INTRODUCTIONA ceramic so white that it was comparable only to snow, so strong that vessels

needed walls only 2–3 mm thick and consequently light could shine through it. So continuous was the internal structure that a dish, if lightly struck would ring like a bell.

This is porcelain!It could be said that the ceramic material known as porcelain holds a special

place in dentistry because, not withstanding the many advances made in composites and glass–ionomers, it is still considered to produce aesthetically the most pleasing result. Its colour, translucency and vitality cannot as yet be matched by any material except other ceramics. Most ceramics for metal-ceramic restorations contain from 15 to 25 vol% leucite as their major crystalline phase, but changes in the leucite volume fraction can occur during thermal treatment of dental porcelains (Mackert and Evans, 1991a,b). Leucite is a potassium alumino-silicate with a high thermal expansion coefficient (Mackert et al, 1986a). Materials for all-ceramic restorations use a wider variety of crystalline phases as reinforcing agents and contain up to 90% by volume of crystalline phase. The nature, amount, and particle size distribution of the crystalline phase directly influence the mechanical and optical properties of the material (Morena et al, 1986b; Kon et al, 1994). The match between the refractive indices of the crystalline phase and glassy matrix is a key factor for controlling the translucency of the porcelain. Similarly, the match between the thermal expansion coefficients of the crystalline phase and glassy matrix is critical in controlling residual thermal stresses within the porcelain (Mackert, 1988). The first glass-ceramics were developed in the late 1950s (Stookey, 1959). Glass-ceramics are polycrystalline solids prepared by the controlled crystallization of glasses" (McMillan, 1979). The crystallization is achieved when the glass is submitted to a heat treatment during which crystal nucleation and growth are thermodynamically possible. Proper control of the crystallization heat treatment is necessary to ensure the nucleation of a sufficient number of crystals and their growth to an effective size. The dual nature of glass-ceramic materials confers upon them the esthetic, mechanical, and chemical qualities of ceramics as well as the ability to be cast and processed as glasses. These characteristics are of great interest for dental applications. Machinability is another property desirable for the maximum utility of glassceramics as dental materials. The ability of glass-ceramics to be machined is closely related to the nature and particle size of the crystalline phase that develops during the crystallization heat treatment (Utsumi and Sakka, 1970). Machinable glass-ceramics for industrial as well as dental applications often contain mica as a major crystalline phase.

Hot-pressed ceramics constitute another application of high technology to dentistry. This process relies on the application of external pressure at elevated temperatures to obtain sintering of the ceramic body. Hotpressed ceramics are also called "heat-pressed" ceramics. Hot-pressing classically helps avoid large pores caused by non-uniform mixing. It also prevents extensive grain growth or secondary crystallization, considering the temperature at which sintering is obtained. The mechanical properties of many ceramic systems are maximized with high density and

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small grain size. Therefore, optimum properties can be obtained by hot-pressing techniques (Kingery et al, 1976).

In spite of their excellent esthetic qualities and their good biological compatibility, dental ceramics, like all ceramic materials, are brittle. They are susceptible to fracture at the time of placement or during function.

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HISTORICAL PERSPECTIVECeramics are the earliest group of inorganic materials to be structurally

modified by man and his early history is principally traced through these materials. The origin of glazing techniques is probably the most interesting advancement.

Ceramic objects have been constructed for thousands of years. The earlier techniques usually consisted of shaping the item in clay or soil and then backing it to fuse the particles together. The initial attempts resulted primarily in coarse and some what porous products, such as goblets and other forms of pottery. Later developments led to unite detailed stone ware items.1774: Nicholas Dubais de Chemant, a surgeon dentist of Paris is credited with

making porcelain dentures.1791: Dechemont – obtains both French and English patent for dental porcelain.1792: John Woodforde – manufactured porcelain pastes1808: Giuseppangelo Fonzi – An Italian dentist produced porcelain metal backed

artificial teeth.1850: First commercial production of porcelain denture teeth by white.1860: Introduction of tube tooth and pivot crown in England.1889: Porcelain inlays and jacket crowns introduced platinum matrix for fusing

porcelain inlays and crowns developed by Land in USA.1903: Dr. Hugh Avery – Introduced new porcelain inlay technique 1905: Electric porcelain furnace1908: Dr. A. Eschneider – Baked porcelain jacket crown1923: Casting of dental porcelain for inlays and crown by lost technique.1925: Dr. Albert LE Gro’s used porcelain by high fusing method. Jan Adrianasen –

pioneered the technique of building up porcelain with a brush.1940: Vacuum firing of dental porcelain1942: Fluorescent dental porcelain introduced1962: Gold alloy for porcelain bonding were used1963: Development of dental aluminous porcelain by McLean and Hughes1968: Use of photosensitive glass ceramic in dentistry by Macclloch1974: Palladium silver alloy introduced for porcelain fused to metal1976: Platinum bonded alumina crown was used by McLean and Seed.1983: High expansion core material by O’Brien1984: First commercial castable dental glass ceramic

The earliest glazing technique was a Summerian invention made famous about 4000 BC as a Egyptian blue faience. More than 10,000 years ago stone age people also used ceramics.

As early as the second half of the eighteen century, Fauchard and others attempted to use porcelain for dental applications. Their efforts, working in the demanding and potentially destructive intraoral environment, were largely unsuccessful.

Porcelain was, however, successfully used for dental prosthesis by the end of the 1800s, when the technique to fire all porcelain jacket crowns on a platinum matrix was first developed but it was not until the mid 1950s that a dental porcelain was

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

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TERMINOLOGIES

Alumina core: A ceramic containing sufficient crystalline alumina (Al2O3) to achieve adequate strength and opacity when used for the production of a core for ceramic jacket crowns.

Aluminous porcelain: A ceramic composed of a glass matrix phase and 35 vol% of more of Al2O3.

CAD-CAM ceramic: A machinable ceramic material formulated for the production of inlays and crowns through the use of a computer aided design, computer aided machining process.

Castable dental ceramic: A dental ceramic specially formulated to be cast using a lost wax process.

Ceramic: A compound of metallic and nonmetallic elements.

Ceramic, dental: A compound of metals (such as aluminium, calcium, lithium, magnesium, potassium, sodium, tin, titanium, and zirconium) and non metals (such as silicon, boron, fluorine, and oxygen) that may be used as a single structural component, such as when used in a CAD-CAM inlay, or as one of several layers that are used in the fabrication of a ceramic based prosthesis. Dental ceramics are formulated to provide one or more of the following properties, castability, moldability, injectability, color, opacity, translucency, machinability, abrasion resistance, strength and toughness. Note: All porcelains and glass ceramics are ceramics, but not all ceramics are porcelains or glass ceramics.

Ceramic jacket crown (CJC): An all ceramic crown without a supporting metal substrate that is made from a ceramic with a substantial crystal content (> 50 vol%) from which its higher strength and/or toughness is derived. These crowns are distinguished from porcelain jacket crowns that are made with porcelain to produce an aesthetic porcelain margin as an alternative to a metal margin on a metal ceramic crown.

Sintering: The process of heating closely packed particles to achieve interparticle bonding and sufficient diffusion to decrease the surface area or increase the density of the structure. For products such as In-Ceram and In-Ceram Spinel, surface contact sintering and minimal density change are required.

Spinel or Spinelle: A hard crystalline mineral (MgAl2O4) consisting of magnesium and aluminium. Also, any of a group of mineral oxides of ferrous iron, magnesium, manganese or zinc.

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Stain: A mixture of one or more pigmented metal oxides and usually a low fusing glass that when dispersed in an aqueous slurry or monomer medium, applied to the surface of porcelain or other specialized ceramic dried or light cured and fired, will modify the shade of the ceramic based restoration. One product is supplied in a light curable binder. These stain product are also called surface colorants or characterization porcelains.

Thermal compatibility: The desirable condition of low transient and residual tensile stress in porcelain adjacent to a metal coping that is associated with a small difference in the thermal contraction coefficients between the metal and the veneering porcelains. The contraction coefficient of the metal should be slightly greater than that of the porcelains so that residual axial and knoop compressive stresses are produced. This condition will ensure the cooling of metal ceramic prostheses without immediate crack formation or delayed fracture caused by residual tensile stresses in porcelain.

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CLASSIFICATION OF CERAMICS: 1. By content :

- Regular feldspathic porcelain- Aluminous porcelain- Leucite reinforced porcelain- Glass infiltrated alumina- Glass infiltrated spinel

2. By use:- Artificial teeth - Core ceramic- Veneer ceramic

3. By processing method :- Sintering - Casting- Machining

4. By their firing temperature : - High fusing : 1300 c - Medium fusing : 1100-13000 c- Low fusing : 850 – 1100 c - Ultra low fusing : less than 850C

5. By method of firing ;- Air fired - Vaccum fired - Diffusable gas

6. By their area of application :- Core porcelain- Body dentin porcelain- Gingival dentin porcelain- Incisal enamel

COMPOSITION OF CERAMICS:1. Feldspar:

When mixed with metal oxides and fired, it forms a glass phase that is able to soften and flow slightly

This softening of glass allows porcelain particles to coalesce together. This is called sintering sintering

Seen in concentration of 75-85 %.2. Kaolin / clay:

It acts as the binder. When mixed with water , it forms a sticky mass which allows unfired

porcelain to be easily worked and molded. On heating it reacts with feldspar and gives rigidity. Its white in color and reduces translucency .so its added only in

concentration of 4-5 %.

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3. Quartz: It imparts more strength, firmness and translucency. It gives stability of mass during heating by providing a frame work.13-14%

• GLAZES: It decreases pores on the surface of fired porcelain. • This increases strength by decreasing the crack propagation. if glaze is removed by grinding, the transverse strength is half of glazed porcelain.1. Self glazing:

External glaze layer is not applied here. The completed restorations is subjected to glazing here.

2. Add on glazes: External glaze layer is applied here. They are uncolored glasses whose fusing temperature is lowered by the

addition of glass modifiers. Disadvantages: Low chemical durability, difficulty to apply evenly, difficult to get exact surface characteristics.

6. Colouring agents: These coloring pigments are produced by fusing metallic oxides together with

fine glass and feldspar -Ex : iron / nickel oxides- brown , copper oxides-green, titanium oxide –yellowish brown, cobalt oxide – blue.

7. Opacifying agents: a. Opacifying agents consists of a metal oxide ground to a very fine particle

size. ex :cerium oxide, titanium oxide, zirconium oxide –most popular.8. Stains:

These powder is mixed with water and the wet mix is applied with brush to the surface of porcelain before glazing.

Internal staining is preferred as it gives life like results and prevents direct damage to stains by surrounding environment.

9. Glass former:Glass formers are silica. 1. Crystalline quartz2. Crystalline cristobalite3. Crystalline tridymite4. Non crystalline fused silica

The vitreous matrix is made of silicate glass. Silica forms sio4 ions with oxygen and is thus highly charged and fills the space between 4 oxygen atoms.the tetrahydra must permit sharing of oxygen atoms to permit the formation of sio4 groups thus resulting in polymerization and a three dimensional network. 10. Glass modifiers :

Potassium oxide, Sodium oxide, Calcium oxide are used as glass modifiers They act as fluxes by lowering the softening temperature of a glass

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When sodium oxide is introduced, instead of bridging the atoms together, it contributes a oxygen atom which act as a non bridging oxygen and as a result a gap is produced in the sio4 network. So the silica tetrahydra thus obtained is able to move more easily at lower temperature than the earlier network.

11. Intermediate oxides : Glass modifiers reduces the viscousity of porcelain. It needs a high viscosity as well as low firing temperature. This is done by

the addition of Aluminium oxide and boric oxides.

The composition of the ceramic generally corresponds to that of the glasses in table, except for an increased alkali content. The addition of greater quantities of soda, potash, and/or leucite is necessary to increase the thermal expansion to a level compatible with the metal coping. The opaque porcelains also contain relatively large amounts of metallic oxide opacifiers to conceal the underlying metal and to minimize the thickness of the opaque layer.

The high contraction porcelains have a greater tendency to devitrify because of their alkali content. They should not be subjected to repeated firing, because this may increase the risk for cloudiness within the porcelain, as well as changes in the thermal contraction behavior. Thus it is obvious that a proper matching of the properties of the alloy and porcelain is imperative to success. Criteria and test methods for determining metal porcelain compatibility have been suggested. Testing methods are focused on the measurement of coefficients of thermal expansion and contraction, thermal shock resistance, and the strength of the bond, which are discussed later.

Conventional dental porcelain is a vitreous ceramic based on a silica (SiO2) network and potash feldspar (K2O.Al2O3.6SiO2) or soda feldspar (Na2O.Al2O3.6SiO2) or both. Pigments, opacifiers, and glasses are added to control the fusion temperature, sintering temperature, thermal contraction coefficient, and solubility. The feldspars used for dental porcelains are relatively pure and colorless. Thus pigments must be added to produce the hues of natural teeth or the color appearance of tooth-colored restorative materials that may exist in adjacent teeth.

Silica (SiO2) can exist in four different forms: crystalline quartz, crystalline cristobalite, crystalline tridymite, and noncrystalline fused silica. Fused silica is a material whose high-melting temperature is attributed to the three - dimensional network of covalent bonds between silica tetrahedral, which are the basic structural the temperature required to sinter the porcelain powder particles together at low enough temperatures so that the allow to which it is fired does not melt or sustain sag (flextural creep).

Glass Modifiers:The sintering temperature of crystalline silica is too high for use in veneering

aesthetic layers bonded to metal substrates. At such temperatures the alloys would melt. In addition, the thermal contraction coefficient of crystalline silica is too low for these alloys. Bonds between the silica terahedra can be broken by the addition of

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alkali metal ions such as sodium, potassium, and calcium. These ions are associated with the oxygen atoms at the corners of the tetrachedra and interrupt the oxygen silicon bonds. As a result, the three-dimensional silica network contains many linear chains of silica tetrahedral that are able to move more easily at lower temperatures than the atoms that are locked into the three-dimensional structures of silica tetrahedral. This ease of movement is responsible for the increased fluidity (decreased viscosity), lower softening temperature, and increased thermal expansion conferred by glass modifiers. Too high a modifier concentration, however, reduces the chemical durability (resistance to attack by water, acids, and alkalis) of the glass. In addition, if too many tetrahedral are disrupted, the glass may crystallizer (devitrify) during porcelain firing operations. Hence, a balance between a suitable melting range and good chemical durability must be maintained.

Manufactures employs glass modifiers to produce dental porcelains with different firing temperatures. Dental porcelains are classified according to their firing temperatures. A typical classification is as follows:

High fusing 13000 C (23720F)Medium fusing 11010 - 13000 C ( 20130 - 20720 F)Low fusing 8500 - 11000 C ( 15620 - 20120 F)Ultra – low fusing < 8500 C (15620F)

The medium – fusing and high – fusing types are used for the production of denture teeth. The low – fusing ultraslow – fusing porcelains are used for crown and bridge construction. Some of the ultraslow – fusing porcelains are used for titanium and titanium alloys because of their low contraction coefficients that closely match those of these metals and because the low firing temperatures reduce the risk for growth of the metal oxide. However, some of these ultraslow – fusing porcelains conation enough leucite to raise porcelains. The potential advantage of ultraslow- fusing veneering ceramics are the reduction in sintering times, decrease in sag deformation of FPD frameworks, less thermal degradation of ceramic firing ovens, and less wear of opposing enamel surfaces.

Because commercial dental laboratories do not fabricate denture teeth for complete denture or removable partial dentures, it has become more common to classify crown and bridge porcelains as high – fusing (850 - 11000C) and low – fusing (> 8500 C). However, this change in classification has not been universally adopted Thus, to avoid confusion, the sintering temperature range should be identified (at least initial) in discussions between dentists and dental technicians so that the less –abrasive benefit claimed for ultraslow – fusing porcelains that were used exclusively between the 1960s and 1990s.

Because it ensures adequate chemical durability, self – glazing of porcelain is preferred to an add – on glaze. A thin external layer of glassy material is formed during a self – glass phase and settling of crystalline particles within the surface for an applied glaze procedure contains more glass modifiers and thus has a lower firing

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temperature. However, a higher proportion of glass modifiers tends to reduce the resistance of the applied glazes to leaching by oral fluids.

Another important glass modifier is water, although it is not an intentional addition to dental porcelain. The hydronium ion, H3O+, can replace sodium or other metal i8o0ns in a ceramic that contains glass modifiers. This fact accounts for the phenomenon of “slow crack growth” of ceramics that are exposed to tensile stresses and moist environments. It also may account for the occasional long-term failure of porcelain restorations after several years of service.

Feldspathic Porcelains Potassium and sodium feldspar are naturally occurring minerals composed

primarily of potash (K2O) and soda (Na2O), respectively. The also contain alumina (Al2O3) , and silica (SiO2) components. Feldspars are used in the preparation of man dental porcelains designed for metal-ceramic crowns and many other dental glasses and ceramics. When potassium feldspar is mixed with various metal oxides and fired to high temperatures, lit can form leucite and a glass phase that will soften and flow slightly. The softening of this glass phase during porcelain firing allows the porcelain powder particles coalesce is called liquid-phase sintering, a process controlled by diffusion between particles at a temperature sufficiently high to form a dense solid. The driving force for sintering is the decrease in energy caused by a reduction in surface area. As explained in the key terms section, section, three dental products (In-ceram Alumina, spinell, and Zirconia) are slightly sintered to produce interconnected pore channels that are necessary for subsequent glass infiltration.

Another important property of feldspar is its tendency to form the crystalline mineral leucite when melted. Leucite is a potassium-aluminum-silicate mineral with a large coefficient of thermal expansion (20to25ppm/oC) compared with feldspar glasses (which have coefficients of thermal expansion less than 10ppm/oC). When feldspar is heated at temperatures between 1150 o C and 1530 it undergoes incongruent melting to form crystals of leucite in a liquid glass. Incongruent melting is the process by which one material melts to form a liquid plus a different crystalline material. This tendency of feldspar to form leucite during incongruent melting is used to advantage in the manufacture of porcelains for metal bonding. Further information is provided in the sintering of porcelain section.

Man dental glasses do not contain leucite as a raw material. Since feldspar is not essential as a precursor to the formation of leucite, as described earlier, these glasses are modified with additions of leucite to control their thermal contraction coefficients.

Feldspathic porcelains contain a variety of oxide components, including SiO2

(52-62 wt% ) , AlOO (11-16 wt%),k2O(9-11 wt%), Na2 O (5-7 wt%), and certain additives, including Li2 O and B2O3. These ceramics are called porcelains because they contain a glass matrix and one more crystal phases. They cannot be classified as glass – ceramics because crystal formation does not occur through controlled nucleation and crystal formation and growth. there are four types of veneering ceramics. These include (1) low-fusing ceramics (feldspar – based porcelain and nepheline senate- based porcelain); (2) ultra low-fusing ceramics (porcelains and

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glasses); (3) stains; and (4) glazes ( self – glaze and add – on glaze). The particle type and size of crystal particles, if present, will greatly influence the potential abrasives of the ceramic prosthesis.

The thermal expansion coefficients of some ultraslow – fusing ceramics (sintering temperatures be below 8500 C) and low-fusing ceramics are listed. These ultra low-fusing ceramics represent an exciting new fail of ceramic core and veneering materials because of their microstructural features. The ontain either a well-distributed dispersion of small crystal particles or few or no crystals, depending on the whether the ceramic is to be used as a veneer or glaze. Initial results of wear studies are promising in several cases relative to reduced enamel wear caused these ceramics. These results are summarized in a later section of this chapter (see wear of Enamel by ceramic Products and Other Restorative Materials).

Other Additives:Other metallic oxides can be introduced, as indicated in Table 21-1. Boric oxide (B2O3) behaves as a glass modifier, that is, it decreases viscosity, lowers the softening temperature and forms its own glass network. Because boric oxide forms a separate lattice interspersed with the silica lattice, it still interrupts the more rigid silica network and lowers the softening point of the glass. Alumina is not considered a true glass former by itself because of the dimensions of the ion and the oxygen/aluminum ratio. Nevertheless, it can take part in the glass network to alter the softening point and viscosity.1. Pigmenting oxides are added to obtain the various shades needed to

simulate natural teeth. These coloring pigments are produced by fusing metallic oxides together with fine glass and feldspar and then regrinding to a powder. These powders are blended with the unpigmented powdered frit to provide the proper hue and chrome. Examples of metallic oxides and their respective color contributions to oxide (yellowish brown) manganese oxide (lavender), and oxide (green) titanium oxide (yellowish brown), manganese oxide (lavender), and cobalt oxide (blue). Opacity may be achieved by the addition of cerium oxide, zirconium oxide, titanium oxide, or tin oxide.

2. Composition:Dental porcelains are essentially mixture of fine particles of Feldspar and

quartz. However the general trend towards the use of less kaolin (clay) with an increase in the feldspar content in order to improve translucency suggests that dental porcelain should be more correctly described as glasses. The feldspar melts first to provide a glossy matrix for the quartz. The quartz thus act as a filler to provide strength. The quartz may be replaced by alumina (Al2O3) such a material is referred to as alumonous porcelain

Low fusing dental porcelain:Oxide Weight %SiO2 69.36B2O3 7.53

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CaO 1.85K2O 8.33Na2O 4.81Al2O3 8.11

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

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Composition of dental ceramics for fusing to high temperature alloys:

CompoundBiodent opaque

BG 2 (%)

Ceramco opaque 60

(%)

VMK opaque 131 (%)

Biodent dentin BD

27 (%)

Ceramco dentin T 69

(%)SiO2 52.0 55.0 52.4 56.9 62.2Al2O3 13.55 11.65 15.15 11.80 13.40CaO - - - 0.61 0.98K2O 11.05 9.6 9.9 10.0 11.3Na2O 5.28 4.75 6.58 5.42 5.37TiO2 3.01 - 2.59 0.61 -ZrO2 3.22 0.16 5.16 1.46 0.34SnO2 6.4 15.0 4.9 - 0.50Rb2O 0.09 0.04 0.08 0.10 0.06BaO 1.09 - - 3.52 -ZnO - 0.26 - - -UO3 - - - - -B2O3, CO2

and H2O4.31 3.54 3.24 9.58 5.85

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PROPERTIESGENERAL PROPERTIES OF CERAMICS 1. PHYSICAL PROPERTIES OF PORCELAINStrength: Porcelain is a material having good strength. However it is brittle and tends to fracture. The strength of dental porcelain is usually measured by terms of this flexure strength or modulus of rupture.

Flexure strength: It is a combination of compressive, tensile, as well as shear strength.Ground – 11,000 PSI (75.8MPa)Glazed – 20,465 PSI (141.1 MPa)

1) Compressive strength of porcelain is 48000 psi (321 MPa) tensile strength [5000 psi (35 MPa)]. Tensile strength is low because of the unavoidable surface defects like porosities and microscopic cracks.Shear strength: It is low and is due to the lack of ductility caused by the complex structure of dental porcelain [6000 PSI (110 MPa)]

Inadequate firing weakens porcelain, the firing also decrease strength as more of the core gets dissolved in the flexure.

2) Surface Hardness: Porcelain is much harder than natural teeth. KHN – 460 (enamel 343).

3) Wear resistance: Porcelain is more resistant to wear than natural teeth.4) Thermal properties: Porcelain has low thermal conductivity, co-efficient of

thermal expansion is close to that of natural teeth 6.4 to 7.8 x 10-6/OC5) Specific gravity: The specific gravity of fired porcelain is usually less, because

of the presence of air voids. It varies from 2.2 to 2.36) Dimensional stability: Porcelain is dimensionally stable after firing.7) Chemical stability: It is insoluble and impermeable to oral fluids. Also it is

resistant to most solvents. However contact with hydrofluoric acid causes etching of the porcelain surface. A source of this is APF (acidulated phosphate fluoride) and stannous fluoride; which are used as topical fluorides.

8) Esthetic properties: The esthetic qualities of porcelain are excellent. It is to match adjacent tooth structure in translucency, color and intensity. In addition, attempts have also been made to match the fluorescent property of natural teeth when placed under ultraviolet light.

9) Biocompatibility: It is compatible with the oral tissues. The margins of finishing line can be even extended to the gingival sulcus

10) Modulous of elasticity: Porcelain has a high modulous of elasticity [10 x 106

PSI (69 GPa)]11) Optical properties: The colors of commercial premixed dental porcelains are in

the yellow to yellow red range. Usually supplied in blue, yellow, pink, orange, brown and grey.

The modifiers are added to the opaque and body porcelain during building of the crown.

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Surface staining: Disadvantages of surface staining are a lowered durability as a result of high solubility and reduction of translucency. Opaque porcelains have very low translucency values to mask metal substructure surfaces. Body porcelain translucency values range between 20% and 35%.

Incisal porcelains have the highest values of translucency and range between 45% and 50%. Since dental enamel is fluorescent under ultraviolet light, uranium oxide have been added to produce fluorescence with porcelain. However because of the low but detectable radioactivity of uranium, newer formulations contain rare earth oxides such as cerium oxide which produce fluorescence.

2. BIOLOGICAL PROPERTIES : They have excellent biocompatibility.

3. CHEMICAL PROPERTIES : It resist attack by chemicals. They have to be roughened by etching with hydrofluoric acid or sand blasting

to improve the retention of a cement to the internal surface of the restoration4. MECHANICAL PROPERTIES:

Low tensile strength Exhibits little plastic deformation Have good compressive strengtha) Compressive strength : 50000psib) tensile strength : 5000psic) Shear strength:16000psid) Elastic modulus : 10 × 106 psi e) Knoop hardness : 460f) C T E :12 × 10 -6psig) R.I : 1.52 – 1.54

5. THERMAL PROPERTIES: They have insulating capacity.

6. OPTICAL PROPERTIES: They have good optical properties They are translucent because of absence of free electrons.

Strength of porcelains: Strength of porcelain is decreased by, 1. By the presence of stress concentration areas2. Porosity, roughness , machine damage3. Sharp line and point angles

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4. Interface between bonded structures where elastic modulus of 2 components are different. More brittle material should have less elastic modulus. So it can transfer stress to one with high modulus of elasticity.

5. Interface between bonded structure where large difference in thermal coefficient. Material should have lower coefficient of thermal expansion, so the other has protective compressive stress.

6. Areas of sharp point contacts on brittle material. Rounding of opposing cusp is done, so that occlusal contacts are large areas.

Methods of strengthening porcelain: 1. Method of strengthening brittle materials2. Method of designing components to minimize the stress concentration and tensile

stress. 1. Method of strengthening brittle materials:

Done in 2 ways1. Development of residual compressive stresses within the surface of the

material 2. Ion exchange mechanism :

This techniques is called chemical tempering. in this procedure, a sodium containing glass is placed in a bath of molten potassium nitrate , potassium ions in the bath exchanges place with some of the sodium ions in the surface of the glass article. The potassium ions being around 35% larger than the sodium ions, squeezes in to the place formerly occupied by the sodium ions this creates large compressive stresses in the surface of the glass these residual stresses produce a strengthening effect. 3. Thermal tempering:

This is the most common form of strengthening. This creates residual surface compressive stresses by rapidly cooling the surface of the object while it is hot and in the softened state. This rapid cooling produces a skin of rigid glass surrounding a soft molten core.as the molten core solidifies, it tends to shrink, but the outer skin remains rigid. The pull of the molten solidifying core as it shrinks, creates residual tensile stresses in the core and residual compressive stresses within the outer surface.

4. Disruption of crack propagation: by 3 ways 1. Crack tip interactions2. Crack tip shielding3. Crack bridging

1. Crack tip interactions: these occur when obstacles in the microstructure act to impede the crack motion. These obstacles are second phase particles and act to deflect the crack out of the crack plane. The re orientation of the crack plane leads to a reduction of the force being exerted on the crack in the area of deflection. When the crack is deflected out of plane, the crack is no longer subjected to pure tensile stresses and will involve some shear displacement thus increasing the difficulty of crack propogation.

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2. Crack tip shielding : By 2 waysa) Transformation tougheningb) Microcrack toughening

a) Transformation toughening: This is most commonly associated with the presence of zirconia. Under

unrestrained conditions, zirconia undergoes a high to low temperature phase transformation which involves 3-5% volume increase. In toughened ceramic, the high temperature phase of zirconia is constrained at room temperature. Applied tensile stresses were to advance the crack plane. In the area directly behind the crack tip, the matrix constrains on zirconia are released, allowing low temperature transformation to take place. The transformed phase occupies a greater volume in the bulk material resulting in compressive forces that tend to counteract any advancing crack tip stresses. b) Micro crack toughening :

The high coefficient of thermal contraction and volume reduction associated with the high to low temperature phase transformation of leucite crystals create a condition which causes the leucite crystals to contract significantly more than the glass matrix. Compressive forces are created in the glass matrix surrounding the particles leading to micro cracking in the leucite phase. The residual compressive stresses in the glass phase around the particles can counteract the tensile stresses which drive the crack forward. 3. Crack tip bridging:

It occurs when a second phase act as a ligament to make it more difficult for the crack faces to open. This is better understood by bonded fiber composites. The fibers act as ligaments which makes it more difficult to open the crack at an applied stress.

Methods of designing components to minimize stress concentrations and tensile stress: 1. Minimizing tensile stresses:

In a full coverage metal restoration with porcelain, the metal being of higher thermal expansion will contract faster than the porcelain as a result the metal is placed in tension and the porcelain in compression. For partial metal coverage restorations, the junction between the metal and the porcelain is a potential site for high stress as, the area with only metal will have no balancing compressive forces.so ideally full coverage restorations are preferred. Porcelain unsupported by metal is more subjective to fracture. Reducing stress raisers:

Stress raisers are discontinuities in ceramic structures and in brittle materials that cause stress concentration. The design of ceramic restorations should avoid stress raisers. Abrupt changes in shape/ thickness in the ceramic contour can act as stress raisers and make the restoration more prone to failure. Notches caused the porcelain

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due to the folds of the underlying platinum foil substrate also is a stress raiser. Sharp line angles, large changes in the thickness of porcelain are factors leading to stress concentration. Usually contact points should be avoided and contact areas should be preferred to avoid localized stress areas. I. Reinforcement of inner surface by a higher strength ceramic: Ex. Aluminous core porcelain

CerestoreReinforced high alumina crown

II. Reinforcement of inner surface by metal bonding:Ex. Platinum foilGold foil swaged gold coping of 0.90-0.14mm thickness (renaissance system)Titanium (procera) e.g. Titanium coping

III. Porcelain fusid to metal restorationsEx. Noble metal alloys – Gold containing alloys

Gold free alloysBase metal alloys Nickel – chromium alloys

Cobalt – chromium alloys(rarely used in ceramic bonding)

IV. Designing of restorations:The design should be such that it should not be subjected to tensile stress. To

avoid stress concentration in porcelain, sharp angles should be avoided and the porcelain should be of uniform thickness.

Tensile stresses can be avoided by having a favorable occlusion in porcelain jacket crown. In porcelain fused to metal restoration, the metal should be strong and ductile not allowing flexing. Contact of opposing tooth or teeth should be either on porcelain or on metal, but not at the junction.Fluorescence and Opalescence:

For clinicians who practice esthetic restorative dentistry, particularly in the field of ceramics, fluorescence is an important physical property. Natural teeth are fluorescent. In other words, they emit visible light when exposed to ultraviolet light. Fluorescence adds to the vitality of a restoration and minimizes the metameric effect between teeth and restorative materials. The components of porcelain consist of agents that cause them to fluoresce; thus, they also will emit visible light when exposed to ultraviolet light. It is important that all the basic components of the porcelain, including the dentins, enamels, stains and even the glazing agents, are fluorescent. Opalescence is the ability of a translucent material to appear blue in reflected light and orange-yellow in transmitted light. Opalescence also contributes to the vitality of a restoration.MODE OF SUPPLY

Dental porcelains are available as fine powders to be used with liquid I (or distilled water). The powders and liquid are mixed to form a plastic mass which is shaped or moulded into a desired shape, it is then fired (or sintered) at a high temperature in order to fuse the particles together to form a ceramic body which is esthetically like a natural crown.

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Porcelain is supplied as a kit containing:1. Fine ceramic powders in different shades:

Enamel Dentin Core

2. Special liquid/distilled water3. Stains of colour pigments4. GlazeCERAMIC PROCESSING METHODS

The single unit crown may be a metal ceramic crown (also called a porcelain fused-to-metal crown), a traditional aluminous porcelain crown based on a core of aluminous porcelain, or the newer ceramic crowns based on a core of leucite reinforced porcelain, injection or pressure molded leucite based ceramic, glass ceramic, sintered aluminous porcelain, sintered aluminum oxide, or glass-ceramic processed from cast glass. The types of restoration, with their variations, are discussed in detail in succeeding sections.

The processing stages of the ceramic core for production of ceramic prostheses are summarized in Table. These seven different processes represent the main procedures that were available in 2003. the quality of the final ceramic prosthesis is dependent on each stage of the fabrication process. Machining or grinding of the core structure is of particular importance since flaws or minute cracks can be introduced that can possibly be propagated to the point of fracture during subsequent intraoral stressing cycles. The use of computer aided manufacture (CAM) bprocesses are most likely to induce such damage, although the ceramics with higher fracture toughness are less likely to exhibit such damage. It is possible that subsequent sintering or veneering procedures can reduce the potential for propagation of cracks in the prostheses while in service. However, insufficient data are available from clinical studies of ceramics.

The processing procedures for these ceramics are as follows. The feldspathic porcelain of traditional PFM restorations, some aluminous porcelains (Vitadur-N, Hi-Ceram), and pure alumina ceramic (Procera AllCeram) are condensed by vibration or dry-pressed (Procera) and sintered at high temperature. Pressable ceramics (e.g., IPS Empress, IPS Empress 2, Finesse All-Ceramic, OPC, and OPC-3G), when heated and subjected to hydrostatic pressure, flow into a mold and after removal and divesting are then veneered. Cast and cerammed crowns, such as the obsolete product Dicor, are made using the lost-wax technique. The molten glass is cast into a mold, heat-treated to form a glass-ceramic, and colored with shading porcelain and surface stains.

For slip cast ceramics (I-Ceram, In-Ceram Spinell, and In-Ceram Zirconia), a slurry of liquid and particles of alumina, magnesia-alumina silicate (spinel), or zirconia is placed on a dry refractory die that draws out the water from the slurry. The slip-cast deposit is sintered on this die, and then it is coated with a slurry of a glass phase layer. During firing, the glass melts and infiltrates the porous ceramic core. Translucent porcelain veneers are then fired onto the core to provide the final contour and color.

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For CAD-CAM processes, the ceramic block materials (Dicor MGC, Vita Cerec Mk I, and Vita Cerec Mk II) are shaped into inlays or crowns using a CAD-CAM system (Cerec). CAM refers to computer-aided milling or machining. This process is sometimes referred to as a CAD-CAM process, where CIM refers to computer-integrated machining or milling. These blocks can also be used in copy milling devices (Celay) that mill or machine blocks into core shapes in a manner similar to that for cutting a key from a key blank, that is, by tracing over a master die of the shape to be produced out of the ceramic.SINTERED PORCELAINSLeucite-reinforced feldspathic porcelain

Optec HSP material (leneric/Pentron, Inc.) is a feldspathic porcelain containing up to 45 vol% tetragonal leucite (Schmid et al, 1992; Piche et al, 1994; Denry and Rosenstiel, 1995). The greater leucite content of Optec HSP porcelain compared with conventional feldspathic porcelain for metal-ceramics leads to a higher modulus of rupture and compressive strength (Vaidyanathan et al, 1989). The large amount of leucite in the material contributes to a high thermal contraction coefficient (Katz, 1989). In addition, the large thermal contraction mismatch between leucite (22 to 25 x 10"6/°C) and the glassy matrix (8 x 10~6/°C) results in the development of tangential compressive stresses in the glass around the leucite crystals when cooled. These stresses can act as crack deflectors and contribute to increase the resistance of the weaker glassy phase to crack propagation. After heat treatment of Optec HSP for one hour at temperatures ranging from 705 to 980°C, a second metastable phase identified as sanidine (KAlSi3O8) forms at the expense of the glassy matrix (Vaidyanathan et al, 1989). The crystallization of sanidine is associated with a modification of the optical properties of the material from translucent to opaque. However, sanidine does not appear when the porcelain is heated to 980°C, since sanidine is metastable in the temperature range 500-925°C. The recipitation of sanidine has been reported as well upon isothermal heat treatment of conventional feldspathic porcelain for metal-ceramics (Mackert et al, 1986b; Mackert, 1988; Barreiro et al, 1989). An isothermal timetemperature- transformation diagram that makes it possible to predict the amount of leucite and sanidine in samples subjected to different thermal histories has been established (Barreiro and Vicente, 1993).

Alumina-based porcelainAluminous core porcelain is a typical example of strengthening by dispersion

of a crystalline phase (McLean and Kedge, 1987). Alumina has a high modulus of elasticity (350 GPa) and high fracture toughness (3.5 to 4 MPa.m05). Its dispersion in a glassy matrix of similar thermal expansion coefficient leads to significant strengthening of the core. The first aluminous core porcelains contained 40 to 50% alumina by weight (McLean and Hughes, 1965). The core was baked on a platinum foil and later veneered with matched-expansion porcelain. Hi-Ceram (Vident, Baldwin Park, CA) is amore recent development of this technique. Aluminous core porcelain is now baked directly onto a refractory die (McLean et al , 1994).

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Magnesia-based core porcelainMagnesia core ceramic was developed as an experimental material in 1985

(O'Brien, 1985). Its high thermal expansion coefficient (14.5 x 10'6/°C) closely matches that of body and incisal porcelains designed for bonding to metal (13.5 x 10"6/°C). The flexural strength of unglazed magnesia core ceramic is twice as high (131 MPa) as that of conventional feldspathic porcelain (65 MPa). The core material is made by reacting magnesia with a silica glass within the 1100-1150°C temperature range. This treatment leads to the formation of forsterite (Mg2Si04) in various amounts, depending on the holding time. The proposed strengthening mechanism is the precipitation of fine forsterite crystals (O'Brien et al, 1993). The magnesia core material can be significantly strengthened by glazing, thereby placing the surface under residual compressive stresses that have to be overcome before fracture can occur (Wagner et al, 1992).

Zirconia-based porcelainMirage II (Myron International, Kansas City, KS) is a conventional feldspathic

porcelain in which tetragonal zirconia fibers have been included. Zirconia undergoes a crystallographic transformation from monoclinic to tetragonal at 1173°C. Partial stabilization can be obtained by using various oxides such as CaO, MgO, Y2O3, and CeO, which allows the high-temperature tetragonal phase to be retained at room temperature. The transformation of partially stabilized tetragonal zirconia into the stable monoclinic form can also occur under stress and is associated with a slight particle volume increase. The result of this transformation is that compressive stresses are established on the crack surface, thereby arresting its growth. This mechanism is called transformation toughening. The addition of yttria-stabilized zirconia to a conventional feldspathic porcelain has been shown to produce substantial improvements in fracture toughness, strength, and thermal shock resistance (Morena et al 1986a; Kon et al, 1990). However, other properties, such as translucency and fusion temperature, can be adversely affected. The modulus of rupture of commercially available zirconia-reinforced feldspathic dental porcelain (Mirage II) is not significantly different from that of conventional feldspathic porcelain (Seghi et al, 1990b).

GLASS-CERAMICSMica-based

As described earlier, glass-ceramics are obtained by controlled devitrification of glasses with a suitable composition including nucleating agents. Depending on the composition of the glass, various crystalline phases can 7(2):134-143 (1996) Crit Rev Oral Biol Med 137 nucleate and grow within the glass. The advantage of this process is that the dental restorations can be cast by means of the lost-wax technique, thus increasing the homogeneity of the final product compared with conventional sintered feldspathic porcelains.Dicor (Dentsply Inc., York, PA) is a mica-based machinable glass-ceramic. The machinability of Dicor glass-ceramic is made possible by the

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presence of a tetrasilicic fluormica (KMg25Si4O10F2) as the major crystalline phase (Grossman and Johnson, 1987). Micas are classified as layer-type silicates. Cleavage planes are situated along the layers, and this specific crystal structure dictates the mechanical properties of the mineral itself. Crack propagation is not likely to occur across the mica crystals and is more probable along the cleavage planes of these layered silicates (Daniels and Moore, 1975). In the glass-ceramic material, the mica crystals are usually highly interlocked within the glassy matrix, achieving a "house of cards" microstructure (Grossman, 1972). The interlocking of the crystals is a key factor in the fracture resistance of the glass-ceramic, and their random orientation makes fracture propagation equally difficult in all directions. After being cast, the Dicor glass is converted into a glass-ceramic by means of a single-step heat treatment with a six-hour dwell at 1070°C. This treatment facilitates controlled nucleation and growth of the mica crystals.However, it is critical to re-invest the cast glass restoration prior to the crystallization heat treatment, to prevent sagging or rounding of the edges at high temperature. The match in the thermal expansion coefficients of the glass and the investment is achieved by use of a leucite based gypsum-bonded investment. The interaction of the glass-ceramic and the investment during the crystallization heat treatment leads to the formation of calcium magnesium silicate at the surface of the glass-ceramic (Denry and Rosenstiel, 1993). This crystalline phase could be formed by decomposition of the mica into magnesium silicate that later reacts with the gypsum-bonded investment. This surface layer, called the"ceram layer", has been reported to decrease the strength of glass ceramic crowns significantly (Campbell and Kelly, 1989; Kelly et al, 1989). The effects of alumina and zirconia additions on the bending strength of Dicor glass-ceramic have been investigated. It was found that alumina additions successfully increase the bending strength of Dicor glass-ceramic, whereas zirconia additions had no effect (Tzengetal, 1993). Hydroxyapatite-based

Cerapearl (Kyocera, San Diego, CA) is a castable glass ceramic in which the main crystalline phase is oxyapatite, transformable into hydroxyapatite when exposed to moisture (Hobo and Iwata, 1985). Lithia-based:

Glass-ceramics can be obtained from a wide variety of compositions, leading to a wide range of mechanical and optical properties, depending on the nature of the crystalline phase nucleating and growing within the glass. Experimental glass-ceramics in the system Li2O-Al2O3- CaO-SiO2 are currently the object of extensive research work. The choice of adequate additives is critical in the development of tougher and higher-strength glassceramics (Anusavice et al. 1994b). Differential thermal analysis can be efficiently used to determine the heat treatment leading to the maximum lithium disilicate crystal population in the shortest amount of time, thereby optimizing the nucleation and crystallization heat treatment of this type of glass-ceramic (Parsell and Anusavice, 1994). Machinable ceramics

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Cerec system:The evolution of CAD-CAM systems for the production of machined inlays, onlays, and crowns led to the development of a new generation of machinable porcelains.There are two popular systems available for machining all-ceramic restorations. The best-known is the Cerec system (Siemens, Bensheim, Germany). It has been marketed for several years, and two materials can be used with this system: Vita Mark II (Vident, Baldwin Park, CA) and Dicor MGC (Dentsply International, Inc., York, PA). Vita Mark II contains sanidine (KAlSi3O8) as a major crystalline phase within a glassy matrix. As explained earlier, the presence of sanidine could explain the lack of translucency of this material. Dicor MGC is a machinable glass-ceramic similar to Dicor, with the exception that the material is cast and cerammed by the manufacturer. Colorants have been added to match tooth color. The glass-ceramic contains 70 vol% of the crystalline phase (Grossman, 1991). Manufacturer's control over the processing of this material and the higher volume percent of the crystalline phase could explain the improved mechanical properties of Dicor MGC compared with conventional Dicor glass-ceramic. The use of adhesive resinbased cements has been shown to improve the fracture resistance of all-ceramic crowns (Eden and Kacicz, 1987; Grossman and Nelson, 1987). Other studies have shown that the overall fracture resistance of Dicor MGC was independent of cement film thickness (Scherrer et al., 1994). Presently, the main identified weakness of the Cerec system is the marginal fit of the restorations (Anusavice, 1993). Celay system:

The Celay system (Mikrona Technologie, Spreitenbach, Switzerland) uses a copy-milling technique to manufacture ceramic inlays or onlays from resin analogs. The Celay system is a mechanical device based on pantographic tracing of a resin inlay or onlay fabricated directly onto the prepared tooth or onto the master die (EidenbenzeU/., 1994). As with the Cerec system, the starting material is a ceramic blank available in different shades. One ceramic material currently available for use with the Celay system is Vita-Celay (Vident, Baldwin Park, CA). This material contains sanidine as the major crystalline phase within a glassy matrix. Recently, ln-Ceram pre-sintered slip-cast alumina blocks (Vident, Baldwin Park, CA) have been machined with the Celay copy-milling system used to generate copings for crowns and fixed partial dentures (McLaren and Sorensen, 1995). The alumina copings were further infiltrated with glass following the conventionalln-Ceram technique, resulting in a final marginal accuracy within 50 urn. SLIP-CAST CERAMICSAlumina-based (\n-Ceram)

ln-Ceram (Vident, Baldwin Park, CA) is a slip-cast aluminous porcelain. The alumina-based slip is applied to a gypsum refractory die designed to shrink during firing.The alumina content of the slip is more than 90%, with a particle size between 0.5 and 3.5 micrometers. After being fired for four hours at 1100°C, the porous alumina coping is shaped and infiltrated with a lanthanum-containing glass during a second firing at 1150°C for four hours. After removal of the excess glass, the restoration is veneered with matched expansion veneer porcelain (Probster and Diehl, 1992). This processing technique is unique in dentistry and leads to a high-strength

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material due to the presence of densely packed alumina particles and the reduction of porosity. Two modified porcelain compositions for the In Ceram technique have been recently introduced. In-Ceram Spinell contains a magnesium spinel (MgAl2O4) as the major crystalline phase with traces of alpha-alumina, which seems to improve the translucency of the final restoration. The second material contains tetragonal zirconia and alumina. A variety of alumina-glass dental composites can be prepared by the glass-infiltration process. However, research has shown that the fracture toughness of the composites is relatively insensitive to the volume fraction of alumina in the range investigated (Wolfrtfll., 1993).Hot-pressed, injection-molded ceramics Leucite-based

IPS Empress (Ivoclar USA, Amherst, NY) is a leucite-containing porcelain. Ceramic ingots are pressed at 1150°C (under a pressure of 0.3 to 0.4 MPa) into the refractory mold made by the lost-wax technique. This temperature is held for 20 minutes in a specially designed automatic press furnace (Dong et al, 1992). The ceramic ingots are available in different shades. They are produced by sintering at 1200°C and contain leucite crystals obtained by surface crystallization (Holand et al. , 1995). The leucite crystals are further dispersed by the hot-pressing step. The final microstructure of IPS Empress exhibits 40% by volume of tetragonal leucite. The leucite crystals measure 1-5 um and are dispersed in a glassy matrix. Two finishing techniques can be used with IPS Empress: a staining technique or a layering technique involving the application of veneering porcelain. The two techniques lead to comparable mean flexure strength values for the resulting porcelain composite (Luthy et al, 1993). The thermal expansion coefficient of the IPS Empress material for the veneering technique (14.9 x 10"6/°C) is lower than that of the material for the staining technique (18 x 10~6/°C) to be compatible with the thermal expansion coefficient of the veneering porcelain. The flexural strength of IPS Empress material was significantly improved after additionalfirings (Dong et al, 1992). The strength increase is attributed to a good dispersion of the fine leucite crystals as well as the tangential compressive stresses arising from the thermal contraction mismatch between the leucite crystals and the glassy matrix.Spinel-based

Alceram (Innotek Dental Corp, Lakewood, CO) is a material for injection-molded technology and contains a magnesium spinel (MgAl2O4) as the major crystalline phase (McLean and Kedge, 1987). This system was initially introduced as the "shrink-free" Cerestore system, which relied on the conversion of alumina and magnesium oxide to a magnesium aluminate spinel. One of the recognized advantages of this system was the excellent marginal fit of the restorations (Wohlwend et al, 1989).CONDENSATION (COMPACTION)

The process of packing the powder particles together and removing excess water is known as condensation. Proper condensation gives dense packing and reduce the shrinkage of porcelain and minimize porosity in the fired porcelain.Condensation procedure is followed in application of core, dentin and enamel porcelain either in

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porcelain jacket crown or porcelain fused to metal.The porcelain powder is mixed with distilled water or special liquid supplied by the manufacturer to form a thick paste. Small portions of the paste are then applied to the platinum matrix in jacket crown preparation over the die until the desired shape of the crown has been attained. Excess water is removed by blotting with a linen cloth or similar absorbent material.The remaining water serves as a binder for the powder so that the crown may be properly shaped before firing. Powder consisting of a mixture of particle sizes compact more easily than those with particles of one size only. This reduces the size of the spaces between the particles and thus reduces firing shrinkage.A well compacted crown not only reduces firing shrinkage but also shows a regular contraction over its entire surface.Methods of condensation:1) Vibration: Mild vibration are used to densely pack the wet powder upon the

underlying matrix. The excess water comes to the surface and its is blotted with a tissue paper.

2) Spatulation: A small spatula is used to apply and smoothen the wet porcelain. This action brings excess water to the surface.

3) Wet brush technique: The mix should be creamy and capable of being transferred in small increments to the platinum matrix with hair brush.

4) Ultrasonic: A ceramosonic condenser can induce supersonic vibration in porcelain creates intimate inter relation between metal and opaque porcelain.

5) Gravitational:6) Whipping: Any method may be used for condensation but care is taken not to

allow the porcelain to dry out completely as the porcelain powder is held together due to surface tension of water.

Dry the wet structure in a warm atmosphere before placing into the hot furnace. After condensation the compacted mass supported by the matrix or metal coping should be placed on a fire tray and inserted into the muffle of the ceramic furnace.

Porcelain Condensation Porcelains for ceramic and metal - ceramic prostheses, as well as for other

applications, is supplied as a fine powder that is designed to be mixed with water or anther vehicle and condensed into the desired form (see Fig 21-2). The powder particles are of a particular size distribution to produce the most densely packed porcelain when they are properly condensed. If the produce the densely packed porcelain when they are properly condensed. If the particles are of the same size, the density of packing would not be nearly as high. Thorough condensation is also crucial in obtaining dense packing would not be nearly as high. Thorough condensation is also crucial in obtaining deus packing of the powder particles. Dense packing of the powder particles. Dense packing of the powder particles dense packing of the powder particles. Dense packing of the powder particles provides two benefits: lower firing shrinkage and porosity in the fires porcelain. This packing, or condensation, may be achieved by various methods, including vibration, spatulation, and brush techniques.

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The first method uses mild vibration to pack the wet powder densely on the underlying framework. The excess water is blotted or wiped away with a clean tissue or fine brush, and condensation occurs toward the blotted or bushed area, in the second method a small spatula is used to apply and smooth the it is removed. The second method a small spatula is used to apply and smooth the wet porcelain. The smoothing action brings the excess water to the surface, where it is removed. The third method employs the addition of dry porcelain powder to the surface to absurd the water. A brcelain powder to the surface to accord the water places the dry powder. A brush to the side opposite from an increment of wet porcelain places the dry powder. As the water is drawn toward the dry powder, the wet particles are pulled together. Whichever method is used, it is important to remember that the surface tension of the water is the driving force for condensation, and the porcelain must never be allowed to dry out until condensation is complete.Condensed mass is gradually heated by first placing it in front of the muffle of a preheated furnace and later inserting into the furnace.1) Low bisque stage: The flux begins to melt and flow in between the porcelain

particles. The mass attains some rigidity but very little cohesion. At this stage the material is porous and undergoes minimum of shrinkage. The porcelain do not have translucency and glaze.

2) Medium bisque stage: Here the flux flows freely in between the particles the material is still porous, but there is complete cohesion between the particles and most of the shrinkage is complete. In this stage also there is lack of translucencey and glaze.

3) High bisque stage: Here with shrinkage is completed. There is very little porosity, the mass has attained complete rigidity and smoothness, the body does not appear to be glazed. Most of the addition and alterations are carried out after the porcelain has attained medium bisque stage.

Less the number of firing, higher is the strength and better the esthetics. Too many firings give a life less, over translucent porcelain.

PORCELAIN FURNACEThe ordinary air fire porcelain furnace consists of a muffle, a pyrometer, a

thermocouple and in its most simple form a rheostat or variable transformer for control of firing temperature and sophisticated automatic and programmable time and temperature controller for the most modern furnaces.

The muffle is the heating unit providing necessary high temperature for baking of porcelain. The heating element is a coiled wire of platinum and is embedded into the refractory material of the muffle. The muffle is provided with a door for easy access and to prevent fluctuation of temperature due to heat loss.

The pyrometer is a millivoltmeter calibrated to read in degree of temperature. The thermocouple consists of platinum wire joined at one end with another wire made of 90% platinum and 10% rhodium. The joint is placed inside the muffle, this is known as hot junction of the thermocouple. The free ends of the thermocouple are attached to the pyrometer outside the muffle. When heat is generated inside the muffle, the dissimilar metals of the thermocouple at the hot junction generates and

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electromotive force which deflects the needle of the pyrometer indicating the calibrated temperature.

As the electromotive force varies with variation in temperature inside the muffle, such variations can be measured as temperature on the pyrometer. The temperature controller regulates the current fed to the heating element inside the muffle thereby inducing increase or decrease in muffle temperature.

The main problem in air fired furnace is the opacity of the porcelain due to porosity.Cooling:

The cooling of dental porcelain is complex matter, particularly when the porcelain is fused to metal a metallic substrates.

Multiple firings of metal ceramic restorations can cause the co-efficient of thermal contraction the porcelain to increase and can actually make it more likely to craze or craze because of tensile stress development.

Cooling must be carried out slowly and uniformly. If shrinkage is not uniform it causes cracking and loss of strength. During cooling, subsurface submicroscopic surface cracks occur. Because of the low thermal conductivity of porcelain, the differential between the thermal dimensional change of the outside and inside can introduce stresses which embrittle the porcelain.Different methods and porcelain firings are:2) Air firing3) Pressure firing4) Gas firing5) Vacuum firing1) Air firing:

Air inside a furnace is modulated to the same atmospheric pressure during this procedure. There is more chances of air entrapment in porcelain. We will get more porous, less translucent porcelain.

2) Pressure firing:The air inside the furnace is subjected to a pressure equal to 10 atmosphere as

the porcelain reaches its maturation temperatures. This compresses the air inside the porcelain mass and reduces the size of the air bubble.

3) Gas firing:The air in the furnace is replaced by a diffusible inert gas like argon or

hydrogen which diffuses out through the maturing porcelain.4) Vacuum firing:

Partial vacuum firing reduces air voids, so porosity is reduced, so better translucent effect.

The air from the furnace is evacuated and this eliminating air from porous spaces which collapses on itself. This is the best and widely used method.

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LABORATORY MANUFACTURING PROCEDUREIn the production of porcelain tooth, the powder ingredients are weighted and

mixed with water containing starch, gum tragacanth, or other organic materials to form a putty like mass that can be handled conveniently.

The molding technique varies with different manufacturers. Generally, the split molds are made of bronze and may be separated so that one portion contains the negative pattern for the lingual surface of 12 teeth and the other contains the negative pattern for the labial surface or face of the teeth.

When the two piece molds are used, a thin layer of the enamel mix is placed in the labial mold to provide the enamel color, and the body mix, which forms the bulk of the tooth, is placed over this. Then a thin veneer of enamel mix is placed in the incisal portion of the lingual mold and is also covered with body mix. When combined, the two halves with the porcelain mixes form a tooth with contours and coloration similar to natural teeth.

The technique employs a third portion, which also fits against the labial surface for the purpose of accurately forming the enamel colored porcelain separately before the body portion is added.

The technique for the three piece molds involves placing the dough like enamel mix in the labial half first, pressing the third or blender mold into it and heating the molds until the mix stiffness. They are then opened, the excess mix is trimmed away and the body mix is added to all the second and larger lingual half of the mold.

Small noble metal rings are embedded in the porcelain to provide a base for the gold plated nickel pins used for the retention of the teeth in the denture base. These rings are made of a metal or alloy with a high melting point and usually are split to allow for the shrinkage of the surrounding porcelain during fusion. Before the moulds are filled, the rings are placed over the tips of tapered points that extend into the tooth from the lingual half of the mould.

After the moulds are filled by either method, they are placed in a press and heated until the porcelain mix develops sufficient hardness to allow handling. Each anterior tooth at this stage is approximately one fifth over size to allow for shrinkage.

After 3 stage of firing, the teeth have been cooled slowly to prevent crazing, all that remains and the attachment of the pins. For this operation small bits of solder are stamped to the ends of the gold clad pins and they are inserted, solder downward, to contact the metal ringes at the base of the tapered openings in the lingual body of the teeth. When heated, either in a furnace or electrically, the solder melts and joins the pin firmly to the embedded rings.

CERAMIC PROSTHESES:Aluminous Porcelain Crowns:

Another method of bonding porcelain to metal makes use of tin oxide coatings on platinum foil. The objective of this technique is to improve the aesthetics by a replacement of the thicker metal coping with a thin platinum foil, thus allowing more room for porcelain. The method consists of bonding aluminous porcelain to platinum

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foil copings. Attachment of the porcelain is secured by electroplating the platinum foil with a thin layer of tin and then oxidizing it in a furnance to provide a continuous film of tin oxide for porcelain bonding. The rationale is that the bonded foil will act as an inner skin on the fit surface to reduce subsurface porosity and formation of microcracks in the porcelain, thereby increasing the fracture resistance of crowns and bridges. The clinical performance of these crowns has been excellent for anterior teeth, but approximately 15% of these crowns fractured within 7 years after they were cemented to molar teeth with a glass ionomer cement.

Based on a 1994 survey, metal-ceramic crowns and bridges were used for approximately 90% of all fixed restorations. However, recent developments in ceramic products with improved fracture resistance and excellent aesthetic capability have led to a significant increase in the use of all-ceramic products. Ceramic crowns and bridges have been in widespread use since the beginning of the twentieth century. The ceramics employed in the conventional ceramic crown were high fusing feldspathic porcelains. The relatively low strength of this type of porcelain prompted McLean and Hughes (1965) to develop an alumina-reinforced porcelain core material for the fabrication of ceramic crowns.

The alumina-reinforced crowns are generally regarded as providing slightly better aesthetics for anterior teeth than are the metal-ceramic crowns that employ a metal coping. However, the strength of the core porcelain used for alumina-reinforced crowns is inadequate to warrant the use of these prostheses for posterior teeth. In fact McLean reported a fracture rate of molar aluminous porcelain crowns of approximately 15% after 5 years.Castable and machinable Glass-Ceramics (Dicor and Dicor MGC)

When used for posterior crowns, ceramic crowns are most susceptible to fracture. Shown in Figure 21-6 (see also the color plate) is the stress distribution computed by finite element analysis in a 0.5mm-thick molar Dicor crown loaded on the occlusal surface, just within the marginal ridge area. The maximum tensile stress is located within the internal surface directly below the point of applied force and just above the 50 m-thick layer of resin cement (see the arrow in fig. 21.6). this site represents the critical flaw responsible for crack initiation under an applied intraoral force. The location of initial crack formation was consistent with the location of maximum tensile stress predicted by the finite element calculations as shown in figure 21.6. an SEM image of a fractured clinical crown of Dicor glass-ceramic is shown in fig 21.8. because of the smaller forces exerted on anterior crowns, the risk for fracture of anterior crowns is significantly less than that for posterior crowns.

The first commercially available castable ceramic material for dental use, Dicor, was developed by Corning Glass works and marketed by Dentsply international. Dicro is a castable glass that is formed into an inlay, facial veneer, or full-crown restoration by a lost-was casting process similar to that employed for metals. After the glass casting core or coping is recovered, the glass is sandblasted to remove resideual casting investment and the sprues are gently cut away. The glass is then covered by a protective "embedment" material and subjected to a heat treatment that causes microscopic platelike crystals of crystalline material (mica) to grow within

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the glass matrix. This crystal nucleation and crystal growth process is called ceramming. Once the glass has been cerammed, it is fit on the prepared dies, ground as necessary, and then coated with veneering porcelain (as shown in fig. 21.8) to match the shape and appearance of adjacent teeth. Dicor glass-ceramic is capable of producing surprisingly good aesthetics, perhaps because of the "chameleon" effect, where part of the color of the restoration is picked up from the adjacent teeth as well as from the tinted cements used for luting the restorations.

Dicor glass-ceramic contains about 55 vol% of tetrasilicic fluormica crystals. The ceramming process results in increased strength and toughness, increased resistance to abrasion, thermal shock resistance, chemical durability, and decreased transluency. Dicor MGC is a higher quality product that is crystallized by the manufacturer and provided as CAD-CAM blanks or ingots. The CAD-CAM ceramic Dicor MGC contains 70 vol% of tetrasilicic fluormica platelets, which are approximately 2m in diameter. The mechanical properties of Dicor MGC are similar to those of Dicor glass-ceramaic, although it has less translucency (contrast ratio of 0.41 -0.44 versus 0.56, respectively).

Dicor has recently been discontinued presumably because of low tensile strength and the need to color the prosthesis on the exterior region rather that within the core region, which would more closely resemble a natural tooth. Although Dicor is no longer sold, the principles for selection are useful when products of similar mechanical and physical properties are being considered. The advantages of Dicor glass-ceramic were ease of fabrication, improved aesthetics, minimal processing shrinkage, good marginal fit, moderately high flexural strength, low thermal expansion equal to that of tooth structure, and minimal abrasiveness to tooth enamel.

The disadvantages of Dicor glass-ceramic were its limited use in low-stress areas and its inability to be colored internally. As designed, it was colored with a thin outer layer of shading porcelain and surface stain to ieve acceptable aesthetics. However, Dicor MGC ingots have been supplied in light and dark shades, making it possible for technicians to build depth of color into the fabrication process.

Although both of the Dicor products were based on a glass-ceramic core that was minimally abrasive to opposing tooth enamel, the required shaduing or veneering porcelains were more abrasive. Aesthetically, Dicor crowns were more lifelike than metal-ceramic crowns, which often exhibit a metal collar, a gray shadow subginigivally, or poor translucency. The life expectancy of Dicor crowns in high-stress areas is not as good as that of PEM crowns. Two veneering materials were used to improve the color of Dicor crowns: Dicor Plus, which consisted a pigmented feldspathic porcelain veneer, and Willi's Glass, a veneer of Vitadur N aluminous porcelain.

Tooth preparation for glass-ceramic of this type is the same as that required for metal-ceramic prostheses except that, for first and second molars a reduction of 2mm is recommended. Occlusal surfaces and incisal edges must be reduced a minimum of 1.5mm. Axial surfaces should be reduced a minimum of 1.0mm. The preparation should be either a shoulder with a rounded gingivoaxial line angle or a heavy chamfer.

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Pressable Glass-Ceramics:A glass-ceramic is a material that is formed into the desired shape as a glass,

then subjected to a heat treatment to induce partial devitrification (i.e., loss of glassy structure by crystallization of the glass). The crystalline particles, needles, or plates formed during this ceramming process serve to interrupt the propagation of cracks in the material when an intraoral force is applied, thereby causing increased strength and toughness. The use of glass-ceramics in dentistry was first proposed by MacCulloch in 1968. He used a continuous glass-molding process to produce denture teeth. He also suggested that it should be possible to fabricate crowns and inlays by centrifugal casting of molten glass.

Pressure molding is used to make small, intricate objects. This method uses a piston to force a heated ceramic ingot through a heated tube into a molk, where the ceramic form cools and hardens to the shape of the mold. When the object has solidified, the refractory mold (investment) is broken apart and the ceramic piece is removed. It is then debrided and either stained and glazed (certain inlays) or veneered with one or more layers of a thermally compatible ceramic.

IPS Empress is a glass-ceramic provided as core ingots that are heated and pressed until the ingot flows into a mold. It contains a higher concentration of leucite crystals that increase the resistance to crack propagation (fracture). The hot-pressing process occurs over a 45 min period at a high temperature to produce the ceramic substructure. This crown form can be either stained and glazed or build up using a conventional layering technique.

The advantages of this ceramic are its lack of metal, a translucent ceramic core, a moderately high flexural strength (similar to that of Optimal Pressable ceramic), excellent fit, and excellent aesthetics. The disadvantages are its potential to fracture in posterior areas and the need to use a resin cement to bond the crown micromechanically to tooth structure.

IPS Empress and IPS Empress2 are typical products representative of several other leucite-reinforced and lithia disilicate-reinforced glass-ceramics, respectively. Some properties of IPS Empress and IPS Empress2 glass-ceramic core materials are listed in table. 21.6. IPS Empress is a leucite-containing glass-ceramic that contains about 35 vol% of leucite (KAISI2O6) crystals, which increases the resistance to crack propagation (fracture). The veneering ceramic also contains leucite crystals in a glass matrix. After hot pressing, divesting, and separation of the ceramic units the sprue segments, they are veneered with porcelain containing leucite crystals in a glass matrix.

A cross-sectional illustration of an IPS Empress crown is illustrated in fig. 21.9. The IPS Empress2 is similar except that the core consists of lithia disilicate crystals in a glass matrix and the veneering ceramic contains apatite crystals. The very small apatite crystals cause light scattering in a way that resemble by the structure and components of tooth enamel. The coefficient of expansion of the apatite glass-ceramic veneering ceramic is 9.7 ppm/0 C, which is similar to that of IPS Empress2 core ceramic (10.6 ppm/0 C). Obviously, this veneering ceramic should not

33

be used with the IPS Empress core ceramic that has a much higher expansion coefficient (150 ppm/0 C).

The core microstructure of IPS Empress2 glass ceramic is quite different from that of IPS Empress, as evidenced by the 70 vol% of elongated lithia disilicate crystals in IPS Empress2. The primary crystal particles in IPS Empress2 are 0.5 to 4m in length. A smaller concentration of lithium orthophosphate crystals (Li2 Si2 O5) approximately 0.1 to 0.3m in diameter has also been reported (Holand et al., 2000). The microstructural difference between IPS Empress and IPS empress2 results in a slight decrease in translucency for IPS Cmpress2 (0.55) (Holland et al., 2000). As is the case for most pressable glass-ceramics, the advantages of IPS empress and IPS Empress2 glass-ceramic core materials are their potential for accurate fit, excellent transluency and overall aesthetics, and a metal-free structure. Disadvantages are their low to moderately high flexural strength and fracture toughness. These properties limit their use to conservative designs in low to moderate stress environments. Shown in fig. 21-10, 21-11 and 21-12 are three-unit glass-ceramic FPDs made from a lithia-disilicate-based core material. The FPD shown in fig. 21-12 was made without a veneering ceramic to enhance the fracture resistnce. A summary of important properties is presented in Table 21-7 for a variety of dental ceramics. A list of pressable ceramics and their veneering ceramics is summarized.

OPC and OPC 3G are two pressable ceramics that are similar in nature to IPS Empress and IPS Empress2, respectively. OPC is a leucite-containing ceramic and OPC 3G contains lithia disilicate crystals. The ultralow-fusing temperature of the veneering porcelain suggests a low level of wear of opposing enamel. However, insufficient clinical data are available to support this hypothesis.In-Ceram Alumina, In-Ceram Spinell, and In-Ceram Zirconia

In-Ceram is supplied as one of three core ceramics: (1) In-Ceram spinell (2) in-Ceram Alumina, and (3) in-Ceram Zirconia. A slurry of one of these materials is slip-cast on a porous refractory die and heated in a furnace to produce a partially sintered coping or framework. The partially sintered core is infiltrated with glass at 11000 C for 4 hr to eliminate porosity and to strengthen the slip-cast core. The initial sintering process for the alumina core produces a minimal shrinkage because the temperature and time are sufficient only to cause bonding between particles and to produce a desired level of sintering. Thus the marginal adaptation and fit of this core material should be adequate because little shrinkage occurs. The flexural strength (modulus or rupture) values of the glass-infiltrated core materials are approximately 350 Mpa for in-Ceram spinell (ICS), 500 Mpa for In-Ceram Alumina (ICA)and 700 Mpa for In-Ceram Zirconia (ICZ) compared with strengths of 100 to 400 Mpa for Dicor, Optec Pressable Ceramic, IPS Empress and IPS Empress2. Despite the relatively high strength of these materials, failures can still occur in single crowns as well as FPDs.

Because of the variation in strength, the primary indications for these core ceramics vary as shown in Table 21-9. For example, ICS is indicated for use as anterior single-unit inlays, onlays, crowns, and veneers, ICA is indicated for anterior and posterior crowns and anterior three-unit FPDs. Because of its high level of

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opacity, ICZ is not recommended for anterior prostheses. However, because of its extremely high strength and fracture toughness, it can be used for posterior crowns and posterior FPDs. As suggested in chapter 4, it is essential that the gingival embrasure areas of ceramic FPD connectors be designed with a large radius of curvature to minimize the stress-raiser effect in areas of moderate to high tensile stress. The connectors also should be sufficiently thick to minimize stresses during loading. For Empress and Empress2 ceramics used in molar areas, the connector height should be at least 4mm.3 tables: Page No. 687 to 689

Until in-Ceram was introduced, aluminous porcelain had not been used successfully to produce FPDs because of low flexural strength and high sintering shrinkage. Thus the principal indications for aluminous porcelain crowns were the restoration of maxillary anterior crowns when aesthetics was important and their use in patients with allergies to metals. Its advantages and disadvantages are summerized in the following.

A schematic drawing of an In-Ceram crown is shown in Fig 21.13 The same diagram can be used to illustrate crowns made with In-Ceram Spinell (ICS) and In-Ceram Zirconia (ICZ), which will be discussed below. The three In-Ceram ceramics are glass-infiltrated core materials used for single anterior crowns (all three products), posterior crowns (In-Ceram Alumina and in-Ceram Zirconia), anterior three-unit FPDs (In-Ceram Alumina), and three-unit posteriro bridges (In-Ceram Zirconia).

The most translucent of the three ceramics- In-Ceramics, In-Ceram Spinell, was introduced as an alternative to in-Ceram Alumina. This ceramic has a lower flexural strength, but its increased translucency provides improved aesthetics in clinical situations in which the adjacent teeth or restorations are quite translucent. The core of ICS is MgAl1O4 and that for ICZ is a mixture of Al2O3 and ZrO2. These core ceramics are also infiltrated with glass, and they are fabricated in a manner similar to that for ICA, although the firing temperatures and times may be different.

The final ICA core consists pf 70 wt% alumina infiltrated with 30 wt% sodium lanthanum glass. The final ICS core consists of glass-infiltrated magnesium spindl (MgAl2O4). ICZ contains approximately 30wt% zirconia and 70 wt% alumina. The power-liquid slurry is slip cast onto a porous die that absorbs water from the slurry, thereby densifying the agglomeration of particles onto the die. Steps for fabricating in-Ceram prostheses are as follows: (1) prepare teeth with an occlusal reduction of 1.5 to 2.0mm and a heavy circumferential chamfer (1.2mm), (2) make an impression and pour two dies, (3) apply Al2O3 on a porous duplicate die, (4) heat at 1200 C for 2 hours to dry Al2O3 , (5) sinter the coping for 10 hours at 11200 C, (6) apply a sodium lanthanum glass slurry mixture on the coping, (7) fire for 4 hours at 11200 C to allow infiltration of glass, (8) trim excess glass from the coping with diamond burs, (9) build up the core with dentin and enamel porcelain, (10) fire in the oven, grind in the anatomy and occlusion, finish, and glaze.

The advantages of ICA include a moderately high flexural strength and fracture toughness, a metal-free structure, and an ability to be used successfully with conventional luting agents (Type 1 cements). The collective advantages of the three

35

glass-infiltrated core materials are their lack of metal, relatively high flexural strength and toughness, and ability to be successfully cemented using any cement.

In spite of this high flexural strength (429 Mpa), the Weibull modulus of ICS is quite low (5.7), which is indicative of a large scatter in the distribution of strength values relative to the probability of fracture. (Tinschert et al., 2000). Its marginal adaptation may not be as good as that achieved with other ceramic products. In one study the mean marginal discrepancies were 83 m for Procera All Ceram, 63 m for IPS Empress, and 161m for In-Ceram Alumina. Other drawbacks of ICA include its relatively high degree of opacity, inability to be etched, technique sensitivity, and the relatively great amount of skilled labor required. These disadvantages apply also to In-Ceram Zirconia. Compared with ICM, the opacities of ICA and ICZ core ceramics are much greater.

Although these newer core ceramics have excellent fracture resistance inproper design of the connector area of a FPD can significantly reduce the fracture resistance and clinical survivability of the prosthesis. Shown in fig. 21-14 is the stress distribution in a three-unit FPD, which shows relatively high principal tensile stress (red area) at the tissue side of the interproximal connector when an occlusal load of 250 N is applied to the occlusal surface of the pontic.

In summary, In-Ceram Spinell (ICS) is a glass-infiltrated core ceramic that offers greater translucency for crowns than either the ICA or ICZ core ceramics. However, ICA has lower strength and toughness compared with ICA and ICZ. Thus the use of ICS is limited to anterior inlays, onlyas, veneers, and anterior crowns. Although ICZ is the strongest and toughest of the three core ceramics, its use is limited to posterior crowns and FPDs because of its high level of core opacity. ICZ is a much stronger and tougher material and has greater opacity than ICA.\Procera AllCeram:

The Procera AllCeram crown is composed of densely sintered, high-purity aluminum oxide core combined with a compatible allCeram veneering porcelain. This ceramic material contains 99.9% alumina, and its hardness is one of the highest among the ceramics used in dentistry. Procera AllCeram can be used for anterior and posterior crowns, veneers, onlays, and inlyas.

A unique feature of the Procera system is the ability of the Procera Scanner to scan the surface of the prepared tooth and transmit the data to the milling unit to produce an enlarged die through a CAD-CAM process. The core ceramic form is dry pressed onto the die, and the core ceramic is then sintered and veneered. Thus the usual 15%-20% shrinkage of the core ceramic during sintering will be compensated by constructing an oversized ceramic pattern, which will shrink during sintering to the desired size to accurately fit the prepared tooth.CAD-CAM Ceramics:

As shown in the ceramic classification chart in all-ceramic cores can be produced by processes of condensation and sintering, casting and ceramming, hotpressing and sintering, sintering and glass infiltration, and CAD-CAM processing for the Cerec DAB-CAM system the internal surface of inlays, onlays, or crowns is ground with diamond disks or other instruments to the dimensions obtained from a

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scanned inage of the preparation. for some systems, the external surface must be ground manually, although some recent CAD-CAM systems are capable of forming the external surface as well.

A milling operation within a Cerec CAD-CAM unit (Siemens Aktiengesellschaft, Bensheim, Germany ). The ceramic lock is being ground by a diamond-coated disk whose translational movements are guided by computer-controlled input. A cerec CAD-CAM ceramic block is shown in Figure 21-16 before milling, at an intermediate milling stage, and after completion of the milling operation for an inlay. These ceramics are supplied as small blocks that can be gound into inlays and veneers in a computer-driven CAN-CAM system. Vitablocs MK II are feldspathic procelains that are used in the same way as is Dicor HGC (machinable glass-ceramic). The disadvantages of CAD-CAM restorations include the need for costly equipment, the lack of computer-controlled processing support for occlusal adjustment, and the technique-sensitive nature of surface imaging required for the prepared teeth. Advantages include negligible porosity levels in the CAD-CAM core ceramics, the freedom from making an impression, reduced assistant time associated with impression procedures, the need for only a single patient appointment (with the Cerec system), and good patient acceptance. A list of CAD-CAM and copy-milled ceramics is given in Table 21-10.

An advantage of CAD-CAM ceramics is that one can select a core ceramic either for strength and fracture resistance, for low abrasiveness, or for translucency. for example, the extensive wear of opposing enamel that occurs when it is opposed by a feldspathic porcelain surface in the absence of posterior occlusion can be minimized by selscting a core ceramic that is minimally abrasive to enamel. Cercon and Lava Zirconia Core Ceramics:

The cercon Zirconia system (Destsply Ceramco, Burlington, NJ) consists of the following procedures for production of zirconia-based prostheses. After preparing the teeth (2.0mm incisal or occlusal reduction and 1.5 mm axial reduction), an impression is made and sent tot he laboratory, where it is poured with a model material. A wax pattern approximately 0.8 mm in thickness is made for each coping on the holding appliance on the left side of the scanning and milling unit (Cercon Brain). A presintered Zirconia blank is attached to the right side of the Brain unit. (Cercon Brain). A presintered zirconia blank is attached to the right side of the Brain unit. The blank has an attached barcode, which contains the enlargement factor and other milling parameters for computer control or the milling procedure After the unit is activated, parameters for computer control of the milling procedure After the unit is activated, the pattern in scanned and the blank is rough-milled and fine-milled on occlusal.

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Ceramic Block Ceramic typeCeramic veneer

Indications Manufacturer

CerAdapt Highly sintered Al2O3 All cream Implant superstructure

Nobel Biocare

Cercon Base Presintered ZrO2; postsintered after milling

Coercion Cream S

Crowns and FPDs

Dentsply Ceramco

DC- Kristen Leucite-base Triceram Crowns DCS Dental AG/Esprident

DC- Zirkon Presintered ZrO2 hot isostatic postcompaction

Vitadur D Triceram

Crowns and FPDs

DCS Dental AG/Vita/Esprident

Denzir Presintered ZrO2; hot isostatic postcompaction

Empress2 Crowns and FPDs

Decim, Ivoclar

LAVA Frame ZrO2; presintered and postsntered

LAVA Ceram

Crowns and FPDs

3M ESPE

ProCad Leucite-based Malthechnik Veneers, inlays, onlays, and crowns

Ivoclar

Procera AllCeram

Al2O3; presintered and postsintered

AllCeram Crowns and FPDs

Nobel Biocare

Synthoceram Al2O3; reinforced; pressed and postsintered

Sintagon Crowns Elephant

VitaBlocs Mark II

Feldspathic porcelain block

Maltechnik Veneers, inlays, onlays, and crowns

Vident

VitaBlocks Alumina

Sintered Al2O3; followed by glass infiltration

Vitadur Alpha

Crowns and FPDs

Crowns and FPDs Vident

VitaBlocs Spinell

Sintered

MgO-Al2O3 spinel followed by glass infiltration

Vitadur Alpha

Crowns Vident

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VitaBlocs Zirconia

Sintered Al2O3/ZrO2

followed by glass infiltration

Vitadur Alpha

Crowns and FPDs

Vident

Zircogon ZrO2; presintered and postsintered

Zircogon Crowns Elephant

And gingival aspects in an enlarged size to compensate for the 20% shrinkage that will occur during subsequent sintering at 13600 C. the processing times for milling are approximately 35 for a crown and 80 min for a four – unit fixed FPD. The milled are approximately 35 min for a crown and 80 min for a four- unit fixed FPD. The milled prosthesis is removed from the and the remaining extraneous extension are removed. The zircon coping or framework is then placed in the Cercon furnace and fired at 13500 C for approximately 6 hours to fully sinter the yttria-stabilized zirconia core coping or framework. The sintering shrinkage is achieved uniformly and linearly in three – dimensional space by the in targeted process of scanning, enlarging the pattern design, controlled milling and sintering.

After any subsequent trimming with a water – cooled, hinge- speed diamond bur the finished ceramic core framework is then veneered with a veneering ceramic (Cercon Ceram S) and stain ceramic

All-ceramic prostheses represent the most aesthetically pleasing, but also the most fracture-prone prostheses. However, with adequate tooth reduction, an excellent quality impression, a skilled technician, and a ceramic with reasonably high flexure strength (250 MPa) and fracture toughness ( 2.5Mpa.m1/2), reasonably high success rates can be achieved. The material that has the greatest potential fracture toughness (9 MPa.m½) and flexural strength that has the greatest potential fracture toughness (9 MPa.m½) and flexural strength (>900 MPa) is pure tetragonal stabilized zirconia (ZrO2). Tinschert et al (200lb) reported that the fracture resistance of three-unit ceramic FPDs (1278 N) made of Cercon zirconia core ceramic (Dentspy Ceramco) was more than twice as great as the values reported for In-Ceram Alumina (514 N) and Empress2 (621 N). Shown in Figure is a comparison of the force required to fracture three-unit FPDs cemented to dies with zinc phosphate cement. The zirconia product (Cercon) would be expected to exhibit less fracture resistance in this case, but clinical data are needed to confirm this hypotehsis.

To ensure maximum survival times, adequate occlusal tooth reduction is essential for posterior teeth. Optimal clinical performance of some ceramic products require a minimal occlusal reduction of 2 mm for molar tooth preparations. If the ceramic will be supported by a material with high elastic modulus such as a ceramic or metal post or an amalgam build-up, less occlusal reduction (1.5 mm) may be possible without compromising the survivability of the crowns. For patients exhibiting extreme brusism, either metal or metal-ceramic prostheses should be used. METHODS OF STRENGTHENING CERAMICS: Minimize the effect of stress Raisers

Why do dental ceramic prostheses fail to exhibit the strengths that we would expect from the high bond forces between atoms? The answer is that numerous

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minute scratches and other defects are present on the surfaces of these materials. These surface flaws behave as sharp notches whose tips may be as narrow as the spacing between several atoms in the material. These stress concentration areas at the tip of each surface flaw can increase the localized stress to the theoretical strength of the material even though a relatively low average stress exists throughout the bulk of the structure. When the induced mechanical stress exceeds the actual strength of the material, the bonds at the notch tip break, forming a crack. This stress concentration phenomenon explains how materials fail at stresses far below their theoretical strength.

Stress raisers are discontinuities the ceramic and metal-ceramic structures and in other brittle materials that cause a stress concentration in these areas. The design of ceramic dental restorations should also avoid stress raisers in the ceramic. Abrupt changes in shape or thickness in the ceramic contour can act as stress raisers and make the restoration more prone to failure. Thus the incisal line angles on an anterior tooth prepared for a ceramic crown should be well rounded.

In ceramic crowns, several conditions can cause stress concentration. Creases or folds of the platinum foil or gold foil substrate that become embedded in the porcelain leave notches that act as stress raisers. Sharp line angles in the preparation also create areas of stress concentration in the restoration. Large changes in porcelain thickness, a factor also determined by the tooth preparation, can create areas of stress concentration.

A small particle of porcelain along the internal porcelain margin of a crown also induces locally high tensile stresses. A stray particle that is fused within the inner surface of a shoulder porcelain margin of a metal-ceramic crown can cause localized tensile stress concentrations in porcelain when an occlusal force is applied to the crown.

Even though a metal-ceramic restoration is generally stronger than most ceramic crowns of the same size and shape, care must be taken to avoid subjecting the porcelain in a PFM to loading the produces large localized stresses. If the occlusion is not adjusted properly on a porcelain surface, contact points rather than contact areas will greatly increase the localized stresses in the porcelain surface as well as within the internal surface of the crown.

Fracture mechanics is a science that allows scientists to analyze the influence of flaw/stress interactions on the probability of crack propagation through an elastic brittle solid. The principles of linear elastic fracture mechanics were developed in the 1950s by Irwin (1957). This pioneering research on fracture phenomena was based on earlier investigations by Griffth (1921) and Orowan (1944, 1949, 1955). Irwin found that when a brittle material was subjected to tensile stresses, specific crack shapes in certain locations were associated with greatly increased stress levels. He also recognized the importance of determining the fracture toughness of these materials as a measure of their ability to resist fracture. The fracture toughness (K IC) of a material represents the resistance of a material to rapid crack propagation. In contrast, the strength of a material depends primarily on the size of the initiating crack that is present. The strength of dental ceramics and other restorative materials is

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controlled by the size of the cracks or defects that are introduced during processing, production and handling. In this chapter a description is given of the processing methods used to produce ceramic prostheses and the potential of these methods to introduce flaws or cracks that may limit their clinical survival.

The brittle fracture behavior of ceramics and their low tensile strengths compared with those predicted from bonds between atoms can be understood by considering stress concentrations around surface flaws. As ceramics tends to have no mechanism for plastically deforming without fracture as do metals, cracks may propagate through a ceramic material at low average stress levels. As a result, ceramics and glasses have tensile strengths that are much lower than their compressive strengths in the oral environment, tensile stresses are usually created by bending forces, and the maximum tensile stress created by the bending forces occurs at the surface of a prosthesis. It is for this reason that surface flaws are of particular importance in determining the strength of ceramics.

As the crack propagates through the material, the stress concentration is maintained at the crack, a pore, or a crystalline particle, which reduces the localized stress. The removal of surface flaws or the reduction of their size and number can produce a very large increase in strength. Reducing the depth of surface flaws in the surface of a ceramic is one of the reasons that polishing and glazing of dental porcelain is so important. The fracture resistance of ceramic prostheses can be increased through one or more of the following six options: (1) select stronger and tougher ceramics; (2) develop residual compressive stresses within the surface of the material by thermal tempering, (3) develop residual compressive stress within interfacial regions of weaker, less tough ceramic layers by properly matching thermal expansion coefficients, (4) reduce the tensile stress in the ceramic by appropriate selection of stiffer supporting materials, (5) minimize the number of porcelain firing cycles, (6) design the ceramic FPD prosthesis with greater bulk and broader radii of curvature to minimize the magnitude of tensile stresses and stress concentrations of curvature to minimize the magnitude of tensile stresses and stress concentrations during function, and (7) adhesively bond ceramic crowns to tooth structure.

Develop Residual Compressive Stresses: One method of strengthening glasses and ceramics is the introduction of

residual compressive stresses within the veneering ceramic. Consider three layers of porcelain: the outer two of the same composition and thermal contraction co-efficient and the middle layer of a different composition and a higher thermal contraction coefficient. Suppose that the layers are bonded together and the bonded structure is allowed to cool to room temperature. The inner layer has a higher coefficient of thermal contraction and thus contracts more as it cools. Hence, on cooling to room temperature, the inner layer produces compressive stresses in the outer layers as previously described for thermal tempering. This three-layer laminate technique is used by Corning Glass works to manufacture dinnerware.

A similar condition can develop in a veneering porcelain bonding to an alloy coping used for metal-ceramic crowns and FPDs and adjacent ceramic layers in all-

41

ceramic prostheses. The metal and porcelain should be selected with a slight mismatch in their thermal contraction coefficients (the metal thermal contraction coefficients (the metal thermal contraction coefficient being slightly larger) so that the metal contracts slightly more than the porcelain on cooling from the firing temperature to room temperature. This mismatch leaves the porcelain in residual compression and provides additional strength for the prosthesis. Examples of how residual tensile stresses can weaken a metal-ceramic crown or FPD and how residual compressive stresses can weaken a metal-ceramic crown or FPD and how residual compressive stresses can increase fracture resistance are shown in Figure.

The same principle applies to ceramic prostheses in which the thermal contraction coefficient of the core ceramic is slightly greater than that of the veneering ceramic (such as opaceous dentin or body/gingival porcelain).

The fabrication of metal-ceramic and all-ceramic prostheses usually involves processing at high temperature, and the process of cooling to room temperature affords the opportunity to take advantage of mismatches in coefficients of thermal contraction of adjacent materials in the ceramic structure. Ideally, the porcelain should sustain slight compression in the final restoration. This objective is accomplished by selecting an alloy that contracts slightly more than the porcelain on cooling to room temperature.

A further, yet fundamentally different, method of strengthening glasses and ceramics is to reinforce them with a dispersed phase of a different material that is capable of hindering a crack from propagating through the material. There are two different types of dispersions used to interrupt crack propagation. One type relies on the toughness of the particle to absorb energy from the crack and deplete its driving force for propagation. The other relies on a crystal structural change under stress to absorb energy from the crack. These methods of strengthening are described below.

Minimize the Number of firing cycles:The purpose of porcelain firing procedures is to densely sinter the particles of

powder together and to produce a relatively smooth, glassy layer (glaze) on the surface. In some cases, a stain layer is applied for shade adjustment or for characterization such as stain lines or fine cracks. Several chemical reactions occur over time at porcelain firing temperatures and of particular importance are increases in the concentration of crystalline leucite in the porcelain designed for fabrication of metal-ceramic restorations. Leucite, K2O-Al2O34SiO2, is a highe-expansion crystal phase, which can greatly affect the thermal contraction coefficient of the porcelain. Changes in the leucite content caused by multiple firings can alter the thermal contraction coefficient of the porcelain. Some porcelains undergo an increase in leucite crystals after multiple firings that will increase their thermal expansion coefficients. If the expansion coefficient increases above the value for the metal, the expansion mismatch between the porcelain and the metal can produce stresses during cooling that are sufficient to cause immediate or delayed crack formation in the porcelain.

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Minimize Tensile Stress Through Optimal Design of Ceramic Prostheses: Tougher and stronger ceramics can sustain higher tensile stresses before

cracks develops in areas of tensile stress. Conventional feldspathic porcelains should not be used as the core of ceramic crowns, especially in posterior areas, because occlusal forces can easily subject them to tensile stresses that exceed the tensile strength of the core ceramic. Of major concern are tensile stresses that are concentrated within the inner surface of posterior ceramic crowns. Sharp line angles in the preparation also will create areas of stress concentration in the restoration, primarily where a tensile component of bending stress develops. A small particle of ceramic along the internal porcelain margin of a crown will also induce locally high tensile stresses. Thus the ceramic surface that will be cemented to the prepared tooth or foundation material should be examined carefully when it is delivered from the laboratory. Furthermore, when grinding of this surface is required for adjustment of fit, one should use the finest grit abrasive that will accomplish the task.

Because the forces on anterior teeth are relatively small, the low to moderate tensile stresses produced can be supported by ceramic crowns more safely. However, if there is a great amount of vertical overlap (overbite) with only a moderate amount of horizontal overlap (overjet), high tensile stresses can be produced. Metal-ceramic crowns use a metal coping as the foundation of the restoration to which the porcelain is fused. The stiff, metal coping minimizes flexure of the porcelain structure of the crown that is associated with tensile stresses.

Most dental restorations containing ceramics should be designed in such a way as to overcome their weaknesses, that is their relatively low tensile strength, their brittleness, and their susceptibility to flaws in the presence of surface flaws. The design should avoid exposure of the ceramic to high tensile stresses. It should also avoid stress concentration at sharp angles or marked changes in thickness. One way to reduce tensile stresses on the cemented surface in the occlusal region of ceramic inlays or crowns is to use the maximum occlusal thickness possible. However, within practical limits of tooth reduction, this thickness is typically 2.0 mm.

Aluminous porcelain crowns are contraindicated for restoring posterior teeth because occlusal forces can induce tensile stresses, which are often concentrated near the internal surface of the crown. Metal-ceramic crowns use a metal coping as the foundation of the restoration to which the porcelain is fused. In an attempt to overcome these stresses, the strong, stiff, yet ductile metal coping minimizes flexure of the porcelain structure of the crown that is associated with tensile stresses. Both the bonded platinum foil aluminous porcelain corwn technique and the swaged gold alloy foil technique are also based on this same concept.

The tensile stresses in a ceramic FPD can be reduced by using a greater connector height and by broadening the radius of curvature of the gingival embrasure portion of the interproximal connector. However, a connector height greater than 4 mm makes the anatomic form in the buccal area of a posterior FPD too bulky and unaesthetic.

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Ion Exchange: The technique of ion exchange is one of the more sophisticated and

effective methods of introducing residual compressive stresses into the surface of a ceramic. The ion-exchange process is sometimes called chemical tempering (Anusavice et al, 1992) and can involve the sodium ion since sodium is a common constituent of a variety of glasses and has a relatively small ionic diameter. If a sodium-containing glass article is placed in a bath of molten potassium nitrate, potassium ions in the bath exchange places with some of the sodium ions in the surface of the glass article and remain in place after cooling. Since the potassium ion is about 35% larger than the sodium ion, the squeezing of the potasium ion into the place formerly occupied by the sodium ion creates very large residual compressive stresses.

The product GC Tuf-Coat (GC Corp., Tokyo, Japan) was a potassium-rich slurry that could be easily applied to a ceramic surface and, when heated to 4500 C for 30 min (in any standard porcelain furnace) caused a sufficient. Increases of 100% or more in flexural strength have been achieved with several porcelain products that contained a significant concentration of small sodium ions. However, the depth of the compression zone is less than 100 m (anusavic et al. 1994). Therefore this strengthening effect could be lost if the porcelain or glass - ceramic surface is ground, worn, or eroded by long - term exposure to certain inorganic acids.

Thermal Tempering: Perhaps the most common method for strengthening glass is by thermal

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

Thermal tempering is used to strengthen glass for uses such as automobile windows and windshields, sliding glass doors, and diving masks. Often, jets of air directed at the molten glass surface accomplish the rapid cooling of the outer skin. If one observes the rear window of an automobile through polarized sunglasses, it is usually possible to discern a regular pattern of spots over the entire window. This pattern of spots corresponds to the arrangement of the air jets employed by the manufacturer in the tempering process. For dental applications, it is more effective to quench hot glass phase ceramics in silicone oil or other special liquids rather than using air jest that may not uniformly cool the surface. This thermal tempering treatment induces a protective region of compressive stress within the surface ( DeHoff and anusavic,1992)

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Dispersion Strengthening A further, yet fundamentally different, method of strengthening glasses and

ceramics is to reinforce them with a dispersed phase of a different material that is capable of hindering a crack from propagating through the material. This process is referred to as dispersion strengthening. Almost all of the newer higher- strength ceramics derive there improved fracture resistance from the crack- blocking ability of the crystalline particles. Increasing the crystal containing primarily can strengthen dental ceramics containing primarily a glass phase can be strengthened by increasing the crystal content of leucite (K2O.Al2O3.4SiO2) lithia dislocate (Li2O.2SiO2), alumina ( Al2O3) magnesia - alumina spinel (MgO. Al2O3), zirconia (ZrO2), and other types of crystals. Some crystal phase additions are not as effective as others in toughening the ceramics. Toughening depends on the crystal type, its size, its volume fraction, the interpartics. Toughening, and its relative thermal expansion coefficient relative to the glass matrix. For example, the fracture toughness (KIC) of soda lime silica glass is 0.75 Mpa . m1/2 If one disperses 70 vol% of terasilicic flourmill crystals in the glass.(Dicor MGC glass - ceramic), the toughness increases only to 1.5 Mpa. m1/2. However, by dispersing 70 vol% if lithia dislocate crystals in the glass matrix ( IPS Empress2), KIC increased to 3.3 Mpa. M 1/2 ( Holand and Beall, 2002).

When a tough, crystalline material such as alumina ( Al2O3) is added to a glass the is toughened and strengthened because the crack cannot pass through the alumina [particles as easily as it can pass through the glass matrix. This technique has found application in dentistry in the development of alimonies porcelains ( Al2O3 particles in a glassy porcelain matrix) for porcelain jacket crowns. Most dental ceramics that heave a glassy matrix utilize reinforcement of the glass by a disposed crystalline substance.

Tinschert et al (200lb) evaluated the mean strength and standard deviation values for several ceramics. This mean strength values were as follows; (Mpa ± SD) were Cerec Mark II, 86.3±12.2; In Ceram Alumina, 429.3 ± 87.2; IPS Empress. 83.9 ± 11.3; Vitadur Alpha Core, 131.0 ± 9.5; Vitadur Alpha Dentin, 60.7 ± 6.8; Vita VMK 68, 82.7± 10.0: and Zirconia-TZP, 913.0 ± 50.2. There was no statistically significant difference among the flexure strength of Cerec Mark II, Dicro, IPS Empress, Vitadur Alpha Dentin, and Vita VMK 68 ceramics (P>0.05). The highest Weibull moduli were associated with Cerec Mark II and Zirconia-TZP ceramics (23.6 and 18.4). Dicor glass-ceramic and In-Ceram Alumina had the lowest values of Weibull modules (m) (5.5 and 5.7), whereas intermediate values were observed for IPS Empress, Vita VMK 68, Vitadur Alpha Dentin, and Vitadur Alpha Core ceramics (8.6, 8.9, 10.0 and 13.0 respectively).

Except for In-Ceram Alumina, Vitadur Alpha, and Zirconia-TZP core ceramics, the investigated ceramic materials fabricated under the condition of a dental laboratory were not stronger or more structurally reliable than Vita VMK 68 veneering porcelain. Only Cerec Mark II and Zirconia-TZP specimens, which were prepared from an industrially optimized ceramic material, exhibited m values greater than 18. Hence, we conclude that industrially prepared ceramics are more

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structurally reliable materials for dental applications, although CAD-CAM procedures may induce surface and subsurface flaws that may offset this benefit.

Transformation Toughening: When small, tough crystals are homogeneously distributed in a glass, the

ceramic structure is toughened and strengthened because cracks cannot penetrate the fine particles as easily as they can penetrate the glass. Dental ceramics are strengthened and toughened by a variety of dispersed crystalline phases including alumina (Vitadur Alpha, Procera AllCeram, In-Ceram alumina), leucite (Optec HSP, IPS Empress, OPC), tetrasilicic fluormica (Dicor, Dicor MGC), lithia disilicate (OPC 3G, IPS Empress2), and magnesia-alumina spinel (In-Ceram spinell). In contrast, dental ceramics based primarily on zirconia crystals (Cercon and Lava) undergo transformation toughening that involves a transformation of ZrO2 from a tetragonal crystal phase to a monoclinic phase at the tips of cracks that are in regions of tensile stress. The unit cells for tetragonal and monoclinic lattices are shown.

When pure ZrO2 is heated to a temperature between 14700 and 20100 C and it is cooled, its crystal structure begins to change from a tetragonal to a monoclinic phase at approximately 11500 C. During cooling to room temperature, a volume increase of several percentage points occurs when it transforms from the tetragonal to monoclinic crystal structure. This polymorphic transformation can be prevented with certain additives such as 3 mol% yttrium oxide (yttria or Y2O3). This material is designated as ZrO2. TZP (tetragonal zirconia polycrystals). The volume increase in this case is constrained if the zirconia crystals are sufficiently small and the microstructure is strong enough to resist the resulting stresses. This material is extremely strong (flexural strength of approximately 900 MPa) and tough (fracture tougheness, KIC, of approximately 9 MPa) and tough (fracture tougheness, KIC, of approximately 9 Mpam1/2).

The toughening mechanism of crack shielding results from the controlled transformation of the metastable tetragonal phase to the stable monoclinic phase. Several types of crack shielding processes are possible, including microcracking, ductile zone formation, and transformation zone formation. By controlling the composition, particle size, and the temperature versus time cycle, zirconia can be densified by sintering at a high temperature and the tetragonal structure can be maintained as individual grains or precipitates as it is cooled to room temperature. The tetragonal phase is not stable at room temperature, and it can transform to the monoclinic phase with a corresponding volume increase under certain conditions. When sufficient stress develops in the tetragonal structure and a crack in the area begins to propagate, the metastable tetragonal crystals (grains) or precipitates next to the crack tip can transform to the stable monoclinic form. In this process a 3 vol% expansion of the ZrO2 crystals or precipitates occurs that place the crack under a state of compressive stress and crack progression is arrested. For this crack to advance further, additional tensile stress would be required. Because of this

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strengthening and toughening mechanism, the yttria-stablilized zirconia ceramic is sometimes referred to as ceramic steel.

Abrasiveness of Dental Ceramics:A review of the factors and material characteristics that cause excessive

wear of enamel by ceramic prostheses is extremely important to optimize the performance of ceramic-based prostheses. Ceramics are generally considered the most biocompatible, durable, and aesthetic materials available for rehabilitation of teeth, occlusal function, and facial appearance. Currently available products exhibit variable mechanical properties (hardeness, flexure strength, fracture toughness, and elastic modulus), physical properties (index of refraction, color parameters, translucency, chemical durability, and thermally compatible expansion coefficients for the substrates. In spite of their overall excellence in meeting the ideal requirements of a prosthetic material, dental ceramics have one major drawback. These materials can cause catastrophic wear of opposing tooth structure under certain conditions. The most extreme damage occurs when a roughened surface contacts tooth enamel or dentin under high occlusal forces, which may occur because of bruxing, premaguided disclusion is ensured, the wear of opposing enamel and dentin will be greatly reduced. In addition, if the occluding ceramic surface area is periodically refinished after occlusal adjustment or frequent exposure to carbonated beverages and/or acidulated phosphate fluoride, the abrasive wear of opposing tooth structure is further reduced.

Abrasive wear mechanisms for dental restorative materials and tooth enamel include (1) adhesion (metal and composites), in which localized bonding of two surfaces occurs, resulting in pullout and transfer of matter from one surface to the other, and (2) microfracture (ceramics and enamel), which results from gouging, asperties, impact, and contact stresses that cause cracks or localized fracture. For ceramic and enamel, two-phase brittle strucutres are involved. The ceramic onsists of a glass matrix that contains variable levels and sizes of crystals. Tooth enamel consists of a small volume fraction of organic phase matrix and a high volume fraction of hydroxyapatite crystals. The wear of either material depends on the ease with which cracks can propagate through the structure. If microscopic cracks are forced to pass around the crystal particles rather than through them, the material will usually be more fracture and cracks through the glass phase, the particles are less fracture resistant than the glass matrix, or excessive voids or other defects exist along the pathway. The relative strengthening effect is dependent on several factors, including the strength of the glass and crystal phases, the size and spacing of crystalline paricles, the inferfacial bond strength of the crystal-glass interphase region, and the type and magnitude of residual stresses in the structure. These factors are beyond the control of the dentist, although the dentist and laboratory technician can select ceramics that are highly fracture-resistant.

The microfracture mechanism is the dominant mechanism responsible for surface breakdown of ceramics and the subsequent damage that a roughened ceramic surface can cause to tooth enamel surfaces. eNamel is also susceptible to

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this kind of microfracture through four specific mechanisms: (1) asperities extending from the ceramic surface that produce high localized stresses and microfracture; (2) gouging that results from high stresses and large hardness differences between two surfaces or particles extending from these surfaces; (3) impact or erosion that occurs through the action of abrasive particles carried in a following liquid such as saliva; and (4) contact stress microfracture that increases localized tensile stress and also enhances the damage caused by asperties, gouging, and impact or erosion. Because of microfracture mechanisms, it may be necessary to polish the ceramic surface periodically to reduce the height of asperities and to minimize enamel wear rates. Of major concern is the potential catastrophic damage that can be incurred by enamel in contact with polycrystalline asperities having high fracture toughness (KIC) values such as alumina (3.5-4.0 Mpa.m1/2) or cerium-stabilized zirconia (10-16 Mpam1/2), yttrium-stabilized zirconia (6-9- Mpa.m1/2), or cerium--stabilized zirconia (10-6 Mpa.m1/2). In contrast glass has a fracture toughness of only 0.75 Mpa.m1/2 and should cause less gouging, contact stress, and impact damage within contacting enamel surfaces.

The abrasiveness of ceramics against enamel is affected by numerous factors and properties of the crystal phase particle and the glass matrix (if present). These include hardness, tensile strength, fracture toughness, fatigue resistance, particle glass bonding, particle-glass interface integrity, chemical durability, exposure frequency to corrosive chemical agents (acidulated phosphate fluoride, carbonated beverages), abrasiveness of foods, residual stress, subsurface quality (voids or other imperfections), magnitude and orientation of applied forces, chewing and bruxing frequency, contacting area, lubrication by saliva, and wear frequency. Thus it is understandable why the hardness of the ceramic is not a good predictor of the potential wear of enamel surfaces by a ceramic. However, the larger the hardness difference between two sliding surfaces, the greater is the degree of gouging.

Wear of ceramics Compared with other MaterialsTo minimize enamel abrasion by a contacting ceramic structure, we should

use a ceramic that exhibits uniform surface microfracture at the same rate as tooth enamel under the same conditions of loading, antagonist structure, food substance abrasiveness, applied forces, and degree of lubrication. The breakdown of the ceramic surface should be uniform so that asperities such as large crystalline inclusions do not project out from the surface.

These asperities produce high stress concentration areas within the opposing enamel surface that lead to gouging, troughing, and greater localized microfracture of the enamel structure. If such nonumiform surface wear of ceramics occurs duraing oral function, the only solutions available to reduce enamel wear are to reduce the occlusal load by occlusal adjustement or to polish the ceramic surface periodically to reduce stress concentrations and the height of these asperities.

Some of the new ultralow-fusing ceramics have a wide range of thermal expansion coefficients as listed. These are approximate values estimated for

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several low-fusing ceramic products produced by the Ducera company: Duceragold (7800 C) and Duceram LFC (Dentsply Ceramco) were introduced between 1991 and 1992. Duceram LFC is classified as a hydrothermal ceramic that was claimed to develop a hydrothermal layer approximately 1m thick vitro. In theory, this property allows a protective layer to seal microscopic surface cracks. Its veneer is a low-fusing ceramic that minimizes shrinkage of the core ceramic during subsequent firings. Duceram LFC and Duceragold do not contain large leucite crystals and therby retain a stable thermal expansion coefficient over several firings. The opalescence the fluorescence are also easier to achieve than for conventional low-fusing fieldspathic porcelains because of the ability to maintain very small crystal particles (400-500nm). Because of its high expansion coefficient, Duceragold is intended as a veneer for high-expansion, low-fusing porcelains, were subsequently developed as veneering ceramics for titanium metals and Procera AllCeram (Nobel Biocare, Goteborg, Sewden) core ceramics, respectively. TiCeram is another ultralow-fusing ceramic and has a firing temperature of approximately 7400C. The initial veneering porcelain for Procera AllCeram was Vitadur N (Vita Zahnfabrik, Bad Sackingen, Germany), a large-particle aluminous porcelain. Currently, AllCeram porcelain (Degussa Dental) is used. Finesse, an ultralow-fusing ceramic (~7600C) that contains larger leucite crystals, was introduced by Dentsply Ceramco, Inc. (E. Windsor, NJ). Vita Omega 900 (Vita Zahnfabrik) is another ultralow-fusing ceramic. For CAD-CAM processing, dicor MGC glass-ceramic (Caulk/Dentsply, Milford, DE), Vita MK I ceramic (Vita Zahnfabrik), and Vita MK II ceramic (Vita Zahnfabrik) blocks are available, which also offer a small-particle distribution of crystals that may reduce wear of opposing enamel surfaces.

It is not known what effect, if any, thermal mismatch differences will produce on the surface quality of these ceramics. Microcracking can lead to surface flaws, loss of surface material, and increased wear of enamel. However, this effect should only occur when a gross mismatch occurs between a core ceramic and its veneering ceramic.

Based on a study of abrasion by a 500-g slurry of glass (Derand and Vereby, 1999), 100g alumina (100 m), and 120g water, the mean wear depths (in microns) after a specified time period for several ceramics and tooth enamel were as follows: enamel (24.3), Finesse (20.3), Vitadur Alpha (16.3), Procera (15.8), Dentsply Ceramco II (13.6), Vita Omega (13.1), Ti-Ceram (12.1), IPS Empress (11.8), Duceragold (11.5), and Creation (10.8). The enamel wear was significantly greater than that of all ceramics tested. The wear depth of Vitadur Alpha was significantly than that of all ceramics tested. The wear depth of Vitadur Aplha was significantly greater than that for IPS Empress, Duceragodl, and Creation ceramics. It is clear that the relative wear rate of enamel by a highly abrasive medium is greater than that of most porcelains. Wear of Enamel by Ceramic Products and other restorative materials

Another factor that can increase wear of ceramics against enamel is the non-uniform distribution or clustering of crystals. IPS Empress after hot-pressing at

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1800 C exhibits clusters of relatively large (5-10 m) leucite crystals (KAISi2O6) with cracks between the crystal agglomerates. This non-interlocking arrangement of elucite crystals also occurs in the veneering ceramic after it is sintered at 9100C. in contrast, IPS Empress2 core ceramic exhibits a uniform dispersion of smaller lithia disilicate (LiSi2O4) crystals after hot pressing at 9200 C and veneering at 8000

C. One should expect greater wear of enamel by IPS Empress compared with

IPS Empress2. IPS Empress ceramic contains 35 5 vol% of leucite crystals that are formed in a noninterlocking particle cluster pattern (Holand et al, 2000). The core microstructrue of IPS Empress2 is quite different from that of IPS Empress, evidenced by elongated lithia disilicate crystals 0.5 to 4 m in length and a smaller concentration of lithium orthophosphate crystals (Li2Si2O5) approximately 0.1 to 0.3 m in diameter (Holand et al, 2000).

Studies of ultralow-fusing ceramics have generally revealed significantly lower enamel wear rates than those produced by conventional low-fusing porcelains. However, the results of a recent study suggest that one of these ceramics, Duceram LFC, caused significantly more enamel wear (0.197 mm3), presumably because of the higher void volume within the surface layer of Duceram (Magne et al, 1999). In this study, the combined enamel/ceramic wear rates were significantly greater for Duceram LFC (0.363 mm3). Veneering ceramics contain either large crystalline filler particles or a glass structure with no crystals or very small crystals.

Similarly, the results from another in vitro study (Al-Hiyasat et al, 1999) of enamel wear (after 25,000 simulated chewing cycles) by ceramics using a corn meal slurry (three-body condition) revealed greater relative wear depth in enamel by Duceram LFC (0.74mm) and Vitadur Alpha (0.80 mm) compared with Vita Cerec Mk II (0.48 mm). The explanation given for the higher wear rate of enamel by Duceram LFC was the presence of porosities within the surface of the ceramic. This result for Duceram LFC is in contrast with two-body wear data (al-Hiyasat et al, 1998a) that indicated significantly less enamel wear by Duceram LFC also Vita Mk II (0.65 mm) and a gold alloy (0.09 mm) and Vita Omega porcelain (0.96 mm). The wear Vitadur Alpha porcelain (0.93 mm) and in distilled water without an abrasive food medium (0.54 mm after 25,000 simulated chewing cycles) compared with Vitadur Alpha porcelain (0.93 mm) and Vita Omega porcelain (0.96 mm). The wear rate of enamel by gold alloy was significantly less than by the four ceramics.

One would expect the latter types of ceramics to cause minimal wear of enamel. Metzler et al (1999) reported that the relative enamel loss was less for two lower fusing ceramics, Finesse (0.56) and Vita Omega 900 (0.60), compared with Dentsply Ceramco II porcelain (0.85), large-particle leucite-based porcelain.

Krejci et al (1994) reported significantly lower estimated 5-year enamel wear rates for amalgam (50 m) and a new Cerec CAD-CAM ceramic (95 m), Vita Cerec Mk II V7K, compared with the original Vita Cerec Mk I ceramic (225 m). In comparison, the wear of enamel by enamel was 107 m. Hacker et al

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(1996) found considerably lower enamel wear rates for a gold-palladium alloy (9 m) compared with AllCeram veneer ceramic (60 m) and Dentsply Ceramco porcelain (230 m).

Jagger et al (1995) reported the following wear depths of dentin after exposure to wear by restorative materials: amalgam (0 m), microfilled composite (7 m), gold alloy (16.7 m), conventional composite (31.7 m), and Vitadur N aluminous porcelain (100 m). these results indicate that direct filling materials are less abrasive to dentin than aluminous porcelain. This result is not surprising. What is of importance of importance is the significant potential benefit of amalgam and microfilled composite as the least abrasive restorative materials fr situations in which dentin is exposed.

Al-Hiyasat et al (1998b) investigated the effect of a carbonated beverage (Coca Cola) on the wear of human enamel and three dental ceramics: Vitadur Alpha (feldspathic porcelain), Duceram LFC (ultralow-fusing porcelain), and Vita Mark II, a machinable ceramic. Tooth and ceramic specimens were tested in a wear machine under a load of 40 N, at 80 cycles per minute, for a total of 25,000 cycles. The test was performed in distilled water or with intermittent exposure to a carbonated beverage (Coca Cola). When tested in water Alpha porcelain caused significantly more enamel wear and also exhibited greater wear than Duceram LFC and Vita Mark II. However, after exposure to the carbonated beverage, the enamel wear produced by Duceram LFC did not differ significantly from that produced by Alpha porcelain. Vita Mark II produced the least amount of enamel wear. Exposure to the carbonated beverage significantly increased the enamel wear. The wear of Duceram LFC and vita Mark II increased with exposure to the carbonated beverage. It was produced by Duceram LFC and Vita Mark II ceramics. Overall, Vita Mark II was the most resistant to wear and also significantly less abrasive than Vitadur Alpha porcelain.

Reducing Abrasiveness of Ceramics by Polishing and Glazing: In theory, the smoothest surface should cause the least wear damage to

opposing surfaces. Depending on the initial surface roughness of the ceramic surface, glazing the surface may not adequately decrease the surface roughness since the glassy layer may be of insufficient thickness to fill in scratches and grooves within the ground surface. Thus, under certain conditions, polishing or polishing followed by glazing may be required.

Jagger and Harrison (1994) reported that the amount of enamel wear produced by both glazed (28.8 m) and unglazed Vitadur N aluminous porcelain (29 m) was similar; however; the wear produced by polished porcelain (12m) was substantially less. Polished or glazed porcelain caused significantly less wear than unglazed porcelain. Polishing was accomplished with 3M Soflex disks and Shofurubber points.

After 25,000 cycles of abrasion testing of various porcelain surface on human enamel in vitro, Al-Hiyasat et al (1997) reported no significant difference between the enamel wear of glazed and polished groups, but wear produced by the

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unglazed groups was significantly higher (P<0.05). Sixty pairs of tooth-porcelain specimens were tested under load in distilled water with and without intermittent exposure to a carbonated beverage. Wear of enamel and Vitadur Alpha porcelain specimens was determined after 5,000, 50,000 and 25,000 cycles. Exposure to carbonated Coca Coal and schweppes beverages significantly increased the amount of enamel wear produced by all porcelain surfaces (P <0.001). The finish of the porcelain surface did not influence its wear resistance under these conditions.

Guidelines for Minimizing Excessive Wear of Enamel by Dental ceramics: To minimize the wear of enamel by dental ceramics, the following steps

should be taken: (1) ensure cuspid-guided disclusion: (2) eliminate occlusal prematurities: (3) use metal in functional bruxing areas; (4) if occlusion in ceramic, use ultralow-fusing ceramics (5) polish functional ceramic surfaces; (6) repolish ceramic surfaces periodically; and (7) readjust occlusion periodically if needed.

Some ultralow-fusing ceramics are less abrasive than traditional low-fusing ceramics, but few clinical studies have been reported on any of these materials to validate the in virto findings. Caution should be exercised in selecting these new ceramics for use since they exhibit widely variable expansion co-efficients and may not be thermally compatible with certain ceramic core materials or metal substrates. Malocclusion is likely the major wear-causing factor that must be avoided to achieve acceptable wear performance with any ceramic product.

Polishing is preferred over glazing as a procedure to reduce abrasion damage of enamel. Ceramic surfaces should be refinished periodically after acid exposure especially acidulated phosphate fluoride. The Shofu porcelain polishing kit followed with diamond paste or SofLex discs (3M) without a diamond paste follow up are useful as effective finishing products.

Ceramic and opposing surfaces should be examined periodically for evidence of excessive wear. Occlusal adjustment and polishing of the ceramic surfaces should be performed to reduce the risk for further surface degradation. Noble metal surfaces are especially indicated for individuals who exhibit evidence of severe bruxing since the wear rates of gold alloys are very low compared with the wear damage caused by either traditional ceramics or recent lower-fusing ceramics.

A rough ceramic surface that is in hyperocclusion with opposing enamel is very likely to cause great abrasive wear of tooth surfaces. to minimize the risk of such wear damage to tooth enamel or other surfaces, the smoothest possible ceramic surface should be produced. This can be accomplished by (1) polishing only, (2) polishing followed by glazing, or (3) glazing only. The second choice is preferred. Glazing is recommended whenever possible before cementation of a prosthesis. if this is not possible, polishing along is acceptable. However, glazing of a very rough ceramic surface may not sufficiently reduce the surface roughness to minimize wear damage. It is clear that a glazed rough, nonglazed surface because the more-abrasive crystalline particles tend to be covered by the lessabrasive glass phase.

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it is not always possible to polish a ceramic surface in the clinic. Because of the heat generated during the polishing of ceramic-based prostheses that require extensive polishing, the temperature increase of pulpal tissue may lead to irreversible pulpitis. This is especially true when the tooth has been greatly reduced in size and the pulp chamber is within 0.5 mm of the external surface of the prepared tooth.

There are several clear indications for polished ceramic surfaces. Polishing of ceramic prostheses should be performed when they cannot be autoglazed. Polishing of ceramic restorations that have functional occlusal pathways or subgingival extensions will ensure optimal smoothness. All CAD-CAM inlays or other ceramic prostheses that will not receive veneering ceramic should also be polished.

Intraoral instrumentation can produce a smoother surface than an autoglazing procedure. Highly polished porcelain may also be naturally glazed or overglazed without significantly increasing the surface roughness. Increased time or cycles of glazing with decrease surface roughness.

Polishing instruments should be selected according to type of ceramic, type of restoration, and level of smoothness desired. If the crown was ground with a 100 m grit diamond, the first polishing abrasive should be 75 m or less. If the abrasive is too fine, more time will be needed to polish the surface.

Hulterstrom and Bergman (1993) found that two of the best polishing systems are Sof-Lex disks (3M Dental) and Shofu Porcelain Laminate Polishing Kit followed by diamond paste. For the Sof-Lex disks, one should start with a disk that is the most effective at removing the initial grinding patterns. If the abrasive grit size on the disk is too small, it will take too long to decrease the roughness. If the grit size on the disk is too small, it will take too long to decrease the roughness. If the grit size is too large (e.g., extra-coarse or coarse), the surface will become rougher. Also, if the polishing procedure is performed in the mouth, care should be taken to avoid heat build-up. Sof-Lex Sof-Lex disk are made to be used in a wet environment, so water coolant should be used whenever possible.

LUTING AGENTSThe development of new materials such as all-ceramic restorations has

brought about a substantially different attitude concerning luting agents.While zinc phosphate cement has been the standard for nearly a century, its overall properties are insufficient for certain types of restorative systems. For example, this traditional cement, which has been quite successful for metal-based restorations, falls far short for porcelain restorations. When zinc phosphate cement is used to lute all-ceramic restorations, bulk fractures can be expected. Specifically, during mastication energy is transmit. A series of cores fabricated by means of a pressing process. A low-fusing porcelain (Finesse, Dentsply Ceramco) fused to a gold metal substrate —a typical example of this material’s potential. In the case of zinc phosphate cement, the energy is distributed in localized regions at the restoration-preparation interface. The application of high levels of stress in localized zones then predisposes the restoration

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to fracture. The fractures associated with the early all-ceramic crowns could be attributed to this phenomenon. The ability to bond not only to the surface of the tooth but also to the internal surface of the ceramic restoration reduces this potential appreciably. When all contact surfaces are bonded as a single structure, the masticatory energies are distributed more uniformly. Consequently, fractures within the ceramic restoration are reduced significantly. It is important, therefore, to select a luting agent that exhibits the appropriate properties. Such a cement should not only bond to the surfaces of the preparation through the hybridizing process but also to the internal surfaces of the ceramic restoration by means of silanation and acid etching. Finally, the cement should exhibit sufficient flexural strength, flexural modulus and fracture toughness. Other desirable properties include color stability and sufficient release of fluoride to offer protection against secondary caries. This luting agent should possess dual-cure potential as well as multiple viscosities for use with different types of restorations. Newer composite-based resins such as Calabra (Dentsply Caulk) show promise in fulfilling these requirements.

PORCELAIN INLAYS/ONLAYSPorcelain is indicated primarily for restoring gingival cavities in the buccal or

labial surfaces of the teeth, where the restoration is free from direct force applications. Cavities are prepared with uniform thickness to the inlay, with the outer walls tapered outward only slightly.

Cavities on the proximal surfaces of the anterior teeth are occasionallyl restored by means of porcelain inlays. For access for impression and placement of inlay labial and lingual walls and the cavity should be removed. Etching the finished inlay with hydrofluoric acid and undercutting the cavity slightly after the impression is taken aid in developing a cement bond for adequate inlay retention.

Incisal angles and edges of the anterior teeth may also be restored with porcelain inlays. Usually a step is prepared in the remaining portion of the tooth and plantium pins are cemented in the dentin to provide adequate retention.

Porcelain is rarely indicated for either occlusal or two surface inlays in the posterior teeth because of its tendency to fracture under direct force application.

For inlay fabrication, the procedure that permits the fusion of the porcelain directly on the ceramic investment die material. Three types of investment materials have been developed for this purpose, including a silicate, a phosphate, and a calcium sulfate bonded type, having differential physical properties.

Although the process eliminates the platinum matrix, it is time consuming and does not effectively shorten procedure. Results on one study did not yield on inlay with significantly improved margin accuracy over that obtained with the conventional platinum matrix technique.

The conventional technique for porcelain inlays, with the platinum matrix, has been modified to what has recently been described as the “rapid inlay technique”. The use of this modification has increased in popularity. Essential to the procedure in the proper cavity design and the formation of the accurate impression on good dies, as well as careful adaptation of the platinum matrix. In this technique, time is varied in

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the three firing operation by insertion of the porcelain to be fused directly into the furnace at 12050C on a special quick heating tray after it has been dried briefly by being placed in front of the furnace door. The firing time at this temperature is about 25 seconds, after which it is air cooled to room temperature. The second and third applications of porcelain are made with comparable firing time for each. This method has significantly shortened the inlay firing operation.

The cementing medium in an important factor in the final appearance of the restoration, since shrinkage tends to make porcelain inlays undersize and a fine cement line may become visible at the margins. The more translucent silico phosphate rather than the opaque zinc cements is sometimes selected as a cementing medium for porcelain restorations.PORCELAIN VENEERSIndications:1. Discoloration – Tetracycline, revitalization, fluorosis and even teeth darken with

age.2. Enamel defects like hypoplasia and malformations3. Diastemata4. Malposition and malocclusion5. Poor restorations6. Aging7. Wear pattern8. Agenesis of lateral incisorContraindications:1. Non-availability of enamel2. Oral habits like bruxism, tooth to foreign body object habits.Advantages:1. Color and esthetics 2. Bond strength3. Periodontal health4. Resistance to abrasion5. Inherent porcelain strength6. Resistance to fluid absorption Disadvantages:1. Time involved2. Difficulty in repair3. Technique sensitive4. Tooth preparation5. Fragility of material6. CostTooth reduction:1. Labial reduction 0.3 to 0.5mm2. Incisal or lingual finishing line should be modified chamfer3. Sulcular marginal placement exactly at gingival margin with rounded shoulder

preparation.

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Methods of fabrication:1. Refractory investing technique2. Platinum foil technique3. Cost ceramic laminate techniquea. Castable ceramic (Dicor)b. Castable appetite (cerapearl)

Etching of enamel:1. Neighbouring teeth are protected by mylar strips or a dead soft metal matrix band2. Etched with 30-37% phosphoric acid solution for 15 seconds.3. Washed with copious amount of water for 30 seconds.Etching of porcelain veneer:1. Etch the inner surface using a etching gel 7.5% hydrofluoric acid constantly

brushed upwards to the periphery of the laminate and allow to stand for 7-10 minutes.

2. Completely submerge the veneer in a 10% solution of baking soda and water until the acid is neutralized

Bonding and porcelain veneers forms the following layers (from tooth surface outside)1. Etched enamel2. Combined E-D bonding agent (Light activated)3. Composite resin luting agent layer4. Unfilled resin layer5. Silane coupling agent to enhance adhesive properly of resin6. Etched porcelain veneer

Expanded use of porcelains:1. Conventional – lingual etched porcelain retainers2. For posteriors – buccal etched porcelain retainers3. Combination of buccal and lingual etched retainers4. Interproximal retainers5. Etched porcelain pieces.

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PORCELAIN JACKET CROWN AND PLATINUM REINFORCED CROWNSThe original porcelain jacket crown, made of feldspathic porcelain, possessed

excellent esthetic but was very prone to fracture. With the development of aluminous reinforcement, the restoration again generated interest amongst dentists. It is still an inherently weak restoration, however its use should be restricted to incisors, where a maximum cosmetic result is necessary.

More than any other restoration, the porcelain jacket crown depends on tooth support which is more critical for the fracture resistance of the restoration than the bulk of porcelain. The “Crescent moon fracture” often seen in this type of restoration is a direct result of inadequate preparation length.

The recommended incisal reduction range from 1 to 1.4 to 2mm. To produce adequate esthetic result, it is best to reduce the incisal edge by 2mm. Any greater reduction will increase the stress in the facial surface which can result in the facial half moon fracture.

Axial reduction if facial and lingual surfaces should about 1mm. Recommendations for reduction of the lingual surface range from 0.5 to 1mm. All the line angles to be round off.

A shoulder 1mm wide is generally preferring shoulders narrower than 1mm should be reserved for those teeth too small to safely permit more than minimal destruction of tooth structure.

Finally sufficient dentin must remain to provide a strong core.Maximum accuracy requires dies that is hard enough to with stand the

pressures of fabrication, an acrylic resin backed, silver plated die should always be used. Undercuts below the shoulder must be blocked out by acrylic resin prior to foil adaptation. If undercut is not blocked it makes difficult to remove and causes distortion on removal.

The finished jacket preparation therefore should have the appearance of a miniature tooth, with no undercuts and shoulder about 0.5 to 1mm wide placed slightly beneath the gingival tissues around the entire tooth.

The separating medium is applied to all parts of the die that will be contacted by block out material. The block out material and excess foil will be removed later for final finishing and evaluation.

Foil adaptation:To fabricate the porcelain jacket crown a piece of 0.025mm or 0.001 inch

platinum foil of correct shape and size is selected. The foil is annealed in a Bunsen burner flame, quenched and carefully wrapped around the die to give the basic matrix form. The shoulder area can be formed with the thumbnail. A pair of fine scissors are used to remove excess foil. A tinner’s joint approximately 1.5mm wide is formed on the mesial or distal aspect and along the incisal edge.

The uppermost piece of foil is reduced by half, and a wedge shaped section axio-incisal corner. A wedge may also be removed at the axiiogingival corner. The longer edge of the foil is folded over the shorter one and the tinner’s joint closed.

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The foil is burnished gently but firmly with a beaver foil burnisher. Burnish from the incisal edge to the shoulder, beginning at the joint and then around the preparation from the joint. once the basic shape is established, the foil is removed from the die and excess foil removed from the skirt leaving a 1-2mm extension. The skirt adds strength and stability to the matrix. Reannel the foil, replace it on the die and complete the burnishing.

If a wedge shaped section was not removed from the tinner’s joint at the axiogingival corner of the shoulder, then four thickness of foil will remain. If so, the area can be carefully reduced with a rubber wheel with care not to perforate the foil. Remove and reanneal the foil to burn of any remaining debris. The procedure of annealing is to remove gaseous impurities from the platinum. The matrix is degassed in an oven at 21000C F for 6 minutes under vacuum and replace the matrix on the die.

Platinum reinforced crowns:For the fabrication of a platinum reinforced crown either – a twin foil

technique.Single foil technique maybe usedA twin foil technique requires the fabrication of the second foil with the

Tinner’s joint on the opposite side from the first to be adapted on the die. The skirt of the second foil is removed. The first foil replaced on the die, and a second foil pushed into place over it. Since it is the second foil that will later remain in the crown. It is removed, abraded tin-plated, oxidized and replaced.

In the single foil technique, atleast one day prior to foil fabrication, the die, except for the shoulder portion is spaced with die relief material equal to one thickness of foil. After the foil is adapted, the should skirt portions are protected prior to tin plating by covering them with either inlay wax or cyanocrylate gel. This burns of during oxidation, leaving an area of clean foil to which the porcelain will not bond. After the finished crown has been glazed, this area of foil is removed with a sharp scalpel and the die spacer removed from the die to produce an accurate fit.

Aluminous Core Porcelain Build Up:Although the core material is different, the technique for core fabrication is the

same for both the standard porcelain jacket crown and the platinum reinforced crown. Platinum reinforced core material is slightly more opaque to mask out the gray colour of the oxidized foil.

The core material is mixed to a stiff consistency with the recommended liquid, blotted to remove excess liquid applied to the foil. An even thickness of 0.5mm should cover all of the foil with the exception of the skirt. The core material should be well condensed and excess moisture removed by blotting. After the basic core buildup. The die is replaced on the cast and the occlusion adjusted as necessary. More porcelain may be added to reinforce high stress areas, such as the cingulum of maxillary anteriors or the marginal ridge area of premolars. The lingual area should also be thickened, as with a metal coping, for extra strength.

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In core design, the high stress cervical areas should be taken into consideration. If these are areas of excess tooth reduction, the core thickness is increased to compensate and allow even and optimum veneer porcelain thickness of approximately 1mm. After completing the core build up, excess material is removed from the skirt and the porcelain slightly ditched with the tip of the brush. A ditch is made in the porcelain all around the shoulder to prevent contraction of the shoulder portion of the matrix during firing. The porcelain is condensed well by vibrating and blotting to avoid porosity. The core is then fired. Aluminous core porcelain is fired under vacuum to 11200F with the first firing.

In other method of firing system, the porcelain is fired from 15000F to 19000F for six minutes. The vacuum is broken and the porcelain is raised to 20000F and held for 15 minutes. With the first firing, there will be some shrinkage away from the shoulder and perhaps some fissuring. The surface of the porcelain should be ground with a clean diamond stone and any fissures opened slightly. The foil is checked to ensure adaptation and reburnished if necessary, it is then placed in an ultrasonic cleaner.

A more fluid porcelain mixture is used for the second core bake to allow the material to flow into any small discrepancies and ditched area. The porcelain is condensed and fired. The second core firing is better done in air because subsequent core firings in vacuum may cause ‘bloating’ or bubbling of the porcelain. The core porcelain is allowed to mature in air at 11200C for 15 minutes to obtain higher density and therefore higher strength.

Firing requirements of core porcelain:1. Raise firing temperature slowly 1500C per minute, rapid drying and firing

increase the risk of fissuring in the core.2. Break vacuum immediately on reaching maximum maturing temperature.

Continues firing in vacuum will cause hone comb porosity.3. Air fire for at least 15 minutes at maximum maturing temperature or even

higher for extra strength.Alumina crystals improves density and bonding of crystals to the porcelain

matrix and strength can be greatly increased.4. After cooling check degree of fusion by immersing in ink surface in dense with

no honey comb porosity, the dye wash off clearly.

Dentin porcelain:Mix dentin porcelain with distilled water on a glass slab to a creamy

consistency. Moisten the core porcelain with distilled water and apply the weak porcelain in small increments to the gingival area using the moist brush, smooth the porcelain towards the incisal area so that it is chamfered to a fine edge about 1/3rd the distance from the cervical margin. Make certain that the porcelain has a gradual chamfer since a sharp angle can create demarcation lines. Condense the porcelain by vibrating and drying with a paper tissue.

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Gingival or cervical dentin porcelain and body porcelain are different in colour bleeding.

Apply the body dentin porcelain to the remaining core porcelain using the incremental brush technique work the porcelain from the incisal edge first and carry it down the labial surface towards the gingival porcelain. In this way the body porcelain will blend easily with the slightly moist gingival porcelain.

Continue to build around the approximal areas but do not fill in approximal space of the gingival. Always maintain the correct contour. Extend the porcelain incisally to a length of 1mm to allow for shrinkage. Finally build up the lingual portion of porcelain by dentine porcelain to the correct occlusal relationship.

Porcelain will always shrink towards its greatest bulk and these areas are to be found at the incisal, the proximal and at the supra-bulge, line and point angles.

Enamel porcelain:Before mixing the enamel and translucent porcelains make sure that the dentin

build up is kept moist. Check that atleast 0.5mm of dentin porcelain is covering the core at the incisal with a periodontal probe. Remove the other half of the incisal dentin and smooth the surface using brush. Apply enamel porcelain to the surface, making sure that each increment is positioned very gently.

Smooth the enamel porcelain towards the dentin blend line and extend it proximally, do not alter the original contour of the tooth. The finished enamel should be extended by 1 to 1.5mm beyond the adjoining central incisor. Thicker enamel reduce value and increase translucency.

The restoration is dried and then placed in the oven to 15000CF to 19500F under total vacuum. It is held at that temperature for one minute and then removed.

The restoration is adjusted, tried in the mouth and final morphological features are added, it is glazed by placing it in the muffle at 19500F is held at that temperature for two to four minutes without vacuum before removal from the furnace.

The matrix is then removed by prying it away from the shoulder with a sharp instrument. The loose edge is grasped firmly with cotton pliers and the platinum is twisted out.Aesthetic Potential of Metal - Ceramic crowns Versus All - Ceramic Crowns:

Although metal -ceramic prostheses account for about 70 % of all fixed restorations a metal - ceramic (MC) crown is not the best aesthetic choice for restoring a single maxillary anterior tooth. A ceramic crown offers a greater potential for success in matching the appearance of the adjacent natural tooth, but ceramic crowns are more susceptible to fracture, especially in posterior sites. A dark line at the facial margin of a MC crown occasionally associated with a metal collar or minimized margin is of great concern when gingival recession occurs. This effect can be minimized by using a ceramic margin or by using a very thin knife - edge margin of Mattel coated with opaque shoulder porcelain. The technician should polish and glaze this margin to or with very thin knife - edge metal margins on the facial surface are successful procedures for improving the aesthetics.

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Sintering of porcelain The thermocemical reactions between the porcelain powers components are

virtually completed during the original manufacturing process. Therefore the purpose of firing is simply to sinter the particles of powder together properly to form the prosthesis. Some chemical reactions occur during prolonged firing times or multiple firings. Of particular importance are the observed changes in the Lucite content of the porcelains designed for fabrication of metal - ceramic restorations. Locution is a high - expansion ( and hinge- contraction) crystal phase whose volume fraction in the glass matrix can greatly affect the thimble contraction coefficient of the porcelain. Changes in the Lucite content can cause the development and the metal, which can produce tensile stresses during cooling that are sufficient to cause crack formation in the porcelain.

The condensed porcelain mass is placed in front of or below the muffle of a preheated furnace at approximately 6500 C (12000 F) for low- fusing porcelain. This preheating procedure permits the remaining water vapor water vapor to dissipate. Placement of the conduced mass directly into even a moderately warm furnace results in a rapid production of steam, there by introducing voids fracturing large sections of the veneer. After preheating for approximately 5min, the porcelain is placed into the furnace, and the firing cycle is initiated.

The size of the powder particles influences not only the degree of condensation of the porcelain but also the soundness 0r apparent density of the final product. At the initial firing temperature, the voids are occupied by the atmosphere of the furnace. As sintering ;;of ;the particles begins, the porcelain particles bond at their points of contact. As the temperature is raised, the sintered glass gradually flows to fill up the air spaces. However, air becomes trapped in the form of voids because the fused mass is too viscous to allow all the air to escape. An aid in the reduction of porosity in dental porcelain is vacuum firing.

Vacuum firing reduces porosity in the following way. When the porcelain is plac4d in the furnace, the power particles are packed together with air channels around them. As the air pressure inside the furnace muffle is reduced to about one-tenth of atmospheric pressure by the vacuum pump, the air around the particles is also reduced to this pressure. As the temperature rises, the particles sinter together, and closed voids are formed within the porcelain mass. The air inside these closed voids is isolated from the furnace atmosphere. At a temperature about 55o C (99oF) below the upper firing temperature, the vacuum is released and the pressure inside the furnace increases by a factor of 10, from 0.1 to 1 atm. Because the pressure is increased by a factor of 10, the voids are compressed to one-tenth of their original size, and the total volume of porosity is accordingly reduced. Not all the air can be evacuated from the furnace. Therefore a few bubbles are present in vacuum-sintered porcelains, but they are markedly smaller thank the ones obtained by air-firing. A finished metal-ceramic multiple unit bridge. Overglazing and shading ceramics As shown in Table 21-2, natural glazed (autoglazed or self-glazed),medium-fusing feldspathic porcelain is much stronger than ground, rough, nonglazed porcelain.If the glaze is removed by grinding, the transverse

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strength may be 40 to 46% less than that of the porcelain with the glaze layer intact. The glaze is effective in reducing crack propagaion within the outer surface because the surface flaws may bebridged and the surface will be under a state of compressive stressive stress. However, the results from one study indicate that porcelains with highly polished surfaces (1-umabrasive paste) have comparable strength to that of specimens that were polished and glazed (Fairhurst et al, 1992). This observation is of clinical importance because after the porcelain prosthesis is cemented kin the mouth, it is common practice for the dentist to adjust the occlusion by grinding the surface of the porcelain with a diamond bur, unfortunately, this procedure weakens the porcelain markedly if the glaze is removed and the surface is left in a rough condition.

If the porcelain surface is rough, a natural glaze treatment is recommended since the fracture resistance of the surface is greater than that of unglazed porcelains. Porcelains for metal-ceramic and ceramic prostheses, porcelain veneers, or denture teeth may be characterized with stains and glazes to provide a more lifelike appearance. The fusing temperatures of glazes are reduced by the addition of glass modifiers that lower the chemical durability of glazes somewhat. Stains are simply glazes and are subject to the same chemical durability problem However, most of the currently available glazes have adequate durability if they are as thick as 50 um or more.One method for ensuring that the applied characterizing stains will be permanent is to use them internally. Internal staining and characterization can produce a lifelike result, particularly when simulated enamel craze lines and other features are built into the porcelain rather than merely applied to the surface. The disadvantage of internal staining and characterization is unsuitable.

It is logical to assume that fine polishing of a roughened surface followed by glazing produces smoother surface than polishing alone, sandblasting followed by glazing, or diamond grinding followed by glazing. A highly polished and glazed surface is smoother than the surfaces of glazed specimens that have been sandblasted or roughened with a diamond followed by glazing.Cooling of Metal-Ceramic Prostheses

The proper cooling of a porcelain prosthesis from its firing temperature to room temperature is the subject of considerable controversy. The catastrophic fracture of glass that has been subjected to sudden changes in temperature is a familiare experience. The cooling of dental porcelain is a complex matter, particularly when the porcelain is fused to a metallic substrate. Multiple firings of a metal-ceramic restoration can cause the coefficient of thermal contraction of the porcelain to increase and can actually make it more likely to crack because of tensile stress development.

The chief limitation to the use of an all-porcelain crown in fixed prosthodontics is its lack of tensile strength. A method for minimizing this disadvantage is to fuse the porcelain directly to a metal coping that fits the prepared tooth. Such a metal-ceramic prosthesis is shown schematically in figure 21-1. The metal on the facial side is approximately 0.3 to 0.5mm thick. It is veneered with opaque porcelain approximately 0.3mm in thickness. The body porcelain is about 1mm thick.

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If a stronger material is used as an inner core of a ceramic crown, cracks can develop only when the stronger material is deformed or broken, assuming that the veneering porcelain is firmly bonded to the stronger substrate. With proper design and physical properties of the porcelainand metal, the porcelain is rein-forced so that brittle fracture can be abided or at least minimized when these crowns are restricted to anterior teeth. Although most metal-ceramic prostheses involve cast melt copings, several novel noncast approaches (sintering, machining, swaging, and burnishing) to coping fabrication have been developed in recent yearsCreep or Sag Resistance

Unfortunately, high-temperature creep or sag of some high noble and noble alloys occurs when the temperature approaches 9800 C(18000 F). The creep can be reduced if the metal has the proper composition so that a dispersion strengthening effect occurs at the high temperature. When such gold alloy is heated to 9800 C or higher, a second phase is precipitated that can harden or strengthen the alloy such creep has been reduced in some of the commercial alloys, but it apparently cannot be eliminated. The solidus temperature (the lower end of the melting range) of base metall alloys, such as nickel-chromium, is higher than that ofgold alloys; hence, base metal alloys are les susceptible to sag than are gold-based alloys.

However, the newer ultralow-fusing veneering ceramics that were introduced in the early 1990s are fired at lower temperatures(sintering temperatures below 8500C) than traditional low-fusing porcelains. For alloys with solidus temperatures of 10000C or higher, creep (sag) deformation should be negligible.

Copings for Metal-Ceramic Prostheses: Four types of process for producing a metal coping for metal-ceramic prostheses are available: (1) electrodeposition of gold or other metal on a duplicate die, (2) burnishing and heat-treating metal foils on a die, (3) CAD-CAM processing of a metal ingot, and (4) casting of a pure metal (CP Ti) or an alloy (high noble, noble, or predominantly base metal) through the lost-wax process. This discussion focuses primarily on the third option since it is the most widely used process.

The traditional development of the metal-ceramic prosthesis was the result of advances in the formulation of both alloys and porcelains. To bond a ceramic veneer to cast alloy copings, a ceramic must have a fusion temperature well above its sintering temperature and, it also must have a co-efficient of thermal contraction that is closely matched to that of the alloys. A metal oxide is necessary to promote chemical bonding of the ceramic veneer to the metal substrate. Foil copings may or may not require a metal oxide or a bonding agent to ensure retention of the veneering ceramic. The use of one type of foil coping is discussed in the following section.

The gold alloys developed for porcelain bonding have higher melting ranges than typical gold alloys for cast metal prostheses; the higher melting ranges are necessary to prevent sag, creep, or melting of the coping during porcelain firing. These gold alloys contain small amounts (about 1%) of base metals such as iron, indium, and tin, as discussed in Chapter 19. The base metals form a surface oxide layer during the so-called "degassing" treatment, and this surface oxide is responsible

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

The alloys and porcelains used for the construction of such restorations have a number of rather stringent requirements. For example, if undesirable residual tensile stresses in the porcelain are to be avoided, both the metal and the ceramic must have thermal contraction profiles on cooling that are closely matched or with the metal exhibiting a slightly higher contraction. If the contraction differences are large, stresses may occur that weaken both the porcelain and the bond. For example a difference in the coefficients of thermal contraction of 1.7 ppm/0 C can produce a shear stress of 280 mPa (39,800psi) in porcelain next to the gold-porcelain interface when the porcelain is cooled from 9540C (17500 F) to room temperature. Because the shear resistance to failure is far less than 280 mPa, these thermal stresses could cause spontaneous bond failure.

High tensile stresses are known to develop in porcelain veneers from a contraction coefficient mismatch between alloy and porcelain. The tensile stresses induced within the porcelain by occlusal forces would, of course, be added to residual thermal tensile stresses. However, when the metal and its porcelain veneer exhkibit similar contraction curves and an average contraction coefficient difference of 0.5ppm/0C or less (between the porcelain"s glass transition temperature and room temperature), fracture is unlikely to occur except in cases of extreme stress concentration or extremely high intraoral forces. These metal-ceramic combinations are know as thermally compatible systems. Many prostheses made from metal and porcelain materials having contraction coefficient differences between 0.5 and 1.0ppm/0C are known to survive for many years. These results are explainable by survival probability analyses that assume that the maximukm biting forcxes on anterior crowns rarely exceeds 2224 N (500lb). In fact, the Guinness book of records (1993) cites the maximum clenching force ever recorded for posterior teeth as 4337 N (975lb) between molar teeth and much lower forces between premolars and between anterior teeth. Thus a rather small number of patients have bite force capabilities that are likely to cause fracture of metal ceramic crowns or bridges even when residual thermal incompatibility stresses are present. As general rule , lower forces are generated by younger children versus older children, female patients versus male patients, a more closed bite versus a raised bite table, occlusion between natural teeth and denture teeth compared with the force generated between natural teeth against natural teeth.

Another equally important property of metal-ceramic systems is that the alloy should have a high proportional limit and, particularly, a high modulus of elasticity. Alloys with a high modulus of elasticity also share a greater proprtion of stress compared with the adjacent porcelain. The metal framework must not melt during porcelain firing and also must resist high -temperature"sag" deformation sag or flexural creep can occur only at high temperatures. It does not occur at oral temperatures.

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The metal-ceramic prosthesis is generally fabricated by a dental technician. The casting procedures are similar to those described for the casting of inlays and crowns. Because of the high melting temperature of the alloys, a phosphate-bonded investment must be used.

The casting should be carefully cleaned to ensure a strong bond to the porcelain. For example, an alloy such as Olympia (heraeus Kulzer), a gold-palladium, silver-free alloy, is heated in the porcelain furnace to a temperature of 10380C (19000

F) to burn off any remaining impurities and to form a thin oxide layer. In many alloy systems, this so-called degassing treatment does not actuallyk degas the interior structure of the alloy, but it does produce an oxide layer on the alloy surface that is essential for the formation of the porcelain-metal bond.

The need for a clean metal surface cannot be overemphasized. The surface may be cleansed adequately by finishing with clean ceramic-bonded stones or sintered diamonds, which are used exclusively for finishing. Final sandblasting with high purity alumina abrasive ensures that the porcelain is bonded to a clean and mechanically retentive surface.

Opaque porcelain is condensed with a thickness of approximately 0.3mm and is then fired to its maturing temperature. Translucent porcelain is then applied, and the tooth form is built. Porcelain powder is applied by the condensation methods previously described. The unit is again fired. Several cycles of porcelain application and firing may be necessary to complete the prosthesis. A final glaze is then obtained.

Metal-Ceramic Crowns Based on Burnished Foil Copings:Captek (Precious Chemicals Company Inc.) is a technology that is based on

the principle of capillary attraction to produce a gold composite metal. The Captek P and G metals can produce thin metal copings for single crowns or frameworks for metalceramic fixed partial dentures (FPDs) with a maximum span length of 18mm (that allows space for up to two pontics). Captek is an acronym for "capillary casting technology". The finished metal coping may be described as a composite material consisting of a gold matric reinforced with small particles of a Pt-Pd-Au alloy. The inner and outer surfaces contain approximately 97% Au. The grain size of the foil is 15 to 20 m.

Malleable Captek metal strips are burnished on a refractory die to fabricate the metal coping of a metal-ceramic crown without the use of a melting and casting process. Examples of Captek metal-ceramic crowns are shown in Figure. The procedural steps are described below.

Starting with a master model of the prepared tooth or teeth, a Captek refractory die is produced. The Capsil relief liquid is sprayed onto the stone die(s) to reduce surface tension, allowing Capsil impression material or Capvest refractory die material to flow freely and to reduce bubbles and imperfections. After die spacer is applied to the master die, undercuts are blocked out. The master die is placed in the proper size-duplication flask, and Capsil silicone material is poured into the flask around the master die(s). The master die is removed from the hardened silicone, and an accurate high-temperature refractory is puoured into the new silicone mold.

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The refractory die is heat-treated, the margins are marked with a red pencil, and an adhesive (Captek Adhesive) is applied to the die. The adhesive aids the adhesion of Captek material to the die and also enhances capillary attraction.

The next step requires the application of Captek p, a highly malleable gold, platinum, and palladium alloy. This internal reinforcing skeleton provides a three dimensional network of capillaries that will be eventually filled by Captek G material forming a composite high gold metal alloy. This layer is burnished on the die, and after the margins are trimmed, it is sintered in a porcelain furnace on the recommended Captek thermal processing cycle.

This capillary structure is next infused with molten gold that is supplied by the Captek P material. The Captek P is pressed in place using uniform firm pressure so it conforms to the shape of die. Excessive pressure or pulling of the material will cause it to tear or break. After trimming away any excess material, any voids are filled with trimmings of Captek P segments.

The crown and/or bridge units are now ready for firing. The pieces are heated at a rate of 550 to 800 C/minute to the recommended firing temperature. Next, Captek G strips are applied. The Captek G metal strip contains 97.5 wt% gold and 2.5 wt% silver. The copings and/or bridge components are fired again in the furnace according to the recommended Captek firing cycle. After processing, Captek G forms a high gold metal alloy composite through capillary attraction. The capillary action is used as the joining method of the hard particles and the resilient particles of metal present n the final coping. The gold melts, yet the Captek P structure remains stable. The Captek composite metal coping is now complete and ready for refractory die removal. The coping is now complete and ready for refractory die removal. The coping is divested, and the margins are finished. For the production of FPDs (bridges), Captek pontics are used. These are specially designed pontics that are precast using a metal-ceramic alloy and plated with pure gold.

The copings and/or pontics are coated with a mixture of a powder and liquid (Capbond), which will provide a thin covering of gold material to enhance areas of Captek P that have been ground during adjustment. The Capbond will also provide a gold color that is identical to that of areas that have not been ground.

Capcon liquid and powder are applied to areas between pontics and abutments, as well as pontic surface areas. The powder contains an alloy of gold, platinum, and palladium to provide the same base material structure as Captek P. Carcon absorbs, through capillary attraction, the Capfil material in the same way that Captek G is absorbed into Captek p, producing a gold connecting area that enhances the color of the pontic surface. Capcon is also used to enhance the buccal or labial pontic areas of Captek preformed pontics.

The units are veneered with tow thin layers of opaque porcelain and other veneering porcelain layers. Before the opaque layer is applied, the finished Captek coping has a thickness of approximately 0.25 mm. Thus this technique provides a much thinner coping thickness than traditional cast metals (0.5mm) and will provide additional space for veneering porcelain. The marginal adaptation is dependent on the skill of the technician in trimming the burnished materials. Unlike traditional cast

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metals that provide atomic bonding to opaque porcelain through an external oxide layer, Captek metal achieves bonding to opaque porcelain through as external oxide layer, Captek metal achieves bonding through a combination of surface interlocking and residual stresses produced by slight differences in thermal expansion coefficients. Although Captek metal is indicated for crowns and FPDs, no long-term clinical data are available to determine the survivability of Captek ceramic prostheses.

Bonding Porcelain to Metal: The primary requirements for the success of a metal-ceramic prosthesis is the

development of a durable bond between the porcelain and the alloy. Once such a bond is achieved, there is an opportunity to introduce stresses in the prosthesis during the porcelain firing procedures. An unfavorable stress distribution during the cooling process can result in cracking of the porcelain, and delayed fracture can also occur. Thus, for a successful metal-ceramic prosthesis to be realized, both a strong interface bond and thermal compatibility are required.

Theories of metal-ceramic bonding have historically fallen into two groups: (1) mechanical interlocking between porcelain and metal and (2) chemical bonding across the metal-porcelain interface. Although chemical bonding is generally regarded to be responsible for metal-porcelain adherence, evidence exists that, for a few systems, mechanical interlocking may provide the principal bond. The oxidation behavior of these alloys largely determines their potential for bonding with porcelain. Research into the nature of metal-porcelain adherence has indicated that those alloys that form adherent oxides during the degassing cycle also form a good bond to porcelain, whereas those alloys form no external oxide at all but rather oxidize internally. It is for these alloys that mechanical bonding is needed.

A variety of tests have been advocated for measuring the bond strength. None can be regarded as an exact measure of the adhesion of porcelain to metal except in cases in which the metal-porcelain couple is matched thermally so that porcelain adjacent to the interface is essentially stress-free. This is a situation virtually impossible to attain because the metal exhibits a linear contraction behavior as a function of temperature and the porcelain exhibits a nonlinear contraction plot.

Clinical fractures of metal-ceramic restorations, although rare, still occur, especially when a new alloy or porcelain is being used or when a new coping technology has been adopted. As is generally true for all dental materials, there is a learning curve associated with the initial use of new products. When fractures occur, it is a good idea to make a vinyl polysiloxane impression of the fracture site for future fractographic analysis. All information on the crown or bridge should be recorded. Including the visual appearance of the fracture site. Although there are in infinite number of fracture paths that may occur, three types are of particular importance in diagnosis the cause of fracture. Shown in figure are fracture paths that have occurred primarily at three sites: (1) along the interfacial region between opaque porcelain (P) and the interaction zone (1) between opaque porcelain and the metal substrate (top); (2) within the interaction zone (center); and (3) along the interfacial region between the metal and the interaction zone (bottom). For conventional metal-ceramic crowns

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made from cast copings, the interaction zone is usually synonymous with the metal oxide layer.

For coping made using atypical methods such as the technologies associated with the Captek system, and electroforming processes, bonding to porcelain is achieved either through a combination of mechanical interlocking and residual stresses that occur because of a metal-ceramic contraction mismatch or through the application of a bonding agent. To characterize the principal site of fracture, magnification of 3 to 100 times is required because a thin layer of retained porcelain may not be visible without magnification. Each of the three principal fracture paths in figure may be caused by excessive stress development, a material deficiency, of a processing deficiency. Bonding of Porcelain to Metal Using Electrodeposited Substrates:

Ceramic bonding to metals in certain cases requires the electrodeposition of metal coatings and heating to form suitable metal oxides. Deposition of a layer of pure gold onto the cast metal and a subsequent short “flashing” deposition of tin have been shown to improve the wetting of porcelain interface. In addition, the electrodeposited layer acts as a barrier between the metal casting and the porcelain to inhibit diffusion of atoms from the metal into the porcelain, within the normal limits of porcelain firing cycles.

A light color of the oxide film enhances the vitality of the porcelain when compared with the dark oxides that require heavy opaque layers to mask dark unaesthetic oxides. Because the activated surface can be controlled from golden reddish-brown to gray, an additional dimension is available for color control of the porcelain.

Alloys and metals such as cobalt-chromium, stainless steel, palladium-silver, high-and low-gold-content alloys, and titanium all have been successful electroplated and tin-coated to achieve satisfactory ceramic bonding. Various proprietar agents are also available that are intended for application to the metal surface before condensation of the opaque porcelain layer. These are applied as a thin liquid to the metal surface and are fired in a manner similar to that of opaque porcelain.

The function of these agents is twofold: (1) they are intended to improve metal ceramic bonding by limiting the build-up of an oxide layer on the base metal surface during firing, and (2) they can improve aesthetics by helping to block the color of the dark metal oxide.Benefits and Drawbacks of Metal-Ceramics:

The properly made crown is stronger and more durable than the ordinary aluminous porcelain crown. However, a long-span bridge of this type may be subject to bending strains, and the porcelain ma crack or fracture because of its low ductility. These difficulties can be partly overcome with proper prosthesis design, as discussed earlier. Proper occlusal relationships are also particularl important for this type of prosthesis.

The most outstanding advantages of metal-ceramic prostheses are the permantnt aesthetic quality of the properly designed reinforced ceramic unit and their resistance to fracture. Unlike similar acrylic resin veneered structures, almost no wear

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of the porcelain occurs by abrasion and there is no staining along the interface between the veneer and the metal. Furthermore, as shown in a clinical stud, the fracture rate of metal-ceramic crowns and bridges is as low as 2.3% after 7.5 years (Coornaert et al, 1984).

A slight advantage of metal-ceramic prostheses over ceramic prostheses is that less tooth structure needs to be removed to provide the proper bulk for the crown, especially if metal only is used on occlusal and lingual surfaces. As previously noted, high rigidity of the structure is needed to prevent fracture of the porcelain. Ver little flexibility can be sustained by dental porcelains because of their moderately high modulus of elasticity e.g., 69 Gpa compared with values of 99.3 Gpa for a Type IV gold alloy, 22.4 Gpa for amalgam, and 16.6 Gpa for a resin-based composite) and their relatively low tensile strength. As a result, only limited elastic deformation of the porcelain approximately (less than 0.1% strain) can be tolerated before fracture occurs. It follows, therefore, that a sufficient bulk metal is necessary to provide the proper rigidity. The minimal metal coping thickness necessary in the occlusal region is approximately 0.3mm. the shape of the crown cannot be conspicuously out of line with the anatomic form of adjacent teeth. Therefore the bulk of the natural tooth may need to be sacrificed to provide adequate space to ensure adequate fracture resistance and aesthetics.REVIEW OF MATERIALS1) Optek HSP (Leucite-Reinforced porcelain):

Optek HSP (Jeneric Pentron Inc. Walingord CT) is a high leucite porcelain designed to make an all ceramic crown without a core. A very esthetic system, it can be built on foil or on a refractory die. Although the strength as improved over a normal feldspathic porcelain, it is not expected to be as strong as those built with a core.Advantages: Offers a very esthetic crown system.Disadvantage: A slightly higher rate of fracture exposed.

2) The aluminous porcelain jacket crown:Porcelain jacket restoration has been in use since Land introduced it in 1903.

Fracture of the restoration was a problem and, in 1965 McLean and Hughes introduced the aluminous porcelain jacket crown. Porcelain was strengthened by dispersion of alumina particles in a “core” porcelain which could be built over platinum foil to a thickness of 0.5 to 1.0mm. Traditional (feldspathic) porcelain could then be build over this core. The core contained 40% to 50% alumina, which strengthened the porcelain jacket crown by about 50%, while still allowing good light penetration and esthetics.

The composition of aluminous porcelain is similar to that of conventional porcelain except for the increased alumina content (40-50%). The alumina particles are much stronger than quartz and have a higher modulus of elasticity. Since the fracture of porcelain is essentially caused by propagation of cracks through the structure, the alumina crystals tend to obstruct the path of the crack.

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Aluminous porcelain is an opaque porcelain used to construct the core layer of the porcelain jacket crown as an initial layer to mask the color of the tooth dentin or the color of the metal. If it is metal ceramic restoration, the opaque agent is usually the ziroconium oxide.

Advantage: The aluminous porcelain jacket crown offers excellent esthetics, improved strength.Disadvantages: This material is not indicated for posterior teeth, fixed partial dentures, or in cases of heavy bruxism. The core is sometimes bright at the neck.

3) Hi-Cream:The authors Southan and Jorgensen found that a refractory die could get better

marginal adaptation by wetting it directly than by using the platinum foil technique. Subsequently, Hi-Cream system was developed to make an aluminous porcelain jacket crown on a refractory die.Advantages: The crowns are designed to be built on a refractory die.Disadvantages: Material not indicated for posterior teeth. FPD and heavy bruxism.

4) Al-Cream (Cerestore, Cast core):In 1983 Sozio and Riley and Coors Biomedical Company (Lakewood CO)

introduced the Cerestore system. It employs a heat stable epoxy die and an aluminous core porcelain which is injection moulded by controlling time and temperature of the firing cycle, it was believed that a shrink free ceramic system resulted.

Injection moulded technique:The coping wax pattern is fabricated on a heat stable epoxy resin die. The die

and wax pattern are invested in a special flask by plaster, and the wax can be removed with boiling water and the flask is oven heated at 1800C for 40 minutes.

In this system aluminous magnesium core material containing about 70% alumina with a thermosetting binder having a transition temperature of 1600C is used, which is heat softened and injection moulded under air pressure in to the preheated mould cavity, formed by the lost wax process on a specially formulated epoxy die material compatible with this technique.

This special epoxy die is heat stable at the moulding temperature and undergoes uncontrolled expansion during curing of the epoxy.

On cooling the thermoset material sets and is disinvested from the mould. The green state ceramic core still on the epoxy die and is then adjusted by core is then removed from the die and fired in a furnace under precise time and temperature control for several hours upto a maximum of 13000C. After firing the alumina magnesium core is then veneered, with a special veneer porcelain similar to aluminous porcelain following a conventional technique.

Thus expanded epoxy die in combination with magnesium core, provide for compensation of firing shrinkage. Because the first layer of ceramic in this technique is molded directly against a die of the preparation, an excellent marginal fit is

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possible. Its compressive strength does not differ significantly from that of conventional aluminous porcelain crowns. At present time, this system is used for single unit restorations.Advantages: Offers improved strength, excellent esthetics and marginal adaptation, shrinkage compensation mechanism.Disadvantages: Cost, time of manufacture, unavailable for purchase today.

5) In cream:In cream composed of fine sized particles of aluminium oxide core with

significantly improved strength, developed by Dr. Sadoun in Paris and recently tested by Seghi et al. The core is fired for 10 hours period in a special furnace on a refractory die. Conventional veneer porcelain can then be build to contour.Advantages: System offer strength (4 times greater than regular) and excellent marginal adaptation.Indications: May be used for anterior esthetic areas, possible posterior use and FPD.

6) Dicor (Cast Glass Ceramic/Cast Ceramic Crown):Dicor (Dentsply International, York, PA) is a castable glass system composed

of SiO2, K2O, MgO, MgF2 which is manufactured by Corning Glass Works. Hence the full contour of the crown is built with Dicor.Technique of fabrication:

Crowns are formed by full contoured anatomic wax pattern which are invested in a phosphate bonded investment. Once the wax has been eliminated in a burnout furnace, a castable glass ceramic material is heated to a molten state (A special glass is cast at about 14000C) and cast in the mold on a centrifigual casting machine. After cooling to room temperature, the casting of transparent amorphous glass is removed from the investment, cleaned and trimmed then subjected to heat treatment.

For the heat treatment reinvest in a refracgtory material and fire it in a special furnace. This converts transparent week restoration into a semicrystalline translucent restoration. The maximum temperature required for this firing is 10750C for several hours. This increase the strength and promotes the development of a crystalline phase of tetrasilica (Mica) within a glass matrix.

When the colour is deep within the tooth, this method fall short. An alternative technique is to cast the glass as a core and bake veneer porcelain over it (Dicor plus, Will’s glass).

In other system the castable ceramic material developed in Japan, the ceramming process produce hydroxyapatite crystal in the glass matrix.Advantages: Ease of fabrication, chameleon effect, conservation of tooth structure (1.2 to 1.3mm reduction is adequate), flexibility of fabrication, adequate strength for anterior teeth, good marginal adaptation, excellent biocompatibility, less plaque accumulation (7 times less according to Sevit et al). 7) Sunrise:

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Another gold foil supported ceramic system that uses ceramico II porcelain. If either sunrise or renaissance system could be used with a core porcelain, strength would be appreciably high, but no such research has been found in the literature. 8) Procera (Noble Pharma, Chicago, USA IL):

Promotes the use of commercially pure titanium coping which is fabricated through milling and spark erosion process. The process takes the stone die and through the use of a copy milling machine, makes three graphite copies and one plastic *Ureol) copy die. On the plastic die the contours of a routine metal coping are waxed. This die with the newly added external coping contour, is placed in the copy milling machine and a copy of the external contours is milled in 99.6% commercially pure titanium. At this time, only the external contours have been duplicated, leaving the titanium copy still solid. Next, the solid titanium coping is inverted into low fusing metal which holds it in place for the spark erosion process. The graphite die is placed in the spark erosion unit and used as template for the internal contours of the coping. Two to four graphite dies are needed to ensure accurate replication of the contours of the preparation. This is because of the degradation of the die from the heat generated by the spark erosion process. The coping is now hand finished on the die and Ti-Cream is applied as specified.

A low fusing porcelain Ti-cream is developed specially for this process applied in a conventional manner. The advantages proclaimed by the manufacturer are:2) Consistency – none of the problems associated with casting of alloys.3) Biocompatibility of pure titanium4) A porcelain kinder to opposing natural dentition.5) Accurate unit relationships through laser welding on the master castDisadvantages:2) No esthetic advantage over other metal ceramic system3) Limited number of laboratories offers this systemIndication: Procera might be used in metal sensitive patients.IPS empress (Ivoclar USA, Amherst, NY):

A new addition for metal free ceramic restoration, uses a wax up, lost wax technique. The ceramic is leucite reinforced and comes in pre cerammed cylinders in various shades. The wax is invested in a special flask with a newly developed investment and placed into the base of the compress press-furnace. The correct shade cylinder is heated to 11000C at which the cylinder becomes plasticized. The cylinder is then heat pressed under a vacuum into the mould where it is hold under pressure to allow complete and accurate fill of the investment cavity. The system uses a special light cure die material and a new Chromas Cop shade guide.Advantages: High flexure strength, no shrinkage after pressing procedure, dimensionally stable after several firings. Used for inlays, onlays, crowns and veneers.Disadvantages: System is expensive.

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Chan and Weber studied plaque retention with different crown systems and found ceramic crowns exhibited little plaque retention, whereas ceramic metal crowns retained less plaque than natural teeth.

Wear Rate: Monasky and Taylor found that the rougher the porcelain surface, the faster the wear rate. Porcelains should be highly polished before glazing to minimize wear of opposing tooth structure.RECENT ADVANCES IN PORCELAIN1. Heat pressed ceramics (IPS empress):

IPS empress ceramic system used to fabricate crowns, inlays, onlays and veneers. In this system a ceramic ingot using high temperature (11000C) pressing fabrication procedure is converted to a ceramic restoration. The technique consists of slowly forcing a softened ceramic into a mould made by traditional lost wax process. Esthetics can be enhanced by applying an enamel layers of feldspathic porcelain or surface stains on the facial surface of anterior crowns and veneers.

By heat pressing, thermal expansion of glass and ceramic are said to induce compressive stresses at the interface and strengthens the restoration.

Heat pressing significantly improves the flexure strength, whereas heat treatment alone does not affect the strength.Procedure:1) Wax the restoration to final contour, sprue and invest as with conventional gold

castings.If the veneering technique is used, only the body porcelain shape is waxed.

2) Heat the investment to 8000C to burn out the wax pattern.3) Insert a ceramic ingot and alumina plunger in the sprue and place the refractory in

the special pressing furnace.4) After heating to 11500C slowly press the softened ceramic into the mold under

vacuum.5) After pressing recover the restoration from the investment by air-abrasion, remove

the spure and refit it to the die.

2. Slip cost alumina crowns and FPD’s (Inceram):The inceram system uses a fine grain alumina core material.

Technique:1) Inceram coping produced directly on special plaster die by brush technique2) Heat pressed coping prepared by injection moulded technique.Moulded technique:

The inceram system is used in the slip cost alumina technique. Here a fine grain alumina core prepared on the porous plaster die, duplicated from the working cost. The alumina slip is ultrasonically mixed and applied to the plaster die and subsequently fired in a special porcelain furnace. The resultant porous coping is infused with a special glass to give a high strength core material. Later this core is veneered with an esthetic feldspathic porcelain.

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Procedure:1) Duplicate the working die with an elastomeric impression material and pour it

with the special plaster. Any undercuts must be blocked out first and two coats of die spacer applied. When the plaster has fully set (2 hours) remove the die, mark the margins, and apply the wetting agent.

2) Mix the appropriate shade of alumina slip with ultrasonic agitation, place the mixture under a vacuum, brush apply it to the plaster die and shape with a blade, trimming back to the margins carefully.

3) The slip is fired in a special furnace initially through a prolonged drying cycle to 1200C that dries the die material, which shrinks away from the core. Then the alumina is fired at 11200C (20480F).The resulting core is porous and weak at this stage but can be carefully transferred to the master die after the die spacer is removed. The relatively low sintering shrinkage (about 0.3%) is compensated for by an expansion of the special plaster.

4) Paint a thick coat of the approapriate shade of glass mixture onto the surface of the core and fire at 11000C (20120F).

5) Remove excess glass from the core by grinding and air abrasion.6) Apply body and incisal porcelain to the core in the same manner as described for

traditional porcelain jacket crowns.CAD-CAM restorations (Computer Assisted Design/Computer Assisted Manufacture):All ceramic restorations:

Since 1985, 4 major CAD-CAM systems for fabricating prosthesis have emerged. The Duret (French) system, the Minnesota system (Denticad), Cicero (Dritch) system are the only three capable of producing complete crowns and FPD’s. The cerac system (Swiss) is limited to inlays.

Cerac system is a CADCAM ceramic restoration with a laser imaging camera that constructs the tooth preparation in 3 dimensions. Here a special video camera is used to scan the prepared tooth data from the image is manipulated by the dentist to design a restoration that is machined from a ceramic block using computerized milling machine. Operator can also program by design of inlay/onlay in the same manner. Presently layers of colors and thus esthetics cannot be built into the restoration. The surface colouration concept similar to Dicor’s seems the best available esthetic choice today. Dicor restorations are made of a castable glass ceramic having an advantage (1) low abrasion of opposing enamel, (2) Low plaque accumulation.Advantages: CAD-CAM eliminates the problems that arise from direct application techniques currently employed.Disadvantage: High cost, inaccuracy of marginal adaptation, and inability to built layers of color and translucency.CADCAM (Ceramic fused to metal restorations):

The Cicero system for the production of ceramic fused to meal restoration used the optical scanning, nearly, net shaped metal and ceramic sintering and computer aided crown fabrication techniques.

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This system produces crowns, FPD’s with different layers such as metal, dentin and incisal porcelain for proximal strength and esthetics .DEFECTS IN CERAMICS

Fabrication defects are created during processing and consist of voids or inclusions generated during sintering. Microcracks develop upon cooling in feldspathic porcelains and are due to thermal contraction mismatch between either the crystals and the glassy matrix (Mackert et al, 1994) or between the porcelain and the metal substrate. Condensation of a ceramic slurry by hand prior to sintering may introduce porosity. Sintering under vacuum has been shown to reduce the amount of porosity in dental porcelains from 5.6 to 0.56% (lones and Wilson, 1975). Porosity on the internal side of clinically failed Dicor glass-ceramic restorations has been shown to constitute a fracture initiation site (Kelly et al 1990). Surface cracks are induced by machining or grinding.The average natural flaw size varies from 20 to 50 micrometers (Anusavice et al, 1991). Usually, failure of the ceramic originates from the most severe flaw. The size and spatial distribution of the flaws justify the necessity of a statistical approach to failure analysis (Weibull, 1939). Surface crystallization of leucite can be induced by seeding the surface of a feldspathic glass with leucite particles (Holand et al, 1995). Ceramic materials for all ceramic restorations are in contact with refractory die materials during firing or pressing at high temperatures. Surface reactions have also been reported between glass-ceramics and the refractory embedment used during the crystallization process, thereby modifying the mechanical properties of the final product (Campbell and Kelly, 1989; Denry and Rosenstiel, 1993). Diffusion processes are temperature-dependent, and surface reactions are likely to occur between the porcelain and the refractory die material.

Degradability of Dental Ceramics:The degradation of dental ceramics generally occurs because of mechanical

forces or chemical attack. The possible physiological side-effects of ceramics are their tendency to abrade opposing dental structures, the emission of radiation from radioactive components, the roughening of their surfaces by chemical attack with a corresponding increase in plaque retention, and the release of potentially unsafe concentrations of elements as a result of abrasion and dissolution. The chemical durability of dental ceramics is excellent. With the exception of the excessive exposure to acidulated fluoride, ammonium bifluoride, or hydrofluoric acid, there is little risk of surface degradation of virtually all current dental ceramics. Extensive exposure to acidulated fluoride is a possible problem for individuals with head and/or neck cancer who have received large doses of radiation. Such fluoride treatment is necessary to minimize tooth demineralization when saliva flow rates have been reduced because of radiation exposure to salivary glands. Porcelain surface stains are also lost occasionally when abraded by prophylaxis pastes and/or acidulated fluoride. In each case, the solutes are usually not ingested. Further research that uses standardized testing procedures is needed on the chemical durability of dental

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

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