The nomenclature for casting alloys usually creates confusion. Classifying noble and base metal casting alloys according to the mechanism for corrosion resistance is the preferred method. The gold-based and palladium-based noble metal casting alloys achieve corrosion resistance because of the inherent nobility of the gold and palladium atoms, which do not form stable oxides at room temperature. In contrast, the base metal casting alloys, where nickel and cobalt are the principal elements, oxidize rapidly to form a passivating chromium oxide surface layer that blocks the diffusion of oxygen and prevents corrosion of the underlying metal.
Historically, terms such as precious, semiprecious, and nonprecious have been used to describe casting alloys. Such precious or semiprecious alloys usually contained a greater quantity of silver, along with more palladium and a reduced gold content. Silver, which is not a noble metal in the oral environment, assumes some noble metal character in the presence of palladium. The terms precious, semiprecious, and nonprecious, which refer to unit metal cost, are much less preferable than the classification of noble and base metals, which refers to the electrochemical character of the alloys.
The major noble metals in dental alloys are gold, platinum, and palladium. The total percentage of these elements is referred to as the noble metal content of the alloy. Iridium (much less than 1% by weight) and ruthenium (up to about 1%) are used, respectively, as grain-refining elements in gold-based and palladium-based casting alloys. The original metal-ceramic alloy compositions (e.g., Jelenko "O", shown in Table 19-1) had approximately 98% noble metal content by weight. Rapid increases in the price of gold during the 1970s stimulated the development of lower-gold content (from about 85% to 50% by weight) alloys and base-metal alloys for fixed prosthodontics." During the 1980s, the highpalladium alloys were developed as economic alternatives to the gold-based alloys."
A classification system'-' developed by the American Dental Association for casting alloys is presented in Table 19-2 and includes alloys for all-metal and metal-ceramic restorations. Because the classification is based solely on noble metal content and ignores other, often critically important, alloying ele
*Recently, the price of palladium has greatly increased. In January 1997, palladium was $120 an ounce; by February 2000, this had increased to $800 an ounce. However, by April 2000, the price has decreased to $560 an ounce. Rapid increases in alloy prices can cause many problems for dentists and the dental laboratory industry.
ments, general statements cannot he made about mechanical properties, clinical performance, and biocompatibility, even within each of the three groups in Table 19-2. Hundreds of dental alloys are commercially available, and appropriate testing is necessary to characterize the properties, safety, and efficacy of each. However, when each of these major groups is further subdivided into important alloy types, some accurate generalizations are possible. They are discussed in the following sections.
High Noble Metal Alloys. The high-noble metal content alloys contain a minimum of 60% by weight of noble elements; at least 40% is gold. There are three systems in this class: gold-platinum-palladium, gold-palladium-silver, and gold-palladium (in the historical order of their development). Table 19-1 lists some mechanical properties and the density for representative alloys of each system.
Au-Pt-Pd. As previously noted, these were the first casting alloys formulated to bond with dental porcelain. Because of concern about adverse effects on the color of dental porcelain, copper, which was traditionally used for strengthening the highgold casting alloys for all-metal restorations, could not be incorporated in the ceramic alloy compositions. Instead, these alloys were strengthened by precipitates of an Fe-Pt intermetallic compound.16 Porcelain adherence was achieved by incorporating tin and indium in the alloys. During the initial alloy oxidation step for the porcelain firing cycles, these elements (as well as some iron) diffused to the alloy surface and became oxidized. Subsequent chemical bonding was achieved between this oxide layer and the dental porcelain (see Chapter 22). Although these alloys have excellent corrosion resistance, they are susceptible to some dimensional changes during the porcelain firing cycles and are not recommended for multiple-unit FPD restorations.
Au-Pd-Ag, These were the first lower-gold content alternative alloys to be widely used in the 1970s. Platinum was eliminated from the alloy compositions, and the gold content was reduced to about 50%, with corresponding increases in the amounts of ssification for Denta Casting Alloys palladium and silver. 17,18 Some alloy strengthening was achieved by solid solution hardening from the dissimilar atomic sizes of the three major elements (gold, palladium, and silver), which form solutions with each other. Additional solid solution strengthening was hypothesized from tin or indium, which were again incorporated as oxidizable elements to provide porcelain bonding. Further alloy strengthening may occur from precipitates formed by these elements. Although these alloys have excellent mechanical properties and porcelain adherence, green discoloration (resulting from diffusion of silver atoms into the porcelain) has been reported for some alloy-porcelain combinations. 19 Possible reasons for this effect may be the high sodium concentration of the porcelain or the relative sizes of the metal ions in the porcelain. The discolored region can be ground away, but this involves an additional processing step. In addition, silver vapor generated in the porcelain furnace during processing can contaminate the muffle, and periodic purging of the furnace with a carbon block is required. Green discoloration has apparently been eliminated in some porcelain compositions by substituting potassium ions for sodium ions; the larger potassium ions impede the diffusion of silver into the porcelain.
Au-Pd. Gold-palladium alloys that are silver-free were developed during the late 1970s and have become very popular. Alloy strengthening is achieved with a combination of solid solution hardening and microstructural precipitates. The hardness (assumed to be related to strength) of these alloys is independent of heat-treatment temperature within the porcelain-firing range, unlike Au-Pd-Ag alloys.16 The Au-Pd alloys have excellent mechanical properties, elevated-temperature creep behav-ior,20 and porcelain adherence ,21 without the green discoloration associated with Au-Pd-Ag alloys.
Discussion. The data in Table 19-1 show that the Au-Pd and Au-Pd-Ag alloys, when compared to the Au-Pt-Pd alloys, have higher values of yield strength and modulus of elasticity, along with lower density. Consequently, FPD restorations fabricated from alloys in the former two groups will be more resistant to masticatory forces and undergo less bending deflection. They also have the economic advantage of more restorations per unit of alloy cost. Selection of the proper porcelain for Au-Pd-Ag alloys is essential if discoloration problems are to be avoided.
Noble Metal Alloys. The noble-metal alloys have a minimum of 25% by weight of noble metal, with no requirement for gold percentage. There are three alloy systems in this class: palladium-silver, palladium-copper-gallium, and palladium-gallium (in the historical order of their development). Table 19-1 lists some mechanical properties and the density for representative alloys of each system.
Pd-Ag. These alloys, developed in the 1970s, continued the trend by manufacturers of reducing the gold content (to between 0% and 2% by weight), with corresponding increases in the palladium and silver contents . 22 A small percentage of gold in these alloys and the high-palladium alloys has little effect on their properties but may facilitate third-party payments. As previously noted, in the presence of palladium, silver appears to assume noble metal character, which is beneficial for corrosion resistance. Because of their high silver content (30% to 35% by weight), these alloys have been called semiprecious, a term that should no longer be used. Compared to the Au-Pd-Ag and Au-Pd alloys, the Pd-Ag alloys have similar values of yield strength and modulus of elasticity and much lower density values. Because of their high silver contents, porcelain greening and furnace contamination can result during fabrication of FPD restorations, unless the porcelain is carefully selected. Nevertheless, these alloys are frequently chosen as a compromise between the more expensive high-noble alloys and the relatively inexpensive base metal alloys.
Pd-Cu-Ga. The Pd-Cu-Ga alloys contain more than 70% by weight of palladium and were developed in the early 1980s as economical alternatives to the gold-based alloys .'z The melting point of palladium (1555° C) is much higher than that of gold (1064° C); gallium has a melting point of 30° C. The addition of gallium to palladium yields high-palladium alloys that can be fused and cast with the same dental laboratory technology developed for the gold-based casting alloys. Multi-orifice torches are required to fuse the high-palladium alloys, and the use of ceramic crucibles dedicated to individual alloys is recommended." Carbon-containing investments should not be used, because the incorporation of very small amounts of carbon in these alloys degrades the bond strength with porcelain. 13 The Pd-Cu-Ga alloys appear to have castability and casting accuracy comparable to the high noble metal alloys 24
Recent measurements25 of the mechanical properties of some Pd-Cu-Ga alloys have produced values of yield strength, modulus of elasticity, and percentage elongation that differ from values in Table 19-1. This suggests some technique-sensitivity in the fabrication of cast specimens for the tension test. In a recent study, values of yield strength and tensile strength were found to be higher for the Pd-Cu-Ga alloys, compared to Au-Pd-Ag, Au-Pd, and Pd-Ag alloys, while the values for modulus of elasticity and ductility were similar when the same simulated porcelain-firing heat-treatment condition was compared. Although a near-surface eutectic structure was present in Pd-Cu-Ga alloy castings that simulated copings for maxillary incisors," this constituent was absent in the 3-mm diameter cast specimens for the tension test. 23 Some Pd-Cu-Ga alloys have hardness values comparable to or exceeding that for tooth enamel, and castings from these alloys may be difficult to finish in the dental laboratory. In addition, chairside adjustments may be difficult for patients. However, substitution of indium for tin yields Pd-Cu-Ga alloys with much lower hardness (VHN approximately 270).26 All these alloys achieve substantial hardening by solid-solution incorporation of other elements within the palladium crystal structure. The hardest Pd-Cu-Ga alloys (VHN exceeding 300) contain a hard grain boundary phase whose composition is close to that of Pd5Ga2.24 Transmission electron microscopic studies indicate that representative highpalladium alloys have the same bulk ultrastructure. 27 X-ray diffraction analyses have revealed that oxidized Pd-Cu-Ga alloys have complex internal oxidation regions that can contain up to five different oxide phases. Oxides of copper, gallium, tin, indium, and even palladium formed under the conditions present in the porcelain furnace were subsequently detected in the oxidized alloys at room temperature. The results of creep experiments on the Pd-Cu-Ga alloys have been mixed.18 The creep rates associated with relatively high thermal incompatibility stresses near the glass transition temperature of dental porcelain were high for two Pd-Cu-Ga alloys, whereas these alloys had excellent creep resistance at high temperatures and low stresses simulating the deflection of a long-span FPD due to gravity during processing.
Pd-Ga. The copper-free Pd-Ga alloys were subsequently developed during the 1980s to provide compositions with lower hardness than that of the initial Pd-Cu-Ga formulations. The hard Pd5Ga2 phase is absent in these alloys, which are strengthened by solid solution hardening. The alloys have a complex fine precipitate structure at the grain boundaries, and their mechanical properties are generally more similar to those of Pd-Ag alloys rather than the Pd-Cu-Ga alloys. Compared to the Pd-Ga alloys, porcelain adherence is superior for the Pd-Cu-Ga alloys 21 The Pd-Ga-Co alloy30 in Table 19-1 has a particularly dark oxide that is more difficult to mask with dental porcelain. This alloy has not yet achieved widespread clinical acceptance.
Discussion. A recent study comparing the dimensional changes at various stages of the simu lated porcelain firing cycles for copings for metal-ceramic single-unit restorations of selected highpalladium alloys found that most of the selected high-palladium alloys had acceptable high-temperature distortion .25,31 Because of the considerable price volatility for palladium, the unit metal cost for the Pd-Cu-Ga and Pd-Ga alloys has recently become competitive with the very popular Au-Pd alloys. When the high-palladium alloys were introduced in the 1980s, the unit metal cost was between one-half and one-third of the Au-Pd alloys. 2 Consequently, there has been a trend toward the Au-Pd alloys and the much less expensive Pd-Ag alloys; the latter have comparable density to the high-palladium alloys. However, caution is needed with the Pd-Ag alloys to prevent porcelain with green discoloration. Some biocompatibility concerns have been raised about the high-palladium alloys, particularly in Germany with the Pd-Cu-Ga alloys. Recent review articles suggest that there are minimal health hazards associated the high-palladium alloys, although further research in this area is recommended.
Predominantly Base Metal Alloys. Table 19-2 defines these alloys (sometimes termed nonprecious) as having less than 25%, by weight of noble metal with no requirements for gold. Most of these alloys used for fixed prosthodontics are Ni-Cr alloys, but some Co-Cr alloys have also been formulated for porcelain application .29,30
Ni-Cr. Yield strength, hardness, and modulus of elasticity can be greatly affected by small differences in weight percentages of minor elemental components among the compositions of these al-loys.34 Table 19-1 illustrates some of these variations. For example, values of yield strength vary from 260 to 807 MPa, and Vickers hardness varies from 175 to 335. (For comparison, VHN values are 50 to 52, 125 to 127, and 120 to 143 for gold, platinum, and copper, respectively.) Consequently, the selection of a specific brand of Ni-Cr alloy depends on the clinical application. If burnishing or extended finishing of a crown is anticipated, a brand with a relatively low yield strength and hardness should be used.
One benefit of these alloys is their much higher values of modulus of elasticity, compared to the noble metal alloys. Therefore, long-span fixed prosthe-ses fabricated from Ni-Cr alloys will undergo much less flexure than similar prostheses fabricated from noble metal alloys, with less likelihood of fracture of the brittle dental porcelain component. These base metal casting alloys are generally considered more technique-sensitive and difficult to cast that the noble metal casting alloys. However, this assessment may reflect the lack of experience of some dental laboratories with the Ni-Cr alloys. Therefore, the choice of dental laboratory is particularly important when these alloys are selected.
Beryllium. Many Ni-Cr alloy formulations contain up to 2% by weight of beryllium. The major reason for incorporating this element in the alloy is to lower the melting range and to decrease the viscosity of the molten alloy, thereby improving its castability. Beryllium also provides strengthening and affects the thickness of the oxide layer, when the alloy is oxidized for porcelain firing. The latter is an important consideration for base metal casting alloys, which can form much thicker oxide layers than noble metal casting alloys. Fracture through the oxide layer may occur and will cause failure of the base metal-ceramic restoration.
The use of beryllium has created some doubt about the safety of some Ni-Cr alloys. NOTE: When the densities of nickel (8.9 g/cm3) and chromium (7.2 g/cm3) are compared to beryllium (1.8 g/cm3), 2% by weight of beryllium in the alloy composition can be equivalent to nearly 10% beryllium on an atomic basis. Consequently, the atomic proportion of beryllium atoms in these alloy compositions can be relatively large.
Nickel. The U.S. Federal Standard for exposure to metallic nickel and soluble nickel compounds (1 mg/m3) is much greater than the proposed National Institute for Occupational Safety and Health (NIOSH) recommendation for such exposure (15 ug/ms for a 10-hour TWA workday). Occupational exposure of refinery workers to nickel has been associated with lung and nasal cancer. Acute effects of exposure to nickel include skin sensitization that can lead to chronic eczema. Therefore, as a health precaution, an operator should wear a mask and use efficient suction when grinding and finishing a dental nickel-base alloy.
One study reports that 9% of the female population and 0.9% of the male population are sensitive to nickel. This prompts the question: Are such individuals likely to manifest an adverse reaction to dental Ni-Cr alloys? In a 20-patient clinical study36 to investigate this question, each of 10 controls (who had no known sensitivity to nickel) showed a negative dermal response and a negative intraoral response to a dental Ni-Cr alloy. Among 10 patients with a known sensitivity, 8 showed a positive dermal response to the alloy. When these patients wore an intraoral appliance containing the Ni-Cr alloy, 30% manifested an allergic response within 48 hours.
The ADA has issued a labeling requirement for base metal alloys that contain nickel. It states that such alloys should not be used in individuals with known nickel sensitivity. Another question now arises: Can patients who are not allergic to nickel become sensitive to it from fixed prostheses made with nickel-containing alloys?
A recent investigation 31 found that Ni-Cr alloys not containing beryllium were more resistant than beryllium-containing alloys to in vitro corrosion. The four alloys studied showed lower corrosion rates in cell culture solutions after cold solution sterilization. Although the corrosion products released from the alloys did not alter the cellular morphology and viability of human gingival fibroblasts, reductions in cellular proliferation were observed. The authors concluded that biocompatibility concerns still exist relating to the exposure of local and systemic tissues to elevated levels of corrosion products from the Ni-Cr alloys.
Co-Cr. The potential health problems associated with beryllium- and nickel-containing alloys have led to the development of another alternative base metal alloy system: cobalt-chromium . 31,39 The modulus of elasticity of the Co-Cr system is the highest of any of the ceramic alloy systems discussed so far (see Table 19-1). One alloy, similar in composition and properties to Co-Cr and used for RPD frameworks, has an elongation of 1%. This very low value, combined with its high level of hardness, suggests that finishing restorations made with it may be difficult.
Ti. Titanium-based alloys have been studied since the late 1970s as potential casting alloys .40 Advantages of titanium-based casting alloys include excellent biocompatibility and corrosion resistance of titanium, which is due to the presence of a thin, adherent, passivating surface layer of TiO, The low density (4.5 g/cm3 ) of titanium, compared to gold or palladium (19.3 and 12.0 g/cm3, respectively), also results in lighter and potentially less expensive restorations*. Alloys studied have included CP (commercially pure) titanium, Ti-6Al-4V, and a variety of experimental alloys .40-42 The casting of titanium-based dental alloys poses special problems because of titanium's high melting point (1668° C) and its strong tendency to oxidize and react with other materials.
Special casting machines must be used that provide either a vacuum environment or an argon atmosphere. Both vacuum/argon pressure and centrifugal casting machines have been developed, and both argon-arc melting and induction melting have
*However, the laboratory cost of fabricating cast restorations from titanium alloys may be high.
been used to fuse the Ti alloys. Additional studies have discussed the use of face coats on the wax pattern" and casting into low-temperature (350° C), phosphate-bonded investments."" The effects of the argon gas pressure, permeability of the investment, and mold venting have also been studied .45-48 Good accuracy for titanium castings on their dies can be achieved with the proper choice of investment."" Selecting a dental laboratory experienced in fabricating these castings is essential. Further research is needed to optimize the metallurgical structure and casting technology for titanium alloys, which have a dendritic microstructure41 resulting from the lack of a suitable grain-refining element. A very hard near-surface region that can exceed 50 um in thickness is also present on the castings due to reaction of the titanium alloy with the investment and perhaps with the residual atmosphere in the casting machine.41 ,44 To overcome the difficulties in casting titanium, a system has been developed that manufactures copings from blocks of pure titanium by machine duplication and spark erosion (Procera*) 9 (Fig. 19-19). Because copings are machined, some error is intro-
*Nobel Biocare: Chicago, 111.
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