Metal Ceramic Restoration Part 1 PDF

¨ Ceramco crown ¨ porcelain veneer crown (PVC), ¨ porcelain-fused-to-gold (PFG) crown, ¨ porcelain-fusedto-metal (PFM) crown, ¨ Metal-ceramic is a more precise term scientifically Metal-ceramic restoration (MCR)is composed of a metal casting, or coping, that fits over the tooth prepa...

¨ Ceramco crown ¨ porcelain veneer crown (PVC), ¨ porcelain-fused-to-gold (PFG) crown, ¨ porcelain-fusedto-metal (PFM) crown, ¨ Metal-ceramic is a more precise term scientifically Metal-ceramic restoration (MCR)is composed of a metal casting, or coping, that fits over the tooth preparation and ceramic that is fused to the coping. ¨ MCR is covered with three layers of porcelain 1. Opaque porcelain conceals the metal underneath, initiates the development of the shade, and plays an important role in the development of the bond between the ceramic and the metal. 2. Dentin, or body, porcelain makes up the bulk of the restoration, providing most of the color, or shade. 3. Enamel, or incisal, porcelain imparts translucency to the restoration. ¨ MCR remains the “gold standard” of predictability with indirect tooth-colored systems. ¨ It is generally accepted that, in spite of development of high strength ceramic materials, MCRs are more resistant to fracture. q While ceramic technologies are rapidly evolving, the failure rates of various available systems are still higher than with metal-based restorations, particularly in high-stress clinical situations. q An indication for MCRs has been the fabrication of fixed partial dentures in the esthetic zone. The coping must allow the porcelain to remain in compression by supporting the incisal region, the occlusal table, and the marginal ridges. Without any underlying support, the glass would break—and so will unsupported porcelain on a restoration. There are six features of importance to be considered when designing the metal coping for an MCR: ¨ 1. Thickness of the porcelain veneer ¨ 2. Support of the porcelain veneer ¨ 3. Thickness of metal underlying and adjoining the porcelain ¨ 4. Placement of occlusal and proximal contacts ¨ 5. Extent of the area to be veneered for porcelain ¨ 6. Design of the facial margin ¨ Porcelain should be kept at a minimum thickness that is still compatible with good esthetics. ¨ Relatively thin porcelain, of uniform thickness and supported by rigid metal, is strongest. ¨ The absolute minimum thickness of porcelain is 0.7 mm, and the desirable thickness is 1.0 to 1.5 mm. ¨ Extensions of porcelain beyond 2.0 mm are prone to fracture even if these thick areas of porcelain are ¨ not in areas of force concentration ¨ For adequate strength and rigidity, a noble metal coping should be at least 0.3 to 0.5 mm thick. ¨ A base metal alloy with a higher yield strength and elevated melting temperature may be as thin as 0.2 mm. ¨ The thickness of the coping may vary, depending on the configuration of the preparation. ¨ The porcelain-metal junction should be placed 1.0 mm from occlusal contacts at the position of maximal intercuspation ¨ When there is inadequate vertical overlap to place the contact on metal, the porcelain-metal junction is placed far enough gingivally for the contact to occur on porcelain ¨ To place occlusal contacts in metal, the porcelain on the facial surface extends over the cusp tip and about halfway down the palatal incline of the facial cusp on maxillary premolars and molars ¨ Variants for maxillary teeth include porcelain coverage of the mesial marginal ridge up to the middle of the triangular ridge or, for those patients who demand absolute esthetics,complete coverage with porcelain of the occlusal surface of premolars and molars. Four mechanisms have been described to explain the bond between the ceramic veneer and the metal substructure: ¨ 1. Mechanical entrapment ¨ 2. Compressive forces ¨ 3. Van der Waals forces ¨ 4. Chemical bonding ¨ Mechanical entrapment creates attachment by interlocking the ceramic with microabrasions in the surface of the metal coping, which are produced by finishing the metal with noncontaminating stones or disks and air abrasion. ¨ When compared with unprepared metal, surface finishing enhances the metal-ceramic bond. Air abrasion appears to enhance wettability, provide mechanical interlocking, and increase the surface area for chemical bonding. ¨ When the coefficient of thermal expansion of a properly designed metal coping is slightly higher than that of the porcelain veneered over it, compressive forces develop. ¨ The slight difference in coefficients of thermal expansion, or thermal contraction as is the case during cooling, will cause the porcelain to draw toward the metal coping when the restoration cools after firing. ¨ Van der Waals forces comprise an affinity based on a mutual attraction of charged molecules. ¨ Although the molecular attraction makes only a minor contribution to overall bond strength, it is significant in the initiation of the most important mechanism, the chemical bond. ¨ Chemical bonding is indicated by the formation of an oxide layer on the metal and by bond strength that is increased by firing in an oxidizing atmosphere. ¨ When fired in air, trace elements in the gold alloy, such as tin, indium, gallium, or iron, migrate to the surface, form oxides, and subsequently bond to similar oxides in the opaque layer of the porcelain. ¨ A gold alloy containing minute amounts of tin and iron creates a significantly stronger bond with porcelain than a pure gold alloy does ¨ The clean separation of porcelain from the metal coping is evidence of either bond failure from contamination of the coping surface or an excessive oxide layer. ¨ Base metal alloys readily form chromium oxides that bond to the porcelain without the addition of any trace elements.. ¨ Conventional gold alloys have a high coefficient of thermal expansion (14 × 10– 6/°C), while ¨ conventional porcelain possesses a much lower value (2 to 4 × 10–6/°C). ¨ A difference of only 1.7 × 10–6/°C can produce sufficient shear stress to produce failure of the bond. ¨ The optimum difference between the two would be no greater than 1 × 10–6/°C. ¨ The coefficient of thermal expansion of porcelain can be increased to as much as 7 to 8 × 10–6/°C by the addition of an alkali such as lithium carbonate. ¨ At the same time, the coefficient of the metal can be lowered to 7 to 8 × 10–6/°C by adding palladium or platinum. ¨ The melting range of the alloy used in the coping must be 170°C to 280°C (300°F to 500°F) higher than the fusing temperature of the porcelain applied to it. ¨ Porcelains most commonly used for this purpose have a fusing temperature of approximately 870°C to 930°C (1,600°F to 1,710°F), and noble alloys melt at near 1,260°C (2,300°F). ¨ High noble alloys have a noble metal (gold, platinum, palladium) content greater than 60%, with at least 40% gold. ¨ Noble alloys have a noble metal content of at least 25%, and ¨ predominantly base alloys have less than 25% noble metal content. ¨ “Gold-palladium alloys have proven most satisfactory for metal-ceramic crowns and fixed partial dentures”. These alloys are composed of gold (44% to 55%) and palladium (35% to 45%), with small amounts of gallium, indium, and/or tin. ¨ Beryllium, which is added to alloys to control oxide formation, is a carcinogen. ¨ Contact dermatitis from nickel-containing prostheses appears to be a risk to some patients ¨ copper or cobalt, in the alloy caused dark oxide formation and poor high-temperature strength. ¨ Originally, one of the most common disadvantages of the silver-containing alloys was the potential of porcelain discoloration, most commonly described as greening ¨ Fundamentals of fixed prosthodontics / Herbert T. Shillingburg Jr. [et al.]. -- 4th ed. Quintessence Publishing Co, Inc, Illinois.ISBN 978-0-86715-475-7.PAGES: 808-818