Seminar
On

Dental Ceramics – Metal Ceramics
Presented by : Submitted to:
Yashendra. Dr Kamlesh Vasudeva Dr Siddharth Phull Dr Anuj Wangoo Dr Sakshi Malhotra Dr Amit Sharma Dr Shiv Kumar

INDEX Sr. No. | Topic | Page no. | 1 | Introduction | | 2 | History | | 3 | Terminology | | 4 | Classifications of Dental Ceramics | | 5 | Properties of Ceramics | | | Metal Ceramic Prostheses | | 6 | Compositions of Porcelain – for Metal Ceramics | | 7 | Requirements for a metal – ceramic system | | 8 | Effect of design on metal ceramic restorations | | 9 | Tooth preparation for PFM restorations | | 10 | Fabrication of porcelain | | a | Porcelain Condensation | | b | Sintering of Porcelain | | c | Method of fabrication | | 11 | Metal-Ceramic Crowns Based on Burnished Foil Copings | | 12 | Failure and repair of metal ceramic restorations | | 13 | Benefits and Drawbacks of Metal-Ceramics | | a | Aesthetic Potential of Metal-Ceramic Crowns Versus All-Ceramic Crowns | | 14 | Conclusion | | 15 | References | |

INTRODUCTION
The word ceramic is derived from the Greek word keramos which literally means ‘burnt stuff’ but which has come to mean more specifically a material produced by burning or firing. The term ceramic refers to any product made from a non-metallic inorganic material usually processed by firing at a high temperature to achieve desirable properties. This material is opaque, relatively weak and porous and would be unsuitable for dental applications. It consisted mainly of kaolin.
The more restrictive term porcelain refers to a specific compositional range of ceramic materials originally made by mixing kaolin (hydrated aluminosilicate), quartz (silica), and feldspars (potassium and sodium aluminosilicates), and firing at high temperature. Dental ceramics for metal-ceramic restorations belong to this compositional range and are commonly referred to as dental porcelains.
Refinements in metal-ceramic systems dominated dental ceramics research during the past 35 years that resulted in improved alloys, porcelain metal bonding, and porcelains. The introduction of a “shrink-free” all-ceramic crown system and a castable glass-ceramic crown system in the 1980s provided additional flexibility for achieving esthetics results, introduced advanced ceramics with innovative processing methods, and stimulated a renewed interest in all-ceramic prostheses.
However, a variety of machinable ceramics are also available for chair-side fabrication of all-ceramic restorations by computer-aided design/computer aided manufacturing (CAD/CAM). As of today, the last major advance in dental ceramics comes with the introduction of transformation toughened zirconia.
HISTORY
In approximately 700 B.C the Etruscans made teeth of ivory and bone that were held in place by a gold framework. Animal bone and ivory from the hippopotamus or elephant were used for many years thereafter.
In the early 1700s many European rulers were spending enormous sums importing porcelain from China and Japan.
In 1774 de Che´man fabricated a Porcelain complete denture for Duchateau , that represented a huge step forward in personal hygiene. de Che´mant worked to improve translucency moving from the centre of the ternary (three-part) phase diagram towards a feldspar-rich formulation, characteristic of today’s feldspathic materials.
In 1808 Giuseppangelo Fonzi, significantly improved the versatility of ceramics by firing individual denture teeth, each containing a platinum pin allowing teeth to be fixed to metal frameworks.
In 1885 LOGAN fused porcelain to platinum post thus resolving the retention problem. He termed them as “RICHMOND CROWN”. These crowns represent the first innovative use of metal ceramic system.
Dr. Charles Land introduced one of the first ceramic crowns to dentistry in 1903. He described a technique for fabricating ceramic crowns using a platinum foil matrix and high-fusing feldspathic porcelain. These crowns exhibited excellent aesthetics, but the low flexural strength of porcelain resulted in a high incidence of failures.
VACUUM-FIRED PORCELAINS - Vines et al in 1958, developed finer porcelain powders for vacuum firing or low-pressure air firing must be regarded as the first major improvement in the esthetics and especially the translucency of all-porcelain crowns.
1959 - Porcelain fused to metal process & use of palladium based & gold based alloys by Weinstein et al.
METAL_ CERAMICS - The introduction of vacuum-fired porcelains and the bonding of porcelain to gold alloys (introduced by Weinstein et al in the early 1960s) were pivotal breakthroughs in dental esthetics.
LEUCITE FILLER CRYSTALS IN PORCELAIN – METAL-CERAMIC SYSTEMS AND STRENGTHENED ‘PRESSABLE’ CERAMICS - In 1962 with the development of a formulation that could be fired on common dental casting alloys.
A significant irnprovement in the fracture resistance of porcelain crowns was reported by Mclean and Hughes in 1965 when a dental aluminous core ceramic consisting of a glass matrix containing between 40 and 50 wt% Al2O3 was used. Because of inadequate translucency (opaque, chalky-white appearance) of the aluminous porcelain core material, a veneer of feldspathic porcelain was required to achieve acceptable aesthetics.
FOIL-REINFORCED PORCELAIN CROWNS - The first commercially viable foil-reinforced crown system was developed by McLean and Sced in 1976 known as Vita-Pt.
ALUMINA-REINFORCED CERAMICS - In the 1970s, manufacturers made collarless metal-ceramic crowns. Commercial shoulder porcelains were developed for direct-lift techniques.
In 1975, Garvie proposed a model to rationalize the good mechanical properties of zirconia, by virtue of which it has been called ‘‘ceramic steel’’.
Several different approaches were developed beginning in the mid 1980s through to the late 1990s to deal with or avoid shrinkage to provide prostheses that were, what an engineer would call, being made to a ‘net shape’ s/as a pressed ceramic powder ⁄ polymeric binder Cerestore , glass ceramic c/d DICOR, lightly sintering aluminum oxide c/d In-Ceram, pressed solid ingots of filled-glass c/d Empress, CAD/CAM parts from solid, full-dense blocks c/d CEREC or fabrication of an oversized die, pressing of alumina powder onto the die c/d Procera.
In 1987, Mo¨rmann and Brandestini introduced a prototype machine that would capture a 3-D image of a prepared tooth - CEREC I introducing the concept of Chairside CAD⁄CAM.
KEY TERMS * ALUMINOUS PORCELAIN: A ceramic composed of a glass matrix phase and 35% vol or more of AL2O3. * CAD-CAM CERAMIC: A machinable ceramic material formulated for the production of inlays and crowns through the use of a computer-aided design, computer aided machining process. * CASTABLE DENTAL CERAMIC: A dental ceramic specially formulated to be cast using a lost wax process/technique.. * COPY MILLING: A process of machining a structure using a device that traces the surface of a master metal, ceramic or polymer pattern and transfers the traced spatial positions to a cutting station where a blank is cut or ground in a manner similar to a key-cutting procedure. * DEVITRIFICATION: Occurs when there is excessive disruption of the glass forming SiO4 tetrahedra in dental porcelain resulting in the crystallization of glass or “devitrification”. Is often associated with high expansion glasses due to increased introduction of alkali’s (soda-Na2O) to increase the thermal expansion. Increase in sodium and potassium ions can cause too much disruption of the SiO4 tetrhedra and subject to devitrification, which appears as cloudiness in the porcelain, accentuated by repeated firings. Once devitrified, it is increasingly difficult to form a glaze surface on the porcelain. The regular or aluminous porcelain is less susceptible to devitrification due to their higher silica to alkali ratio. Consequently since the soda (Na2O) content is less, there thermal expansion is also lower. * FELDSPATHIC PORCELAIN: A ceramic composed of a glass matrix phase and one or more crystal phases of which the more important phase is leucite, which is used to create high expansion porcelain that is thermally compatible with gold-based, palladium-based and nickel-based alloys. Hence this class of dental ceramics are also called “leucite porcelain”. * FRITTING: The process of blending, melting and quenching the glass components is termed “fritting”. The term “frit” is used to describe the finer glass product. The raw mineral powders are mixed together in a refractory crucible and heated to a temperature well above their ultimate maturing temperature. The oxides melt together producing gases, which are allowed to escape and the melt, is then quenched in water. The red-hot glass striking the cold water immediately breaks up into fragments and this is termed the “frit”. * GLASS CERAMIC: A solid consisting of a glassy matrix and one or more crystal phases produced by the controlled nucleation and growth of crystals in the glass. * INCONGRUENT MELTING: Is the process by which one material melts to form a liquid plus a different crystalline material. * SINTERING: A process of heating closely packed particles to achieve interparticle bonding and sufficient diffusion to decrease the surface area or increase the density of the structure. * VITRIFICATION: Is the development of a liquid phase by reaction or melting, which on cooling provides the glassy phase. The structure is termed “vitreous”.
CLASSIFICATION
* According to Craig : Dental ceramics can be classified according to their applications, fabrication method, or crystalline phase :
Application Fabrication Crystalline phase Products Manufacturers
ALL-CERAMIC Soft-machined Zirconia (3Y-TZP) Cercon Dentsply Intl Lava 3M ESPE IPS e.max ZirCAD IvoclarVivadent In-Ceram YZ Vident Alumina (Al2O3) Procera Nobel Biocare In-Ceram AL Vident Soft-machined & glass-infiltrated Alumina (Al2O3) In-Ceram Alumina Vident Spinel (MgAl2O4) In-Ceram Spinell Vident Zirconia(12Ce-TZP/Al2O3) In-Ceram Zirconia Vident Hard-machined Lithium disilicate(Li2Si2O5) IPS e.max CAD Ivoclar Feldspar((Na, K)AlSi3O8) Vita Mark II Vident Leucite (KAlSi2O6) IPS Empress CAD Ivoclar Vivadent
Slip-cast Alumina (Al2O3) In-Ceram Alumina Vident Spinel (MgAl2O4) In-Ceram Spinell Vident Zirconia(12Ce-TZP/Al2O3) In-Ceram Zirconia Vident
Heat-pressed Leucite (KAlSi2O6) IPS Empress Ivoclar Vivadent Lithium disilicate(Li2Si2O5) IPS e.max Press Ivoclar Vivadent Fluorapatite(Ca5(PO4)3F) IPS e.maxZirPress IvoclarVivadent
Sintered Leucite (KAlSi2O6) IPS Empress layering IvoclarVivadent ceramic Alumina (Al2O3) Procera Allceram Nobel Biocare Fluorapatite(Ca5(PO4)3F) IPS e.max Ceram Ivoclar Vivadent layering ceramic
METAL-CERAMIC Sintered Leucite (KAlSi2O6) VMK-95 Vident
DENTURE TEETH Manufactured Feldspar Trubyte Dentsply Intl Feldspar Vita Lumin Vacuum Vident * Classification by Application : (1) ceramics for metal-ceramic crowns and fpd’s,
(2) all-ceramic crowns inlays, onlays, veneers, & fpd’s.
Additionally, ceramic orthodontic brackets, dental implant abutments, & ceramic denture teeth are available.
On basis of their area of application :
• Core porcelain: is the basis of porcelain jacket crown, must have good mechanical properties.
• Dentin or Body porcelain: more translucent than core porcelain, largely governs the shape and color of restoration.
• Enamel porcelain: is used in areas requiring maximum translucency, for example- at the incisal edge. * Classification by Fabrication Method : The most common fabrication technique for metal ceramic restorations is called sintering.
All-ceramic restorations can also be produced by sintering, but they encompass a wider range of processing techniques, including slip-casting, heat-pressing, and CAD/CAM machining. Some of these techniques, such as machining and heat-pressing, can also be combined to produce the final restoration. * Classification by Crystalline Phase : Depending on the nature and amount of crystalline phase and porosity present, the mechanical and optical properties of dental ceramics vary widely. Increasing the amount of crystalline phase may lead to crystalline reinforcement and increase the resistance to crack propagation but also can decrease translucency.

* According to Phillips’ science of dental materials: * Silicate ceramics - Dental porcelains with SiO2 (amorphous glass phase) as the main component and additions of crystalline Al2O3, MgO, ZrO2 and other oxides are included in this category. * Oxide ceramics - contains a principal crystalline phase of Al2O3 MgO, ThO2 or ZrO2 with either no or a small amount of glass phase. * Non-oxide ceramics - are impractical for use in dentistry due to their high processing temperatures and complex processing methods. Eg: Borides, Carbides, Nitrides * Glass ceramics- Dicor, Dicor MGC. * Also these products can be classified in several possible ways according to their: (1) use or indications (anterior posterior, crowns, veneers, post and cores, FPD’s, stain ceramic, and glaze ceramic);
(2) composition (pure alumina, pure zirconia, silica glass, leucite-based glass-ceramic, and lithia-based glass ceramic);
(3) processing method (sintering, partial sintering and glass infiltration, CAD-CAM, and copy-milling); (4) firing temperature (low -fusing, medium-fusing, and high-fusing);
(5) microstructure (glass, crystalline, and crystal-containing glass);
(6) translucency (opaque, translucent, and transparent);
(7) fracture resistance;
(8) abrasiveness. * Dental porcelains are classified according to their firing temperatures as follows: * High fusing 1300" C (2372" F) * Medium fusing 1101"-1300" C (201 3"-2072" F) * Low fusing 850"-1100" C (1562"-2012" F) * Ultra-low fusing 450" C (1562" F) * MICROSTRUCTURAL CLASSIFICATION : At the microstructural level, we can define ceramics by the nature of their composition of glass-to-crystalline ratio. There can be infinite variability of the microstructures of materials, but they can be broken down into four basic compositional categories, with a few subgroups:
• composition category 1 – glass-based systems (mainly silica),
• composition category 2 – glass-based systems (mainly silica) with fillers, usually crystalline (typically leucite or, more recently, lithium disilicate),
• composition category 3 – crystalline- based systems with glass fillers (mainly alumina)
• composition category 4 – polycrystalline solids (alumina and zirconia).
GENERAL PROPERTIES OF CERAMICS : The properties of dental ceramics depend on their composition, microstructure, and flaw population. The nature and amount of reinforcing crystalline phase present dictate the material’s strength and resistance to crack propagation as well as its optical properties.
1. PHYSICAL PROPERTIES OF PORCELAIN -
a) SURFACE HARDNESS & WEAR RESISTANCE: Porcelain is much harder than natural teeth. KHN – 460 (enamel 343) ;450 VHN (Enamel – 350 VHN). Porcelain is more resistant to wear than natural teeth.
b) THERMAL PROPERTIES: Porcelain has low thermal conductivity 1.05 (W m−1 ºC−1) [as compared to Enamel 0.92, Dentine 0.63], co-efficient of thermal expansion is 4 that is close to that of natural teeth 8 to 11.4 (ppm ºC−1).
Porcelain has excellent thermal properties and is a particularly good thermal insulator. This fact is of importance when gross amounts of enamel and dentine are to be replaced and the residual layer of dentine may be of minimal thickness.
c) OPTICAL PROPERTIES: R.I : 1.52 – 1.54 .They have good optical properties. They are translucent because of absence of free electrons. 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 opaque & body porcelain during building of the crown.
Surface staining: Disadvantages of surface staining are a lowered durability as a result of high solubility and reduction of translucency. Opaque porcelains have very low translucency values to mask metal substructure surfaces. Body porcelain translucency values range between 20% and 35%.Incisal porcelains have the highest values of translucency and range between 45% and 50%.
d) CREEP & Sag – High temperature creep/sag of some high noble /noble alloys occur at 980*C . However solidus temperature of base metal alloys are less susceptible than gold based alloys. New ultra low fusing veneering ceramics are introduced in 1990’s these can be fired at lower temperatures below 850 *C.
e)Specific gravity: The specific gravity of fired porcelain is usually less, because of the presence of air voids. It varies from 2.2 to 2.3
f)Dimensional stability: Porcelain is dimensionally stable after firing.
g)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. 2. BIOLOGICAL PROPERTIES : They have excellent biocompatibility. It is compatible with the oral tissues. The margins of finishing line can be even extended to the gingival sulcus
3.CHEMICAL PROPERTIES : It resist attack by chemicals. It is insoluble and impermeable to oral fluids. Also it is resistant to most solvents. Correctly formulated porcelain is very resistant to chemical attack, being unaffected by the wide variations of pH which may be encountered in the mouth. 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. They have to be roughened by etching with hydrofluoric acid or sand blasting to improve the retention of a cement to the internal surface of the restoration
4.MECHANICAL PROPERTIES:
a) Strength: Porcelain is a material having good strength. However it is brittle and tends to fracture. The strength of dental porcelain is usually measured by terms of 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) Good Compressive strength that is 48000 psi (321 MPa).
Tensile strength [5000 psi (35 MPa)]. 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. b) Exhibits little plastic deformation
Elastic modulus : Porcelain has a high modulus of elasticity [10 x 106 PSI (69 GPa)]
c) Fracture toughness for porcelain is 2.6 ( E- 0.6- 1.8; D- 3.1). The addition of up to 50 wt% zirconia to ceramic increases fracture toughness.
d) The brittleness of dental ceramics is compounded by their tendency to undergo static fatigue. This is a time-dependent decrease in strength, even in the absence of any applied load. The process is thought to occur through alkaline hydrolysis of Si–O groups within the porcelain structure.
METAL-CERAMIC PROSTHESES The more restrictive term porcelain refers to a specific compositional range of ceramic materials made by mixing kaolin, quartz, and feldspar, and firing at high temperature. Dental ceramics for ceramic-metal restorations belong to this compositional range and are commonly referred to as dental porcelains. Dental porcelain is used as veneers on metal frameworks (metal ceramic restoration) and on minimally prepared anterior teeth, and for denture teeth. Conventional dental porcelain is a vitreous ceramic based on a silica (SiO,) network and potash feldspar (K20-Al20,-6Si0,) or soda feldspar (Na20 .A I2O3-6SiO2) or both. Pigments, opacifiers, and glasses are added to control the fusion temperature, sintering temperature, thermal contraction coefficient, and solubility.
COMPOSITION OF PORCELAINS:
Acc. To Craig –
Opaque Dentin
Powder (Body)
Component (O/d Powder (010)
SiO , 50-59 57-62
Al2O3 9-15 11-16
Na,O 5-7 4-9
KzO 9-1 1 10-14
TiO, 0-3 0-0.6
ZnO, 0-5 0.1-1.5
SnO , 5-1 5 0-0.5
Rb,O 0-0.1 0-0.1
CeO - 0-3
Pigments - Trace

Components (%)
Clay Binder
Material (kaolin) Silica (feldspar) Glasses
Decorative porcelain 50 25 25 0
High-fusing dental 4 15 80 0 Low-fusing dental 0 25 60 15 * COMPOSITION OF HIGH-, MEDIUM-, AND LOW-FUSING BODY PORCELAINS (WEIGHT PERCENTAGE) : High-fusing Medium-fusing Low-fusing (vacuum fired) Metal-ceramic SiO2 72.9 63.1 66.5 59.2 Al2O3 15.9 19.8 13.5 18.5 Na2O 1.68 2.0 4.2 4.8 K2O 9.8 7.9 7.1 11.8 B2O3 — 6.8 6.6 4.6 ZnO — 0.25 — 0.58 ZrO2 — — — 0.39 * Composition of Dental Ceramics for Fusing to High-Temperature Alloys : Compound | Biodent opaque BG 2 (%) | Ceramco opaque 60 (%) | VMK opaque 131 (%) | Biodent dentin BD 27 (%) | Ceramco dentin T 69 (%) | SiO2 | 52.0 | 55.0 | 52.4 | 56.9 | 62.2 | Al2O3 | 13.55 | 11.65 | 15.15 | 11.80 | 13.40 | CaO | - | - | - | 0.61 | 0.98 | K2O | 11.05 | 9.6 | 9.9 | 10.0 | 11.3 | Na2O | 5.28 | 4.75 | 6.58 | 5.42 | 5.37 | TiO2 | 3.01 | - | 2.59 | 0.61 | - | ZrO2 | 3.22 | 0.16 | 5.16 | 1.46 | 0.34 | SnO2 | 6.4 | 15.0 | 4.9 | - | 0.50 | Rb2O | 0.09 | 0.04 | 0.08 | 0.10 | 0.06 | BaO | 1.09 | - | - | 3.52 | - | ZnO | - | 0.26 | - | - | - | UO3 | - | - | - | - | - | B2O3, CO2and H2O | 4.31 | 3.54 | 3.24 | 9.58 | 5.85 |
The main ingredients are:
• Feldspar - When potassium feldspar is mixed with various metal oxides and fired to high temperatures, it can form leucite and a glass phase that will soften and flow slightly. The softening of this glass phase during porcelain firing allows the porcelain powder particles to coalesce together. For dental porcelains, the process by which the particles coalesce is called liquid-phase sintering. feldspar melts incongruently at about 1150° C to form a glassy phase with an amorphous structure and a crystalline phase consisting of leucite, a potassium aluminosilicate (KAlSi2O6)
Feldspathic porcelains contain a variety of oxide components, lnclu. SiO2 (52-62 wt%), A1203 (11 -1 6 wt%), K20 (9-11 wt%), Na2O (5-7 wt%), and certain additives, including Li20 and B203.These ceramics are called porcelains because they contain a glass matrix & one or more crystal phases.
There are four types of veneering ceramics :
(1) low-fusing ceramics (feldspar-based porcelain & nepheline syenite-based porcelain);
(2) ultra low-fusing ceramics (porcelains and glasses);
(3) stains, and
(4) glazes (self-glaze and add-on glaze).
• Silica (Quartz or Flint) - Silica (SiO,) 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). * Kaolin (clay) - Kaolin / clay (Al2O3, 2SiO2, 2H2O) serves as a binder. When mixed with water, it forms a sticky mass, which allows the unfired porcelain to be easily worked and molded. On heating, it reacts limitedly with feldspar (known as pyrochemical reaction) and thereby provides rigidly. It also adheres to the framework of quartz particles and shrinks considerably during firing. Unfortunately, pure kaolin is white in color and reduces the translucency of porcelain. The large glassy phases developed by compositions of low kaolin content require closely controlled firing times and temperatures in order to produce an acceptable result. Consequently, it is included only in small concentrations of 4 to 5%. * Fluxes (low-fusing glasses) are included to reduce the temperature required to sinter the porcelain powder particles together at low enough temperatures so that the alloy to which it is fired does not melt or sustain sag (flexural creep) deformation.
• Coloring pigments - are added to obtain the various shades needed to simulate natural teeth. Examples of metallic oxides and their respective color contributions to porcelain include iron or nickel oxide (brown),copper or chromium oxide (green),titanium oxide (yellowish brown), manganese oxide (lavender), and cobalt oxide (blue).
• Opacifiers - achieved by the addition of cerium oxide, zirconium oxide, titanium oxide, or tin oxide.
• Stains & color modifiers - Stains are generally low fusing colored porcelains used as surface colorants or to provide/ imitate markings like enamel check lines, decalcification spots, fluoresced areas etc. Color modifiers are less concentrated than stains, used to obtain gingival effects or highlight body colors, and are best used at the same temperature as dental porcelain.
Stains in finely powdered form are mixed with water/ glycerin or any other special liquid & the wet mix is applied with a brush either on the surface of porcelain before glazing or built into the porcelain (internal staining).
• Glass modifiers - Potassium, sodium and calcium oxide are the most commonly used glass modifiers and act as fluxes by interrupting the integrity of sio4 network.Glass modifiers are used to produce dental porcelains with different firing temperatures.
Boric oxide (B203) behaves as a glass modifier; i.e., it decreases viscosity, lowers the softening temperature, and forms its own glass network. Another important glass modifier is water; although it is not an intentional addition to dental porcelain. The hydronium ion, H30+, can replace sodium or other metal ions in a ceramic that contains glass modifiers.
REQUIREMENTS FOR A METAL – CERAMIC SYSTEM :
1. The alloy must have a high melting temperature. The melting range must be substantially higher (greater than 100° C) than the firing temperature of the veneering porcelain and solders used to join segments of a fixed partial prosthesis.
2. The veneering porcelain must have a low fusing temperature so that no creep, sag, or distortion of the framework takes place during sintering.
3. The porcelain must wet the alloy readily when applied as a slurry to prevent voids forming at the metal-ceramic interface. In general, the contact angle should be 60 degrees or less.
4. A good bond between the ceramic and metal is essential and is achieved by chemical reaction of the porcelain with metal oxides on the surface of metal and by mechanical interlocking made possible by roughening of the metal coping.
5. Coefficients of thermal expansion (CTE) of the porcelain and metal must be compatible so that the veneering porcelain never undergoes tensile stresses, which would lead to cracking. Metal-ceramic systems are therefore designed so that the CTE of the metal is slightly higher than that of the porcelain, thus placing the veneering porcelain in compression (where it is stronger) following cooling. This is assuming that linear coefficients of thermal expansion of both porcelain and metal are identical to linear coefficients of thermal contraction.
6. Adequate stiffness and strength of the metal framework are especially important for fixed partial dental prostheses and posterior crowns. High stiffness of the metal reduces tensile stresses in the porcelain by limiting deflection amplitude and deformation (strain). High strength is essential in the interproximal connector areas of fixed partial prostheses.
7. High resistance to deformation at high temperature is essential. Metal copings are relatively thin (0.4 to 0.5 mm); no distortion should occur during firing of the porcelain, or the fit of the restorations would be compromised.
8. Adequate design of the restoration is critical. The preparation should provide for adequate thickness of the metal coping , as well as enough space for an adequate thickness of the porcelain to yield an esthetic restoration. During preparation of the metal framework, prior to porcelain application, it is important that all sharp angles be eliminated and rounded to later avoid stress concentration in the porcelain. If full porcelain coverage is not used (e.g., a metal occlusal surface), the position of the metal-ceramic junction should be located at least 1.5 mm from all centric occlusal contacts.
EFFECT OF DESIGN ON METAL CERAMIC RESTORATIONS
Because ceramics are weak in tension and can withstand very little strain before fracturing, the metal framework must be rigid to minimize deformation of the porcelain. However, copings should be as thin as possible to allow space for the porcelain to mask the metal framework without over-contouring the porcelain.
The labial margin of metal-ceramic prostheses is a critical area regarding design because there is little porcelain thickness at the margin to mask the appearance of the metal coping and to resist fracture.
Recommended margin designs include a 90-degree shoulder, a 120-degree shoulder, or a shoulder bevel.
Provided that the shoulder depth is at least 1.2 mm, these designs should all provide for sufficient porcelain thickness to minimize the risk of porcelain fracture.
When using partial porcelain coverage, s/ as when a metal occlusal surface is desired, the position of the metal-ceramic joint is critical. Stresses should be minimized by placing the metal-ceramic junction at least 1.5 mm from centric occlusal contacts.
The geometry of the interproximal connector area : The inciso-cervical thickness of the connector should be large enough to prevent deformation or fracture because deflection is decreased as the cube of the thickness; greater thickness will minimize deflection of the framework, which may lead to debonding or fracture of the porcelain.

Tooth preparation for PFM restorations:
During tooth preparation it is necessary to allow about 1.5 mm in thickness for a metal coping (0.3–0.5 mm) and the porcelain veneer (1.0 mm) to achieve optimal aesthetics.
Inadequate space may cause:
# overly bulky restoration with reasonable appearance
# opaque layer of porcelain used to mask the metal coping will ‘shine through’ the surface layers of porcelain producing an opaque white or cream spot.
The margin configuration for a PFM crown is a flat shoulder where there is porcelain and a chamfer or bevel where this is a metal finishing line.
Shoulder porcelains: The appearance of the margins of PFM crowns has been revolutionized by the development of shoulder porcelains. These porcelains have adequate substantivity so that they do not flow significantly during firing. This allows the technician to cut the metal coping back from the edge of the tooth, leaving an adequate bulk of porcelain to give a reasonable marginal fit with much improved aesthetics.
FABRICATION OF PORCELAIN :
Porcelain Condensation
Porcelain 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 another vehicle and condensed into the desired form .
Dense packing of the powder particles provides two benefits: lower firing shrinkage and less porosity in the fired porcelain.
This packing, or condensation, may be achieved by various methods, including vibration, spatulation, and brush techniques.
Vibration : 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 brushed area. Spatulation : 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.
Brush technique : Addition of dry porcelain powder to the surface to absorb the water. The dry powder is placed by a brush to the side opposite from an increment of wet porcelain. As the water is drawn toward the dry powder, the wet particles are pulled together.
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.

Sintering of Porcelain
The purpose of firing is simply to sinter the particles of powder together properly to form the prosthesis. Changes in the leucite content can cause the development of a thermal contraction coefficient mismatch between the porcelain 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 650" C (1200" F) for low-fusing porcelain. This preheating procedure permits the remaining water vapor to dissipate.
After preheating for approximately 5 min, 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 or apparent density of the final product.
Vacuum firing reduces porosity in dental porcelain : When the porcelain is placed in the furnace, the powder 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 55" C (39" F) below the upper firing temperature, the vacuum is released and the pressure inside the furnace increases by a factor of 10, from 0.1 to 1 atm. Because the pressure is increased by a factor of 10, the voids are compressed to one-tenth of their original size, and the total volume of porosity is accordingly reduced. METHOD OF FABRICATION :
Porcelain Application Armamentarium :
• Porcelain modeling liquid • Paper napkin • Glass slab or palette
• Tissues or gauze squares • Two cups of distilled water • Glass spatula
• Serrated instrument • Porcelain tweezers or hemostat
• Ceramist’s sable brushes (Nos. 2, 4, and 6) & whipping brush
• Razor blade or modeling knife • Cyanoacrylate resin • Colored pencil or felt-tip marker
• Articulating tape • Ceramic-bound stones • Flexible thin diamond disk(abt 20 mm dia)
Step-by-step procedure : Metal-ceramic restorations consist of a cast metallic framework (or core) on which at least two layers of ceramic are baked .
The first layer applied is the opaque layer, consisting of porcelain modified with opacifying oxides. Its role is to mask the darkness of the oxidized metal framework to achieve adequate esthetics. This thin opaque layer also contributes to the metal-ceramic bond. The next step is the buildup of dentin and enamel (most translucent) porcelains to obtain an esthetic appearance similar to that of a natural tooth. The dentin or enamel porcelain powder is mixed with modeling liquid (mainly distilled water) to a creamy consistency and is applied on the opaque layer. The porcelain is then condensed by vibration and removal of the excess water with absorbent tissue, and slowly dried to allow for water diffusion and evaporation.
After building up of the porcelain powders, metal-ceramic restorations are sintered under vacuum in a porcelain furnace. Sintering under vacuum helps eliminate pores. As the furnace door closes, the pressure is lowered to 0.01 MPa (0.1 atmosphere). The temperature is raised until the sintering temperature is reached, the vacuum is then released, and the furnace pressure returns to 0.1 MPa (1 atmosphere). Studies have shown that sintering under vacuum reduces the amount of porosity from 5.6% in air-fired porcelains to 0.56%. Opaque, dentin, and enamel porcelains are available in various shades.
Copings for Metal Ceramic Prostheses
Alloys for metal ceramic restorations :

Four types of process for producing a metal coping for metal – ceramic prostheses : 1. Electrodeposition of gold or other metal on duplicate die. 2. Burnishing & heat-treating metal foils on a die. 3. CAD-CAM processing of a metal ingot. 4. Casting of pure metal (CpTi) /an aolly ( Noble/ Base metal) through lost wax technique.
Requirement for bonding between ceramic veneer & coping- # Ceramic must have fusion temperature well above its sintering temperature # Ceramic must have coefficient of thermal contraction close to that of alloy
# Metal oxide necessary to promote chemical bonding of ceramic veneer to metal coping
Ceramics used for porcelain-fused-to metal restorations must fulfill five requirements:
(1) they must simulate the appearance of natural teeth,
(2) they must fuse at relatively low temperatures,
(3) they must have thermal expansion coefficients compatible with the metals used for ceramic-metal bonding,
(4) they must withstand of a nickel-based ceramic-metal crown. the oral environment, and
(5) they must not unduly abrade opposing teeth.
Metal-Ceramic Crowns Based on Burnished Foil Copings
Captek : a technology that is based on the principle of capillary attraction to produce a gold composite metal fie Captek P and C metals can produce thin metal copings for single crowns or frameworks for metal ceramic fixed partial dentures with a minimum span length of 18 mm (that allows space for up to two pontics) .
FAILURE AND REPAIR OF METAL CERAMIC RESTORATIONS
The majority of retreatments are due to biological failures, such as tooth fracture, periodontal disease, and secondary caries. Prosthesis fracture and esthetic failures account for only 20% of retreatment cases for single-unit restorations.
A metal-ceramic bond may fail in any of three possible locations:
The highest bond strength leads to failure within the porcelain when tested this is observed with some alloys that were properly prepared with excellent wetting by the porcelain and is also called a cohesive failure.
Another possible cohesive failure is within the oxide layer .
Failures occurring at the interface between metal and oxide layer are called adhesive failures and are commonly observed with metal alloys that are resistant to forming surface oxides, such as pure gold or platinum, and exhibit poor bonding. Base-metal alloys commonly exhibit failures within the oxide layer if an excessively thick oxide layer is present.
COMMON REASONS FOR FAILURE OF METAL-CERAMIC RESTORATIONS

Failure Reason

Fracture during Improper condensation bisque bake Improper moisture control
Poor framework design
Incompatible metal porcelain
Combination

Bubbles Too many firings
Air entrapment during building of restoration
Improper moisture control
Poor metal preparation
Poor casting technique

Unsatisfactory Poor communication with appearance technician
Inadequate tooth reduction
Opaque too thick
Excessive firing

Clinical fracture Poor framework design
Centric stops too close to metal-ceramic interface
Improper metal preparation
Ideally, the prosthesis should be retrieved, metal surfaces should be cleaned, and a new oxide layer should be formed on the exposed area of metal prior to porcelain application and firing.
Porcelain repair using dental composite: All of these techniques present the challenge of bonding chemically dissimilar materials. When porcelain fragments are available and no functional loading is exerted on the fracture site, silane coupling agents can be used to achieve good adhesion between the composite and porcelain; however, metal alloys have no such bonding agent and this type of repair is considered only temporary.
Systems are available for coating the metal surface with silica particles through airborne particle abrasion. The particles are embedded in the metal surface upon impact, then a silane coupling agent can be applied. Alternatively, base metal alloys can be coated with tin followed by the application of an acidic primer. Both methods achieve adequate bond strength and may delay the eventual need for remaking the prosthesis.
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 may 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 particularly important for this type of prosthesis The most outstanding advantages of metal-ceramic prostheses are the permanent aesthetic quality of the properly designed reinforced ceramic unit and their resistance to fracture. Unlike similar acrylic resin veneered structures, almost no wear of the porcelain occurs by abrasion and there is no staining along the interface between the veneer and the metal. In a clinical study, the fracture rate of metal-ceramic crowns and bridges is as low as 2.1% 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. Very little flexibility can be sustained by dental porcelains because of their moderately high modulus of elasticity 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 of metal is necessary to provide the proper rigidity. The minimal metal coping thickness necessary in the occlusal region is approximately 0.3 mm. 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.
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 metal 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 metal coated with opaque shoulder porcelain. The technician should polish and glaze this margin to avoid a rough surface at the margin. The use of MC crowns with butt-joint margins or with very thin knife-edge metal margins on the facial surface are successful procedures for improving the aesthetics.
Conclusion
Ceramics are widely used in dentistry due to their ability to mimic the optical characteristics of enamel and dentin and their biocompatibility and chemical durability. Most highly esthetic ceramics are filled glass composites based on aluminosilicate glasses derived from mined feldspathic minerals. One common crystalline filler is the mineral leucite, used in relatively low concentrations in porcelains for metal-ceramic systems and in higher concentrations as a strengthening filler in numerous all-ceramic systems. Metal-ceramic restorations remain the most popular materials combination selected for crown and bridge applications and have a 10-year success rate of about 95%.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 may crack or fracture because of its low ductility. These difficulties can be partly overcome with proper prosthesis design. The aesthetic capability of metal-ceramic restorations and their superb survivability are sufficient to overcome the drawbacks of metal-ceramic systems. For these reasons, metal-ceramics represent the most widely used prosthesis system used in fixed prosthodontics today.

References 1. Applied Dental materials - Mc Cabe; 9th Edition. 2. Philips Science of Dental Materials 11th Edition. 3. Craig restorative dental materials 13 th edition. 4. Ceramics in dentistry: Historical roots and current perspectives- J. Robert Kelly; PROSTHET DENT 1996;75:18-32. 5. Evolution of dental ceramics in the twentieth century- John W. McLean; J Prosthet Dent 2001;85:61-6. 6. Ceramic materials in dentistry: historical evolution and current practice - JR Kelly; Australian Dental Journal 2011; 56:(1 Suppl): 84–96 7. Dental ceramics: An update - Arvind Shenoy; J Conserv Dent. 2010 Oct-Dec; 13(4): 195–203. 8. JADA 2008;139(suppl 4):4S-7S Dental Ceramics :J. Robert Kelly. 9. Dental ceramics: current thinking and trends ; J. Robert Kelly ; Dent Clin N Am 48 (2004) 513–530