Metal Ceramic - Dental Ceramic-Metal
Posted by John Doe at Dental Assistant on February 2, 2012.
Categories: Dental Materials
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PREPARATION OF CERAMIC-METAL RESTORATIONS
The processing of the metal coping for ceramic-metal restorations is much like that of all-metal crowns and bridges. One significant difference relates to the reuse of metal. As mentioned earlier, when the metal is melted and cast, certain alloying elements can be lost, especially the elements that readily form oxides. These elements are important for bonding with noble metal alloys. To conserve metal, portions of the casting are commonly remelted. Each time the metal is remelted, some of these easily oxidized elements are lost. Therefore a certain portion of new alloy (usually half) should be added each time the metal is reused to replenish the lost alloying elements.
Surface treatment of the metal coping before ceramic application is important for good bonding. These treatments are used to roughen the coping surface and form surface oxides. The surface may be roughened by blasting with a fine abrasive (25 to 50 um alumina); in some cases this results in a large increase in bond strength. In most cases the metal coping is heat-treated either in air or under partial vacuum to produce a surface oxide to improve bonding. In some palladium alloys, the heat treatment forms not only surface oxides but also internal oxides that penetrate the metal from the surface and effectively roughen the surface, thereby improving bonding. Some base-metal alloys tend to form excessively thick interfacial oxides, which weaken the metal-ceramic bond (see Fig. 19-5). With these alloys, the coping is heat-treated and then blasted to remove enough oxide to achieve higher bonding. If this process is not used, failure through the oxide may occur.
Ceramic application is similar to that described in Chapter 18. However, there are several important considerations. The first layer of ceramic is especially important with ceramic-metal restorations because it must hide the metal; special opaque ceramic must be used (see Table 19-D-After the opaque layer has been applied and fired the dentin (or body) ceramic, which contains less of the opaque oxides (such as Sn02 and Zn02), pigments, and fluorescing oxides, is applied and fired. Finally, once the correct contour has been established an essentially transparent glaze layer is applied and fired. The ceramic-alloy compatibility is another important consideration. As pointed out earlier, the thermal expansions must be matched and the porcelain firing temperatures must be low enough that the alloy will not sag; the manufacturer usually supplies compatibility information. Titanium alloys require special ceramics, otherwise processing of the ceramic is similar to that of the other alloys.
EFFECT OF DESIGN ON CERAMIC-METAL RESTORATIONS
Because ceramics are weak in tension and can withstand very little strain before fracturing, the alloy copings must be rigid to minimize deformation of the ceramic. However, you would like the coping to be as thin as possible to allow space for the ceramic to hide the color of the alloy without overcontouring the ceramic. This consideration is especially true for alloys that are white (gray). This might lead to the conclusion that Ni-Cr or Co-Cr alloys would be superior to the noble alloys because their moduli (stiffness) are 1.5 to 2 times greater and the thickness of the coping could be halved. However, loading the restoration places it in bending, and the bending equation shows that deformation is a function of only the first power of the modulus, whereas it is a function of the cube of the thickness. It can be shown that for a typical dental ceramic-metal restoration, the thickness of a base-metal coping can be reduced only about 7% because of the higher elastic modulus. Thus, the advantage of the higher modulus for the base-metal alloys is minimal.
When full ceramic coverage of the coping is done, the shoulder of the crown should be flat with a rounded angle. This design provides for bulk of ceramic and minimizes fracture compared with a knife-edge shoulder geometry. A somewhat easier preparation is a chamfer, which provides for adequate bulk of ceramic and resists fracture essentially as well as the flat-shoulder geometry. In any case, sharp angles in the ceramic are to be avoided.
When using partial coverage by ceramics, such as when a metal occlusal surface is desired, the position of the ceramic-metal joint is critical. Because of the large difference in modulus of the ceramic and metal, stresses occur at the interface when the restoration is loaded. These stresses can be minimized by placing the ceramic-metal joint as far away as possible from contact with opposing teeth. In the design of a ceramic-metal bridge, the geometry of the interproximal area between the crown and pontic is critical. The occlusal-gingival length of the joint should be as long as clinically feasible; because deflection is decreased as the cube of the length, greater length will minimize deflection of the ceramic. It should be remembered that the bridge is not a uniform beam; maximum deflection on loading will occur at the thinnest cross section, the interproximal area.
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