracks occur. (Because of the low thermal conductivity of porcelain the differential between the thermal dimensional change of the outside and inside can introduce stresses which embrittle the porcelain).Glazing:

Porcelain are glazed to give a smooth and glossy surface, enhance esthetics and help in hygiene.

There are two types of glaze.1. Self glaze.2. Over glaze.

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Self-glaze: All the constituents of porcelain frit are completely melted to form a single phase glass. Then the porcelain is said to be self-glazed. The dental porcelains can be self glazed by heating under controlled condition i.e. it is heated to its fusion temperature and maintained for 5 minutes. This causes the surface layer to fuse more confluently and fill up the irregularities giving a glossy surface. The glass grains flow over the surface to form a vitreous layer, which is a glaze (prolonged heating at higher temperature can cause pyroplastic flow of the material, which causes rounding of sharp angles and edges).Over glaze: Over glaze are ceramic powders containing more glass modifiers thus lowering fusion temperature. It imparts an impervious glossly surface to the restoration. The co-eff of thermal exp of the over glaze should be slightly lower than that of body porcelain.

(Glazed porcelain is much stronger than unglazed. The glaze is also effective in reducing crack propagation. If the glaze is removed by grinding the transverse st reduces to half).Over glaze – Some of these materials are fluxes, not porcelains and tend to wear more rapidly than the natural glaze.

The advantage of over glaze is that it can be matured at a lower temperature than that required to attain the auto glaze. This can be helpful for restorations that have been fired several times without forming a natural over glaze. Also, some stains tend to dilute and display a lower chroma when fired at the natural glass temperature. Therefore only if lower glazing temperature is necessary the over glaze may be used however the natural glaze is preferred.

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Definition: Stain is defined as a mixture of one or more pigmented metal oxides and usually a low fusing glass that when dispersed in a aq. slurry or monomer medium applied to the surface of porcelain or other specialised ceramic dried or light cured and fired will modify the shade of ceramic based restoration. They are also called as surface colorants or characterization porcelain.Staining materials: Ceramic stains are usually coloured metallic oxide pigments. Some are mixed with translucent ceramic powders, fired and dispersed in a ceramic glass base.

Stains can be applied on the natural glazed porcelain or to the surface with the glaze removed (better). The stain will penetrate the unglazed surface to a much greater degree than on a glazed surface and have less tendency to pool.

Stains should not be applied on over glaze. The stains leave the surface in a rougher state than the unstained glazed surface.Metals used in metal ceramic restorations:High NobleGold – Platinum – PalladuimGold – Palladuim – SilverGold – PlalladuimNoblePalladuim SilverHigh PalladuimPredominantly baseNickel Chromium

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Nickel Chromium BerryluimCobalt ChromiumBonding Mechanism:

Four mechanisms have been described to explain the bond between the ceramic verneer and the metal substructure.1. Mechanical entrapment2. Compresive forces3. Van der Wall’s forces4. Chemical bonding.Indications:

1. In case of parafunctional mandibular activity where an esthetic restoration is essential.

2. Lingual clearance is less than 0.8 mm after tooth preparation.3. Abutments and aplinting.4. Posteriors, where full coverage is necessary for esthetics.5. Deep chamfer preparation are necessary.6. Good occlusal surfaces are required.Contra Indications:

1. Adolescent teeth with minimal tooth preparation.2. Adult teeth with high enamel wear and insufficient bulk of the

tooth.3. Anterior where esthetics is of prime importance.Advantages:

High strength (comp)Improved fit.

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Dis-advantages:

Low tensile strength and shear stress.Fit may vary due to distortion of metal.Increased opacity decreased light reflectivity.Metal Ceramic Restorations:

The bonding of porcelain to gold alloys introduced in early 1960 by Weinstein et.al. was a pivotal breakthrough in dental esthetics.

Weinstein et.al. first described the production of metal ceramic restorations by using porcelain powders containing 11% to 15% K2O frits.

When subjected to heat treatments at temperature from 700o C to 1200o C glasses in the Na2O, K2O, Al2O3 – S1O3 system containing not less than 11% K2O produced high exp glasses suitable for bonding to metal. The higher thermal expansion resulted from the crystallization of leucite. The proportion of leucite is governed by the K2O content as well as the temperature and duration of heat treatment.

To achieve a strong bond to metal certain conditions must be met.1. The glass must wet the metal and the stresses resulting from

thermal expansion and contraction should not exceed the tensile strength of the glass.

2. Alloys for attachment to dental porcelain must have high temperature stability and produce thin films of oxide for porcelain bonding (“degassing” treatment) (tin oxide ,iridium oxide in precious metals).

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3. The temperature should be raised to such a level that this oxide is partially dissolved into the glass.

4. Excess oxide production can produce weak bonding as some times occurs when Ni-Cr alloys are used.

High gold alloys containing mininum 84% pure gold still remain alloy of choice.

To bond suitably to the alloys porcelain must be sufficiently low fusing and they also must have a coeff of thermal contraction that is closely matched to that of alloys (Metal should have a slightly higher value).Thermal compatibility systems:

This porcelain metal bond is primarily chemical in nature and is capable of forming even when the metal surface is smooth and little opportunity exists for mechanical interlocking.

The alloy should have a high proportional limit and high modulus of elasticity.

The metal framework must not melt during porcelain firing and must resist high temperature (sag deformation).Bonding of porcelain to metal using electrodeposition:

A layer of pure gold is deposited onto the cast metal followed by a short “flashing” deposition of tin.Advantages are:

1. Bonding is improved because of improved wetting of the metal by the porcelain and the reduced porosity at the porcelain metal interface.

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2. The electro deposited layer acts as a barrier between the metal casting and porcelain to inhibit ion penetration by the metal (within normal limits of porcelain maturation).

3. The gold colour of the oxide film improves the vitality and esthetics of the porcelain when compared to the normal dark oxides which requires thick opaque of porcelain to mask it.

4. The colour of the activated surface can be controlled from golden, reddish brown to gray.

5. The deposited layer acts as a buffer zone to absorb stresses caused by difference in the co-eff of thermal exp between porcelain and metal during cooling.

6. The maturation time and temperature of the porcelain is reduced because of the highly reflective surface of the gold layer.

Metal-ceramic crowns based on swaged foil copings – Schossow 1984.

Renaissance (Unikorn Ltd.) and captex are products designed to fabricate the metal coping of a metal-ceramic crown without the rise of a melting and casting procedure.

It is a laminated gold alloy foil that is delivered to the user in a fluted shape reminiscent of a miniature coffee filter.

This is swaged with a swaging instrument burnished and then flame sintered to form a coping with moderate strength.(Ceraplatin or Ceplatec crown).Compressive strength is inferior: Fit was accurateBonded platinum foil coping:

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The 1st commercially viable foil reinforced crown system was developed by Mclean and Seed in 1976.

The surface to platinum foil was coated with upto 2 mm of tin. Oxidation of the tin coating provided the mechanisms for bonding of the porcelain. (This system was marketed under the trade name Vita-Pt).

A foil thickness of not less than 0.1 mm was sufficient to prevent fracture through metals according to studies.

Thus allows more space for porcelain.An alternative system for reinforcing porcelain crowns was

produced by Captek. The material is supplied in the form of impregnated wax like elastic strips that can be burnished to a refractory die before sintering each layer. The gold alloy powder in the centre of the laminate provides the reinforcing phase.Bond failures in metal ceramics:

Classification (Given by O’brien in 1977).Metal porcelain:

Fracture leaves a clean surface of metal seen when metal surface is devoid of oxides.

May also be due to contaminated or porous metal surface.Metal Oxide – Porcelain:

Porcelain at metal oxide surface leaving oxide firmly attached to metal. Seen more often in base metal alloys.Metal-Metal Oxide:

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Metal oxide breaks away from the metal and is left attached to the porcelain. Seen in base metal alloys due to over production of chromium and nickel oxide.

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Metal Oxide – Metal Oxide:

Occurs through the metal oxide. Results from over production of oxide causing sandwich effect between metal and porcelain.Cohesive within metal:

More common in bridges where the joint area breaks. Rarely seen in single crowns.Cohesive within porcelain:

Tensile failure within porcelain bond strength exceeds strength of porcelain. Seen in high gold containing alloys.Technical considerations for metal ceramic restorations:

1. A clean metal surface is essential for good bonding.2. Final texturing is done with a alumina air abrasive by the

process of sand blasting (Mechanical bonding).3. Casting in some cases is heated at 980o C to burn off the

impurities and degas it.4. Applications of opaquer (0.2 mm) to mask the colour of the

underlying metal.5. The porcelain is then built up and fired.All Ceramic Restorations:

The all ceramic crown in one form or the other has been employed in dentistry for more than 80 years. The use of PJC has been limited owing to its lack of precision and the inherent weakness of dental porcelain. However, it is commonly selected as rest of choice over the metal-ceramic crown when esthetics is of prime importance.

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The ceramics employed in the conventional PJC were high fusing feldspathic porcelains. The relatively low strength of this type of porcelain prompted Mclean and Hughes (1965) to develop an alumina reinforced porcelain core material for the fabrication of PJC. (50% weight fused alumina) Strength of upto 180 Mpa was achieved.Dis-advantages:

Addition of alumina reduces the trasparency of the porcelain. Therefore for full coverage crowns alumina reinforced porcelain is often used as a core which is covered by conventional feldspathic porcelain with a smaller content of alumina.Eg: Hi Ceram.

They have lower incidence of clinical fracture for three important reasons.1. Made up of stronger materials and better fabrication

technology.2. All ceramic restoration can be etched and bonded to the

underlying tooth structure with newer dentinal adhesives.3. Greater tooth reduction than was used previously for PJCLeucite-reinforced porcelain (Optec HSP) (Jeneric pentron):

It is a leucite-reinforced feldspathic porcelain that is condensed and sintered like aluminous porcelains and traditional feldspathic porcelain (1020o C).

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The strength is higher than feldspathic porcelain (flexural st. – 146 MPA).

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Dis-advantages:

Marginal fit inaccuracy; inc wear of opposing tooth requires a special die material.Low fusing porcelains:

Recently a number of low fusing porcelains have been introduced including Finesse Duceram LFC. These porcelains sinter at a lower temperature than normally used for all-ceramic porcelains (660o C).

This effect is obtained mainly by a decreased or even non-existing content of leucite. It has a less potential for abrading opposing teeth.

They are used for veneers, inlays/onlays or together with another material for crowns.High Alumina reinforced crowns:

A technique for manufacturing individual all ceramic crowns composed of a coping of densly sintered high purity alumina was described by Anderson and Oden in 1993. This system was marketed as the Procera-All-Ceramic system.

Procera-All-Ceramic copings are manufactured by compacting high-purity alumina powder Al2O3 99.9% with a dry pressing technique against enlarged models of tooth preparation. The milling machine produces a refactory die that is 20% larger than the original die in order to compensate for the shrinkage of dense sintered power.

It has a flexural strength of 601 Mpa. It is one of the strongest current dental ceramic.

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Conventional etching with HF acid has no effect on alumina, other technique must be used.Alumina Veneer Porcelain:

Aluminous veneer porcelains were designed to have a slightly higher thermal expansion than aluminous core porcelains. During the development of veneer porcelains, the radioactive fluirescue sodium diuranuim septoxide was used. This salt produced a strong greenish-yellow colour, when small amounts of cerium oxide were added a bluish-white flourescence very similar to human teeth was achieved.

Vitadur the 1st commercial porcelain was marketed in 1966.(Radioactive flourescence have now been banned).Glass Ceramics (Castable ceramics):

“A glass ceramic is a material that is formed into the desired shape as a glass then subjected to a heat treatment to induce partial devitrification” (i.e. loss of glassy structure by crystallization of the glass). The crystalline particles, needles or plates formed during this ceramming process serve to interrupt the propagation of cracks in the material thereby increasing strength and toughness.

The use of glass ceramics in dentistry was 1st proposed by Mac Culloch in1968. His pioneering effort in co-operation with the Pilkington Glass Company in St.Helen’s, England received little recognition.

Recently a glass-ceramic material based on the work of Grossman and Adair has been marketed under the trade name Dicor (corning glass works and marketed by Dentsply international) mid 1980.

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The original glass ceramic material contained tetrasilicic fluomica crystals (K2Mg5S1O2OF4) (55%) which because of their plate like morphology (mica) added strength and resistance to fracture propagation. Dicor is a castable glass that is formed into an inlay, facial veneer or full-crown restoration by lost wax casting process. After the casting is over the glass is covered by a protective ‘embedment’ material and subjected to a heat treatment that causes microscopic plate like crystals of crystalline material [mica) to grow within the glass matrix. This crystal nucleation and growth process is called ceramming.

(Mac Culloh reported that shade modification could be achieved only with surface colorants which tend to erode over time). Dicor because of its high transluency has a chemeleon like effect and merges with the surrounding teeth (to overcome the problems associated with surface colorants, Dicor was used as a cast coping that could be veneered with a specially prepared aluminous porcelain. However, thin copings (less than 1 mm thick) tended to crack probably because of mismatches in thermal diffusivity or poor resistance to pyrophastic flow during the firing of the veneer porcelain). Although highly esthetic dicor lacks fracture toughness and is more suitable for the fabrication of glass ceramic inlays. (occ wear of opposing natural tooth is less in case of DICOR).

(Another glass ceramic material was developed by Coors Porcelain Company known as Cerestore – Al2O3 is the principle component 70% and is partially crystallised as alpha – Al2O3. The alpha – Al2O3 and Mg. Aluminate spinal are the strengthening pahse).

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In another type of castable glass ceramic material developed in Japan the ceramming process produces hydroxyapatite crystals in the glass matrix rather than mica crystals in Dicor. It consists of a calcium phosphate based glass. It is known as CERAPEARL.Machinable Ceramics:

These products are supplied as ceramic ingots in various shades and are used either for computer aided designing, computer aided manufacturing procedure on in copy milling technology.(One of the 1st CAD-CAM systems introduced was DURET system by Hennson – Not popular).

In the CAD-CAM technique (Cerec) Seimens developed by Mormann and Brandistini and made commercially available in 1988 the prepared cavity is mapped by a mini camers and fed to a computer linked to a milling machine. This CAD-CAM technique is intended for use in dental office and produces a ceramic inlay in a one visit appointment.

Cerec Vitablocs Mark I was the 1st to be used with the Cerec system. It is a feldspathic porcelain with a composition similar to that of porcelains used for PFM restoration (93 Mpa st). Cerec vitablocs Mark II is a feldspathic porcelain with a finer grain size and allegedly increased strength and decreased invitro abrasive wear of opposing tooth structure.

Dicor MCG is a glass ceramic with fluoromica crystals in a glass matrix.

CAD-CAM technology has mainly been used for inlays/onlays.

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It has greater flexural st (216 Mpa) than castable dicor.In the celay copy milling technology 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. The same ingots are used.Advantages:

Negligible porosity Freedom from making an impression Reduced assistant time Single sitting Good patient acceptance Produces less wear of natural tooth.

The dis-advantages of CAD-CAM restoration include the need for (1) Costly equipment(2) Lack of computer controlled processing(3) Support for occlusal adjustment and(4) The technique sensitive nature of surface imaging that is

required for the prepared teeth.A gap between the restoration and the tooth is evident that is

wider than that in other all ceramic system.To over come this dis-advantages – A newer CAD-CAM

system was introduced by Siemens called Cerec-E or the ceramic recontouring system with electric milling machine. This system produces smaller marginal gaps.Injection moulded glass ceramic:

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Pressable ceramics (IPS-empress).This material was developed by Wohlwend Vivadent at

Dental Institute, Zurich University.IPS empress is a precerramed glass-ceramic that is heated

in a cylinder form and injected under pressure and high temperature into a mold.

It has 30-40% volume of leucite crystals and thus increased. Flexural strength and tends resistance to crack propagation.Dis-advantages:

Abrasion of opp tooth. Special Oven and die material.The core material is a glass ceramic containing lithium

disilicate and lithium orthophosphate crystals, while the veneering material contains fluorapatite crystals.

A higher volume of the crystalline phase results in increased strength of IPS empress compared to the original IPS empress.

Optec OPC is also a leucite containing glass ceramic, which is processed by molding under pressure and heat.

A full crown is waxed, invested and placed in a specialized mold that has a alumina plunger. The ingot is placed under the plunger. The entire assembly is heated to 1150o C and the plunger is released. The plunger presses the molten ceramic into the mould. The pressed ceramic is then baked.Glass infiltrated high alumina core ceramic (In-Ceram):

In-Ceram is a so-called infilterated ceramic and is used as a core material which is later veneered with feldspathic porcelain. The slightly sintered aluminous porcelain core is infilterated with

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glass at 1100o C for 4 hours to eliminate porosity and to strengthen the slip-cast core.

Slip casting is the science of preparing stable suspensions and fabricating structures by building a solid layer on the surface of a porous mould that absorbs the liquid phase by means of capillary forces. The most common mould material used in slip casting is plaster of paris. In 1910 Count Von Schwerin showed that alumina could be plasticized by grinding in acid.

Slip casting is generally carried out with AlS2O3 most of the particles in 1 to 5 mm range few particles exceed 20 mm. A commonly used vehicle is 1% solution of polyvinyl alcohol.

Sadoun refined the slip casting technology to produce a high strength. coping which is marketed under the trade name In-Ceram. He showed that a lightly sintered alumina powder could be infused with a (1100o C – 4 hours) low-firing sodium lanthamim glass to produce a dense composite ceramic of very high strength. 75% of alumina is used. In-Ceram has a high strength of 630 Mpa (450 Mpa).

A more translucent ceramic called In-Ceram spinell has been introduced as an alternative to In-Ceram. The core in In-Ceram spinell is MgAl2O4 infilterated with glass. It has decreased flexural strength.

The powder containing fine grained particles of Al2O3 and Mg2O3 is mixed with H2O to form a suspension which is known as slip. This slip is placed on a gypsum die and baked at 1120o C for 10 hours to produce an opaque porous core. At this stage the material is very fragile and must be handled very careful.

Next an appropriate shade of glass powder is applied to the core and it is baked again at 1100o C for 4 hours. During this

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process the molten glass infilterates the porous alumina by capillary action and increases strength by 20 times.Shrink free alumina (Sozio and Riley 1983):

The alumina ceramic used in this technique is a shrink free composition. Aluminium oxide is the primary component Al2O3

(corundum) dominant phase.It enhances strength.Magnesium Aluminate spinel (One of the mechanically

strongest oxide ceramic material).The ceramic formulation is such that on firing chemical and

crystalline transformations occurs to compensate for the decreased shrinkage volume.Direct Moulding:

Because the alumina ceramic has the unique property of not changing dimensionally from its unfired state to fired state a direct formation approach can be followed rather than the conventional indirect casting procedure.

The moulding procedure is done on the master die. The die material must withstand the temperature of the moulding process without breaking or distorting. A special epoxy resin (cerestore epoxy) was developed for this purpose. Unlike the most conventional dental epoxies this product is heat stable and undergoes permanent controlled expansion during curing.

The ceramic substrate is supplied as a dense pellet of the compacted shrink free formulation. The pellet is heated until it is flowable (160o C) and then transferred by pressure into a suitable mold directly on the master die. In addition to being thermoplastic the ceramic is thermo setting therefore after the flowable ceramic has been injected into the mould and around

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the die it automatically sets. The green substrate is then removed from the die and sintered. The core is called cerestore core.

The opalascent veneer porcelain was invented by The Dentist’s Supply Company of New York, which in 1962 filed the 1st patents on the incorporation of the fine particles (grain size less than 5 mm) of alumina, aluminium or zirconium silicate, zirconium or tin oxide into commercial tooth porcelains. They have good esthetics.Hybrid ceramics (Estenia):

It is a combination of ceramic and composite material.It has 92% wt. filler loading. The particle sizes 0.02 to 2 . It is indicated for jacket crown, inlay, onlay, crown and bridge.Magnesia Ceramic Jacket Crown: Obrien 1985

Magnesia was used as a core material on the basis of high expansion core material because of coeff. of thermal exp. being 13.5x10-6 for magnesia. This diff is explained on the basis that magnesia has FCC structure where as alumina has a hexagonal close packed structure. Modulus of elasticity is same as Alumina reinforced porcelain – 131 MPA.The main Advantages is:

Stronger jacket crown.Exceptional esthetic.No need for special equipment.No long process.Improved flexival strength.Ormocers:

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Ormocers an acronym for organically modified ceramics are a new type of material, which chemically are methacrylate substituted alkosilanes, that is they are organic-inorganic copolymers. The alkylsilyl groups of the silane allow the formation of a inorganic S1-O-S1 network by hydrolysis and condensation. Polymerisation reaction to give cross linked structures and their properties may be modified by filler particle substitution. A material based on this concept has recently become available.Definite: Degussa dental Hanan, Germany.The manufacturers claim:

Low shrinkage, high abrasion resistance condensibility, timeless aesthetics, bio-compatibility and protection against caries. No longer taken clinical trial results are yet available.Bonded porcelain restorations:

The concept of bonding composite resin to acid etched porcelain was 1st reported by Simonsen and Calamia in 1983 and the fabrication of porcelain veneers with this system was reported by Horn in 1983. Ceramic materials that have Al2O3 as the main component are not etched appreciably by HF acid.Hybrid ceramis (Estenia)It is a combination of ceramic and composite material.It has 92% wt filler loading.The particle size is 0.02 mm – 2.0 mm.It is indicated for jacket crowns, inlay, onlay, crown and bridge.Porcelain laminate veneers:

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A laminate veneer is a conservative alternative to full coverage for improving the appearance of an anterior tooth. A porcelain laminate is an extremely thin shell of porcelain applied directly to tooth structure. This restoration may be used to improve the colour of stained teeth, alters contours of misshapen teeth and close interproximal spaces.

The idea of porcelain veneers was given in 1930’s by Dr.Charles Pinceus.

Glazed porcelain which is non-porous, resists abrasion possesses esthetic stability and in well tolerated by gingiva is used. In early 1980’s a method of bonding porcelain to acid etched enamel was developed.

CONCLUSION:

Despite all possible differences in the materials and methods when a choice has to be made, apart from longivity focus should be directed to several other aspects which are important to the pt. for exmaples price, esthetics and number and duration of dental.

REFERENCES:

1. Phillips Science of Dental Material 10th Edition.

2. Recent Advances in Dental Materials – R.Nageswar Rao.

3. The all ceramic restoration – JADA, 97, Vol.128.

4. Ceramics – DCNA.

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