The choice of casting alloy largely determines the selection of investment and casting techniques and therefore is discussed first.
The number and variety of alloys suitable for casting have expanded dramatically, largely because of changes in the price of gold. Many alloys are available, especially for metal-ceramic restorations (see Chapter 19). The dentist must be able to make a rational choice based on current information.
Factors to Be Considered
Intended Use. Traditionally alloys for casting were classified on the basis of their intended use, as follows:
Type I: Simple inlays Type II: Complex inlays Type III: Crowns and FPDs Type IV: RPDs and pinledges Porcelain: Metal-ceramic alloys
Physical Properties. In 1965 the ADA adopted the specifications of the Federation Dentaire Internationale (FDI), which classified casting alloys according to their physical properties (specifically their hardness), as follows: Type I: Soft Type 11: Medium Type III: Hard Type IV: Extra hard
Porcelain-type alloys with a high noble metal content were found to have similar hardness to Type III alloys, and base metal alloys were found to be harder than Type IV alloys (see Chapter 19).
Color. Manufacturers place considerable emphasis on the color of their alloys, and color preference is often given to gold over silver. The patient's views on the subject should be sought if the metal will be visible in the mouth; otherwise, the color of the alloy is irrelevant.
Color is not a good guide to gold content: 9-carat jewelry alloy with only 37.5% gold looks considerably more yellow than a metal-ceramic alloy with 85% gold but no copper.
Composition. To be accepted by the ADA as an alloy suitable for dental restorations,17 the manufacturer must list the percentage composition by weight of the three main ingredients and any noble metal percentage(s). Traditionally the functional characteristics of corrosion resistance and tarnish resistance were predicted on the basis of gold content. In general, if at least half the atoms in the alloy are gold (which would be 75% by weight), good resistance to corrosion and tarnish can be predicted. Nevertheless clinical evaluations have failed to show statistically significant differences in the tarnish resistance of high-gold (77%) and low-gold (59.5% to 27.6%) alloys." However, a poorly formulated alloy, even of high gold content, can rapidly tarnish intraorally.
Cost. Treatment plans are often modified to suit the financial capabilities of the patient or a third party. Base metal alloys have found favor principally because of their low cost. Similarly, alloys containing approximately 50% gold have been found to offer some economic advantage (although the savings are not proportional to the reduced gold content of the
*At the time of writing, palladium is considerably more expensive at $560 per ounce than gold ($280 per ounce). However, it is less dense (12.0 g/ml compared to 19.3 g/ml so an ounce of metal will yield more restorations).
alloy). Alloys containing primarily palladium* and only a small percentage of gold offer an alternative for use in the metal-ceramic technique, although soldering procedures maybe less predictable.
When calculating the intrinsic metal cost of a restoration, determine the volume of the casting rather than its weight. Dental casting alloys can vary considerably in density from 8 g/ml to almost 19 g/ml (see Table 19-1). An "average" restoration has a volume of 0.08 ml; an all-metal pontic may have a volume reaching 0.25 ml. 19 Therefore, it is conceivable that the cost of a large pontic cast in a low-density alloy would be equal to or less than the cost of a complete cast crown fabricated from a high-density alloy. When noble metal prices are high, more sophisticated techniques of scrap recovery become economically attractive. These can range from installing conventional metal catchers in all areas where castings are finished to equipping all work stations with filtered suction machines.
Clinical Performance. In most respects, clinical performance (biologic and mechanical) is more important than cost. Biologic properties that can be evaluated include gingival irritation, recurrent caries, plaque retention, and allergies. Mechanical properties include wear resistance and strength, marginal fit, ceramic bond failure, connector failure, and tarnish and corrosion.
A risk in choosing a new alloy is that defective clinical performance may fail to appear in laboratory testing or short-term animal and clinical trials. For example, manufacturers introduced copper-based casting alloys with very poor corrosion resistance20 when the price of gold was rapidly rising.* Although the clinically established alloys all have disadvantages, their performance is likely to have been well documented, and the prognosis of restorative treatment can be more accurately predicted.
Laboratory Performance. Sound laboratory data are essential in the selection of a casting alloy. Important areas of consideration are casting accuracy, surface roughness, strength, sag resistance, and metal-ceramic bond strength. Presently available data suggest that nickel-chromium alloys have lower casting accuracy21 and greater surface rough-ness22 than gold alloys (Fig. 22-14) but higher strength and sag resistance because of their higher melting ranges 23
*In fact, these formulations were very similar to aluminum-bronze alloys sold as dental gold in the 1920s.
Handling Properties. The ease with which an alloy can be manipulated may influence its selection. An alloy that produces satisfactory clinical results, but only under extremely critical conditions or with expensive equipment, may be rejected in favor of one that produces acceptable results with less critical manipulation.
The ability to burnish an alloy to reduce marginal gap width and thus reduce the exposed thickness of the luting agent is important,24 although the areas where marginal adaptation is clinically most important (interproximally and subgingivally) are usually not very accessible for such manipulation.
Biocompatibility. All materials for intraoral use should be biocompatible. In addition, it should be possible to handle them safely in the office or laboratory. Many hazardous materials are commonly used in dentistry, such as mercury, chloroform, silver cyanide, and hydrofluoric acid. Consequently, restrictions have been imposed on their shipping and use. For instance, asbestos in casting ring liners and uranium salts in dental porcelain are no longer used. There is also concern25 for the possible health hazards (see Chapter 19) associated with alloys containing nickel and beryllium. Although no definite conclusions can be drawn, appropriate safety precautions are advisable when grinding these alloys. Filtered suction units and appropriate barriers (masks) should be used. The ADA26 requires nickel-containing alloys to carry a precautionary label stating that their use should be avoided in patients with a known nickel allergy (Fig. 22-15).
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