Сraig. Dental Materials

Type

Au-Pt-Pd

Au-Pd

Au-Pd-Ag

Pd-Ag

1

I

Au (YO)

84-86

45-52

51-52

-

In, Ga 9

1

1

I

temperature of the alloys. Finally, although considerable Pt and Pd are present, these alloys are still yellow, which makes producing pleasing esthetics with the ceramic easier than with white alloys.

Au-Pd Types This high-noble type with good corrosion resistance has decreased gold but increased palladium content. These alloys contain no platinum or iron and thus are solutionrather than precipitation-hardened. They contain In for bonding, gallium to decrease the fusion temperature, rhenium for grain refining, and ruthenium for castability. Because of their high palladium content, the alloys are white (some call it gray) rather than yellow, even though they contain about 50% gold. This color causes increased difficulty in producing esthetic restorations.

These alloys are stronger, stiffer, and harder than the Au-Pt-Pd type and have higher elongation (more ductile) and casting temperatures (easier to solder). They have lower densities, and when they were introduced were less costly than the Au-Pt-Pd type, because Pd was cheaper than Au. With Pd being more costly than Au, there is no longer a cost advantage. The decrease in density indicates more care should be taken during casting because of the decrease in the force with which the alloy enters the casting ring. However, these alloys are easy to cast, and soldering is easy because of the higher casting temperature.

Au-Pd-AgTypes These alloys contain less palladium than the Au-Pd type; the decrease is made up by adding silver. However, they still have good corrosion resistance. Again, In and Sn are added for bonding with the ceramics, ruthenium (Ru) for castability, and rhenium (Re) for grain refining. Hardening results from solution hardening. As seen in Table 19-3, the properties of the Au-Pd-Ag type are similar to those of the Au-Pd type.

Pd-Ag Types These alloys, which contain no gold and have a moderately high silver content, have the lowest noble-metal content of the

five noble alloys. They contain In and Sn for bonding and Ru for castability. Their properties are similar to the Au-Pd-Ag type, except they are less dense (-11 g/cm3 vs. 14 g/cm3). They were developed at a time when the cost of Au was about $800/oz and the cost of Pd was low; those conditions no longer exist. Some ceramics used with these high-Ag alloys resulted in what was called "greening," really a color shift toward yellow. Contamination and technique was blamed to some extent for this problem.

Pd-Cu Types These alloys contain very high Pd content with 10% to15% Cu. They contain In for bonding and Ga for controlling casting temperature. These alloys have high strength and hardness, moderate stiffness and elongation, and low density. However, they have low sag resistance and form dark oxides. They are white alloys, like all the other types except the yellow Au-Pt-Pd type.

COMPOSITION AND PROPERTIES

OF BASE-METAL ALLOYS

The range of compositions of base metal alloys for ceramic-metal restorations are given for Ni-Cr, Co-Cr, and Ti type in Table 19-4, and typical properties of these alloys are listed in Table 19-5. Considerable variation in composition and properties are shown in these tables.

Ni-Cr Types Chromium provides tarnish and corrosion resistance, whereas alloys containing A1 and Ti are strengthened by the formation of coherent precipitates of Ni3A1 or Ti3A1. Mo is added to decrease the thermal coefficient of expansion, and Be to improve castability (by reducing the melting point) and hardening. Note because of the wide differences in atomic weight of Be, Ni, and Cr, 2 wt%is roughly equal to 6 at%. The use of Be may cause some toxicity problems and surface oxidation at high temperatures.

These alloys are harder than noble alloys but usually have lower yield strengths. They also have higher elastic moduli, and it was hoped thinner copings and frameworks could result. They have much lower densities (7 to 8 g/cm3)

and generally higher casting temperatures. Adequate casting compensation is at times a problem, as is the fit of the coping.

Co-Cr Types Again, Cr provides tarnish and corrosion resistance. Unlike Co-Cr partialdenture alloys, the alloys for ceramic-metal restorations are strengthened by solution hardening rather than carbide formation. Mo helps lower the coefficient of expansion and RLI improves castability. They are stronger and harder than noble and Ni-Cr alloys and have roughly the same densities and casting temperature as Ni-Cr alloys. Casting and soldering of these alloys is more difficult than noble alloys, as is obtaining a high degree of accuracy in the castings.

Ti Types Pure Ti and Ti-6A1-4V may become important for ceramic-metal restorations, but they present processing difficulties, as indicated by casting temperatures of 1760"to 1860' C and their ease of oxidation. However, newer techniques, such as machine duplication and spark erosion to fabricate copings, may increase the use of these metals.

In summary, the noble alloys have good corrosion resistance, but only the Au-Pt-Pd alloys have a desirable yellow color. The other types are white (gray), which is more difficult to mask with the ceramic. The Au-Pd, Au-Pd-Ag, and Pd-Ag types have excellent mechanical properties coupled with high fusion temperatures and ease of casting and soldering. However, the Pd-Ag type has caused some problems with discoloration of the ceramic. The Pd-Cu types are characterized by the formation of dark oxides, which may cause problems with masking by the ceramic. The Ni-Cr and Co-Cr types are noted for high hardness and elastic modulus, although some Ni-Cr alloys have lower yield strengths. They are also noted for high casting temperatures. In general the Ti types have lower mechanical properties than the other base metal alloys but notably lower density and higher casting temperatures. Good bonding of ceramic and alloy can be achieved with all alloys, but bonding with some base metal alloys is more technique sensitive.

PREPARATION OF CE

RESTORATIONS

The processing of the metal coping for ceramicmetal 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 pm alumina); in some cases this results in a large increase in bond strength. In most cases the metal coping is heattreated 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 19-11. After the opaque layer has been applied and fired the dentin (or body) ceramic, which contains less of the opaque oxides (such as SnO, and ZnO,),

588 Chapter 19 CERAMIC-METAL SYSTEMS

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.

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 flatshoulder 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 interproxima1 area.

I SELECTED PROBLEMS

Problem 1

The ceramic of a metal-ceramic crown placed in the mouth fractured from the metal-

lic substructure. What factors might have produced such a failure, and how can it be avoided?

Solution a

Surface contamination of the alloy before placing the ceramic may be a causative factor. Impurities on the metal surface, such as organic powder from the grinding stones or grease and oils from fingers, may prevent a good wetting of the ceramic, and air bubbles will be present at the ceramic-metal interface. To avoid this problem, you should use vitrified grinding stones, protect the metal surface from polishing debris, and not touch the metal with the fingers.

Solution b

Underfired opaquer may be a factor. When the opaque ceramic has not been brought up to its fusing point, a complete fusion into the surface of the metal has not been achieved. Proper firing technique with the opaque material will eliminate this trouble.

Solution c

Another cause for the fracture may be improper metallic thickness. A uniform metal thickness is very important to prevent failures in the metal-ceramic bond. A minimum thickness of 0.4 mm is allowable. Thinner metal substructure will not protect the ceramic from fracture.

Solution d

The reuse of alloys may cause a fracture in the substructure. When sprue buttons are used to cast a new substructure, tin, or indium may be decreased or eliminated, and a very weak bonding with the porcelain is the result. The use of fresh alloys for casting the substructure is ideal, but a combination of a 50% (75% is better) fresh alloy and 25% to 50% used alloy may be used without detrimental effects on the metal-ceramic bond.

Problem 2

When a ceramic-metal restoration was removed from the oven, cracks in the ceramic were observed. What factors may have caused this failure, and how can it be avoided?

Solution a

Improper selection of ceramic and alloy will cause such cracks. Manufacturers often produce ceramic with special characteristics to match the thermal properties of a particular alloy. When another metal is tried with the same ceramic, a mismatch in the thermal expansion may be sufficient to cause cracking. Only the alloy and the ceramic suggested by the manufacturer should be used.

Solution b

Another causative factor may be overglazed or overfired ceramic. An overglazed or overfired ceramic no longer matches the alloy properly. Irregularities on ceramic surfaces should be finished before glazing to avoid overglazing.

Solution c

When the ceramic-metal restoration is allowed to cool in the furnace after the ceramic has been baked, cracks in the ceramic material will be produced. The ceramic should never be cooled in the furnace because slow cooling may change some physical properties of the ceramic, creating a mismatch with the alloy.

Solution d

When hot ceramic is touched with a cold instrument, a thermal shock can produce cracks.

Problem 3

When a metal-ceramic fixed appliance was completed, the shades appeared too gray. What could be the cause of such a dark ceramic, and how can it be avoided?

Solution a

When the coating of opaque ceramic is too thin or incomplete, the transparency of the body ceramic will allow the gray coping to show through. The opaque bake should be examined for gray areas and reopaqued if any are present. Two thin coats of opaque ceramic with separate firings are often recommended.

590 Chapter 19 CERAMIC-METAL SYSTEMS

Solution b

When opaque ceramic has been fired to maturity at the opaque bake, by the time a third or fourth bake is made, the ceramic may have become too glazed and lost some of its opacifying qualities, thereby allowing the metal to reflect through the opaque layer, creating a gray shade. The manufacturer's suggested technique to bake the opaque should be carefully followed.

Solution c

When a crucible contaminated by an alloy containing base metals is used, a dark shade may be obtained. To avoid this problem, d o not use a crucible that has been used to cast any other alloy. Only clean crucibles without ceramic liners or fluxes should be employed to cast the alloys for metal-ceramic restorations.

Solution d

Nonprecious alloys may contaminate the oven. Another source of oven contamination is the formation of volatilized impurities when the ceramic furnace has been used often for degassing and soldering operations. When the contamination accumulates in the oven, ceramic fired there will be dark. To avoid this problem you should purge the ceramic oven frequently.

Problem 4

A metal-ceramic restoration was cemented in the mouth. After insertion, flaking or chipping of the ceramic was observed. What may have been the cause of this failure?

Solution

The main problem with ceramic is it does not withstand much bending without fracture. When fired on a thin or flexible substructure, deformation of the metal under stress may deform the ceramic beyond its limit, and flakes or chips in the ceramic material are produced. The alloy selected to prepare ceramic-metal restorations or appliances should be built with enough bulk and have a high enough rigidity

to withstand masticatory stresses without excessive deformation.

Problem 5

A crown preparation was made on a small lower anterior tooth that allowed little space for a metal coping. A Ni-Cr alloy was selected for the coping because its stiffness was reported to be twice that of the noble alloys. Because of the higher stiffness, the thickness of the coping was halved. In service the ceramic fractured. Why?

Solution

It is a mistake to halve the thickness in response to a higher modulus. In the deflection of the coping, the thickness of the coping is much more important than the modulus. If inadequate space is available for the coping and the desired color of a normally thick coping cannot be achieved without overcontouring the ceramic, an all-ceramic crown should be considered.

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