Alloys for prosthodontic restorations

Alloys for prosthodontic restorations John C. Wataha, DMD, PhDa School of Dentistry, Medical College of Georgia, Augusta, Ga. The numbers and types of alloys for prosthodontic restorations have increased dramatically over the past 25 years, making selection of an alloy for a given clinical situation difficult. Factors such as cost, the need for better strength, and worries about alloy corrosion have pressured the alloy market to change significantly. A number of properties—including yield strength, hardness, elastic modulus, microstructural phases, grain size, corrosion performance, coefficient of thermal expansion, oxide color, and melting range—are relevant to the proper selection of an alloy for a given clinical problem. In this article, a brief historical look at prosthodontic alloys and the nomenclature for alloys is followed by a discussion of the most important physical properties of alloys for clinical practice. A summary of the types of alloys available today and their classification is then presented. Finally, speculations about future trends for alloys are made, and simple guidelines are suggested to help dentists choose appropriate alloys for their practices. This review excludes implant alloys, dental amalgam, and alloys for orthodontic and endodontic applications. (J Prosthet Dent 2002;87:351-63.)

A

lloys for prosthodontic restorations have become increasingly complex in the last 25 years. There are hundreds of choices from dozens of global companies. Today’s alloys have, as their most abundant element, a number of metals that include gold, palladium, silver, nickel, cobalt, and titanium. The metallurgy of each of these alloy systems is generally complex and demanding of the laboratory and the dentist. The proper selection and manipulation of these alloys is imperative if dental prostheses are to perform well with longevity. Improper selection or manipulation by the laboratory or the dentist may result in the failure of expensive restorations, possibly leading to litigation by the patient. In this article, several aspects of dental alloys are presented. A brief historical look at prosthodontic alloys and the nomenclature for alloys is followed by a discussion of the most important physical properties of alloys for clinical practice. A summary of the types of alloys available today and their classification is then presented. Finally, speculations about future trends for alloys are made, and simple guidelines are suggested to help dentists choose appropriate alloys for their practices. For the sake of brevity, this review excludes implant alloys, dental amalgam, and alloys for orthodontic and endodontic applications. What is an alloy? An alloy is a metallic material formed by the combination of 2 or more metals or 1 or more metals with a nonmetal.1 In their molten state, metals dissolve to various degrees in one anothaProfessor,

Departments of Oral Rehabilitation and Oral Biology and Maxillofacial Pathology. Presented at the Academy of Prosthodontics Annual Meeting in Santa Fe, NM, May 18, 2001. Financial support provided by the Metalor Group, Medical College of Georgia Biocompatibility Program.

APRIL 2002

Table I. Tensile strength and hardness of pure gold and various gold alloys53 Tensile strength (MPa)

Cast 24K gold Condensed gold foil Gold + 10 wt% Cu Cast Au-based alloy (soft condition) Cast Au-based alloy (hard condition)

105 250 395 425 525

Hardness (kg/mm2)*

28 (VHN) 60 (VHN) 85 (BHN) 135 (VHN) 195 (VHN)

*Hardness is given in either Brinell hardness number (BHN) or Vickers hardness number (VHN).

er, allowing them to form alloys in the solid state. Just as not all liquids are soluble in one another, not all metals are soluble in one another. Thus, gold and palladium will freely mix together, but copper and silver will not. This extent of solid solubility depends on the relative sizes of the individual atom species, the crystal structure formed by the pure metal components, the valences of the components, and their reactivity.2 It is worthwhile to consider why alloys are used in dentistry. Why not just use pure metals such as gold? Alloys are needed for their superior strength and other physical properties (Table I). The tensile strength and hardness of pure cast gold are approximately 20% and 15%, respectively, of that of a typical gold-based casting alloy. Pure metals do not have appropriate physical, biological, or corrosion properties (other than noble metals for the latter) to serve adequately in the mouth in prosthodontic applications.

ALLOYS IN DENTISTRY: PRE-1975 Before 1975 (an arbitrary date for the purposes of this discussion), prosthodontic alloys could be dividTHE JOURNAL OF PROSTHETIC DENTISTRY 351

THE JOURNAL OF PROSTHETIC DENTISTRY

WATAHA

Table II. Composition and physical properties of goldbased casting alloys, pre-197554 Composition (wt%) Au Ag Pd

Type

I II III IV

81-83 76-78 73-77 71-74

— — 0-1

0.2-0.5 1-3 2-4 2-5

Tensile strength (MPa)*

300 365 530 770

Vickers hardness (kg/mm2)*

65 130 160 235

*Hardened

condition, promoting ordered solution. Note that type I and II alloys had no hard and soft conditions because they contained insufficient copper.

Table III. Composition and physical properties of some porcelain-bonding alloys, pre-197555 Alloy

1 2 3 4 5 6

Composition (wt%) Au Ag Pd

85 50 52 — Ni or Co 68 Ni 65 Co

1

30 Cr 15 18

6 40 28 65 C 0.1 0.1

Tensile strength (MPa)

Vickers hardness (kg/mm2)

490 560 500 650 — 700 680

175 220 220 210 — 290 400

ed into 3 distinct and mutually exclusive groups: alloys for full-cast restorations, porcelain-bonding alloys for metal-ceramic restorations, and alloys for removal partial denture frameworks. Casting alloys were subdivided into 4 types by the American Dental Association (ADA) (Table II), 3 of which (types I-III) were commonly used for cast intracoronal or extracoronal restorations. Type I alloys were used for inlays with little or no occlusal contact. These alloys were relatively weak but highly burnishable, with percentage elongation values (permanent strain at fracture) exceeding 25%. Type II alloys were somewhat stronger, with good values of percentage elongation and sufficient strength to tolerate some occlusal contact such as with onlays. Type III alloys had lower ductility (percentage elongation) but were nearly twice as strong as type I alloys and were the mainstay for full crowns and fixed partial dentures. Remarkably, all of these alloys were gold-based, with only a few compositional differences among them, largely in the amount of copper, silver, and palladium. The most common alloy by far was the type III alloy with a typical approximate composition of 75% gold, 10% silver, 10% copper, 3% palladium, and 2% zinc (values in wt%). It is important to note that, in this pre-1975 system, the alloy “type” indicated both compositional class and physical properties.3 Porcelain-bonding alloys were distinct from fullcast alloys largely because the former required melting 352

Table IV. Important physical properties of removable partial denture alloys, pre-197556 Alloy

Nickel-based* Cobalt-based* Jelenko LG (Co-Cr-Ni) Type IV gold *Includes

Elastic modulus (GPa)

Density (g/cc)

Elongation (%)

186 228 228 90

7.5 7.6 7.5 15.2

1.7 1.5 10.0 6.0

carbon in the composition.

ranges that could survive the application of porcelain (typical firing temperatures of 870°-1370°C).4 All of the alloys in Table II had liquidus temperatures, which represent the upper end of the melting range, of less than 950°C3; these temperatures were well below that necessary for most common porcelain applications. Furthermore, casting alloys commonly contained copper and silver, both of which could present problems with sagging of the substructure (copper)5,6 or greening of the porcelain (silver) during porcelain firing. Five to 6 types of porcelain-bonding alloys were used (Table III), of which 4 were gold- or palladium-based and governed by ANSI/ADA Specification No. 38. The most common alloys were gold-palladium. Compared to their full-cast counterparts, the porcelain-bonding alloys had higher palladium contents to raise the fusing range to at least 1150°C.7 The use of copper was minimized to reduce sagging,5,6 and many laboratories avoided alloys with silver to eliminate greening problems. Although nickel and cobalt alloys existed for metal-ceramic applications at this time, their use was not nearly as common as it is today. Basemetal alloys used for metal-ceramic restorations differed from those used for removable partial dentures by the absence of carbon. Alloys for removable partial dentures consisted of type IV gold (Table II) and nickel- or cobalt-based alloys (Table III). The type IV gold alloys contained enough palladium and copper to provide strength through solid-solution hardening but not enough palladium to cause the alloy to whiten (which occurs above 10 wt.% palladium).8 The nickel- and cobaltbased alloys were similar to their porcelain-bonding counterparts, except that the former contained 0.1 to 0.5 wt% carbon to harden and strengthen the alloy through carbide formation. A hybrid nickel-cobalt alloy (Jelenko LG; Armonk, N.Y.) also was used and contained 27% chromium, 13% nickel, and approximately 50% cobalt. The base-metal alloys were much stronger and harder than the type IV gold-based alloys, but more importantly for removable partial denture frameworks, they possessed relatively high elastic moduli and low densities (Table IV). These properties allowed the base-metal partial framework to VOLUME 87 NUMBER 4

WATAHA

be thinner, lighter, less expensive, and stiffer than its type IV counterpart, which gradually discouraged the use of type IV gold alloys for removable partial dentures. As of 1969, almost 87% of these frameworks were made of base-metal alloys (up from <20% in 1949).9 Type IV gold alloy frameworks are virtually unknown today. Because the majority of alloys for prosthodontic applications were gold-based in the pre-1975 era, the nomenclature of alloys was relatively simple. Restorations were made from either gold-based alloys, often called “precious” alloys, or from base-metal alloys, analogously called “nonprecious.” As the price of gold increased and made palladium-based and reduced gold-based alloys more economically competitive, a third category, often termed “semiprecious,” was added. The use of the terms precious, semiprecious, and nonprecious is now discouraged because they identify alloys based on the cost of the component metals. Given that these costs can vary widely, the terms are inevitably inaccurate. Furthermore, silver is considered a precious metal on the metals commodity market but not within the old nomenclature system since pure silver is subject to corrosion in the oral environment. Finally, the terms precious, semiprecious, and nonprecious do not describe the electrochemical behavior of alloys well. Thus, a new system of nomenclature evolved and will be discussed later in this article.

MAJOR FACTORS THAT AFFECT ALLOY AVAILABILITY Over the past 25 years, several major factors have changed the alloys used for prosthodontic restorations. The first factor has been economic, manifested gradually after the deregulation of the price of gold in 1969 and more recently (1995–2001) by flux in the price of palladium. Previously fixed at USD 35/oz, the price of gold varied widely in the late 1970s and early 1980s, reaching USD 800/oz in 1980 (see www.kitco.com or other Internet sites for commodity metal prices). Although the price of gold has stabilized in more recent years (to approximately USD 265/oz in 2001), previous instability caused manufacturers to seek alloys with less gold content. These lower-gold alloys were less expensive, and their prices were more stable. As a consequence of the fluctuation in gold prices, palladium-based alloys became very common in the late 1980s and early 1990s. Another noble metal, palladium cost only about USD 150/oz at that time. However, in the past 5 to 6 years, the price of palladium has fluctuated greatly (from USD 1000/oz in late 2000 to USD 470/oz in late 2001) because of social and economic turmoil in Russia, where much of the world’s palladium is mined. In the past 3 years, alloy manufacturers have again been presAPRIL 2002

THE JOURNAL OF PROSTHETIC DENTISTRY

sured to find alloy compositions that use less palladium. Silver has also experienced price fluctuations. In the late 1970s, the price of silver quadrupled from approximately USD 4/oz to almost USD 20/oz. The effect of these price changes on the prosthodontic alloy market was smaller because silver was a relatively minor component of most alloys at that time and because even at USD 20/oz, the cost of silver was minor compared to that of gold and platinum. Interestingly, the price of platinum has stayed relatively constant over the years (USD 476/oz in late 2001). A second major factor that has affected the evolution of alloys has been the need for better physical properties. The need for alloys with higher elastic moduli (stiffness) in removable partial denture alloys has already been discussed. Higher elastic modulus is more desirable because a high-modulus alloy will not bend as much when loaded in flexure and will therefore transmit occlusal forces more efficiently to the remaining teeth or other tissues. Alloys with high elastic moduli are also important in metal-ceramic fixed partial dentures. Any flexure of the metal substructure will cause fracture of the overlying porcelain because of the brittle nature of the ceramic. In single units and short-span fixed partial dentures, gold-based alloy systems possess ample elastic modulus to prevent this flexure. However, in longer-span fixed partial dentures, an elastic modulus of 90 GPa is not sufficient to prevent clinically problematic levels of flexure.10 This problem becomes acute if the metal substructure is thin or the connectors between units must be narrow because of anatomical or esthetic constraints. Thus, with an increase in metal-ceramic restorations, the alloy market has been pressured to offer alloys with higher values of elastic modulus. As Table IV shows, the elastic modulus of even the strongest gold-based alloy (old type IV) if only about 50% that of base-metal alloys (185-225 GPa). Palladium-based alloys offer a slightly higher modulus (115-125 GPa) than their gold-based counterparts but still cannot match that of the base-metal alloys. For comparison, the elastic modulus of high-grade steel is approximately 206 GPa.11 A third major factor that has shaped the alloy market is concern about corrosion and biocompatibility. Public awareness of the release of metals from alloys into the body has grown over the past 10 to 15 years and has periodically erupted into an irrational panic, as with the palladium scare in Germany in the early 1990s.12 Although no scientific evidence of serious biological problems (except allergic reactions for some individuals) with the use of palladium in dental alloys has ever existed,13 concerns about palladium became the focus of the public eye, and consumers demanded palladium-free alloys. In response, manu353

THE JOURNAL OF PROSTHETIC DENTISTRY

WATAHA

alloys that are critical to the clinical performance of prosthodontic restorations are reviewed.

Grain size

Fig 1. Photomicrograph of high-noble alloy surface etched with acid to expose alloy grain structure (original magnification × 100). Lines throughout picture are grain boundaries. Each grain is single alloy crystal, and average grain diameter in this alloy is approximately 10 µm. Thus, alloy would be considered fine-grained. Grains appear to have slightly different colors because each crystal is at different orientation to observer and therefore reflects light differently. (Photomicrograph courtesy of Metalor Corporation, Neuchatel, Switzerland)

facturers developed these alloys, which are available today on a global scale. Over the past 10 years, some manufacturers have even sought to develop alloys that release less mass than was accepted previously. This strategy has lead to the use of newer, more complex alloy systems. The development and evolution of alloys for prosthodontics has not been without technical problems. The introduction of new types of alloys has created problems for both the laboratory and the dentist. Newer alloys may be more difficult to cast, not cast as accurately, be more difficult to solder and finish, or involve esthetic challenges, among other problems.14 The development of alloys is complex and requires a knowledgeable company with experience in research and development. The ability of that company to provide reproducible alloy quality and technical assistance is also very important. Even the highest quality alloy, if abused, may not perform well intraorally. The dentist and technician should appreciate these complexities, select quality products that have proven records, and use them exactly as specified by the manufacturer.

CLINICALLY IMPORTANT PROPERTIES OF DENTAL ALLOYS The selection of alloys by the dentist and technician is always a matter of cost. However, selecting alloys with the best physical, chemical, and biological properties for a specific clinical situation should always be the first priority. In this section, several properties of 354

Although knowledge of metallurgy is generally beyond that required by dentists and technicians, there are several metallurgical concepts that can help the practitioner select appropriate alloys. The first concept is that of grains in alloys. When a molten alloy cools to the solid state, crystals form around tiny nuclei (clusters of atoms). As the temperature drops, these crystals grow until the crystal boundaries meet each other in the solid state. At this point, each crystal is called a grain and the boundaries between crystals are grain boundaries (Fig. 1).15 Grains are important for the physical properties of alloys because grain size influences other clinical properties. Small grains have been found to improve the elongation and tensile strength of cast gold alloys (but do not affect hardness or yield strength).16 Grain size is determined by a number of variables, including the cooling rate of the solidifying alloy and the presence of special grain-refining elements in the alloy composition. Most alloy manufacturers use small amounts (50 ppm) of elements such as iridium or ruthenium as grain refiners in gold-based casting alloys. A higher weight percentage (0.5%-1%) of ruthenium is used as a grain-refining element in the high-palladium casting alloys for metal-ceramic restorations.17 Because these elements have high melting points (iridium melts at 2410°C and ruthenium at 2310°C), they remain solid during casting and act as “seeds” around which grains grow as the alloy cools. In general, a grain size of 30 µm or less has been reported to be desirable in dental alloys.18 Grain sizes vary from 10 to 1000 µm. Dendritic structures (roughly analogous to grain structure) may be very large in base-metal alloys, where the size of a single grain can approach the diameter of a removable partial denture framework clasp.19 Under normal conditions, the grain structure of alloys is not visible. Special acid etching and magnification are generally necessary to view grains.

Phase structure A second metallurgical concept important to the clinical performance of alloys is phase structure (also called microstructure, which includes the grain structure of the alloy). Alloys can be either single phase (Fig. 2, A) or multiple phase (Fig. 2, B). Single-phase alloys have essentially the same composition throughout, whereas multiple-phase alloys have areas of composition that differ by microstructural location. Whether an alloy is single- or multiple-phase is dependent on the solubilities of the alloy elements.20 If all elements are completely mutually soluble in the solid state (as is the case with gold, palladium, and copper), VOLUME 87 NUMBER 4

WATAHA

THE JOURNAL OF PROSTHETIC DENTISTRY

A

B Fig 2. Scanning electron micrographs of single-phase (A) and multiple-phase (B) alloys (original magnification × 1000). Total width of each micrograph is approximately 100 µm. In single-phase alloy, little structure is evident aside from polishing scratches because alloy is reasonably homogeneous throughout. In multiple-phase alloy, several different microstructural constituents are visible. Each phase has different composition; this can be verified by quantitative elemental analyses with electron beam microprobe or scanning electron microscope. Each phase in multiple-phase alloy has its own grain structure.

then the alloy will be single-phase. If some elements are not soluble in one another (such as gold and platinum), then the alloy may be multiple-phase. Phase structure affects the corrosion, strength, and etching characteristics of alloys. In general, multiplephase alloys are prone to higher corrosion rates than single-phase alloys because of galvanic effects between APRIL 2002

the microscopic areas of different composition.21,22 However, the presence of multiple phases also allows them to be etched for bonding. Because each phase generally etches at a different rate, etching creates a microscopically rough surface. The effect of phase structure on strength is more complex. In some cases, multiple phases greatly strengthen an alloy, but in oth355

THE JOURNAL OF PROSTHETIC DENTISTRY

Fig 3. Common, clinically relevant forces on alloys. In top illustration, tensile (opposing) forces increase until they begin to distort alloy. Alloy becomes slightly longer. In middle illustration, diamond indenter is subjected to known force to make permanent indentation in alloy surface. Larger indentation equates to softer alloy. In bottom illustration, force is applied to flex alloy reversibly (flexure is denoted by smaller arrow below bar). Greater flexure equates to lower elastic modulus and reduced stiffness of alloy.

ers they weaken the alloy. Strengthening depends on the nature of the second phase (particularly its ductility), its composition, and dispersion throughout the other phases.23 Single-phase alloys are almost always easier to manipulate in the laboratory, have more consistent properties, and are less technique-sensitive. It is for these reasons that manufacturers have preferred to sell single-phase alloys. The phase structure of an alloy is not visible without substantial magnification (alloys generally are viewed under a scanning electron microscope). Small amounts of secondary phases often can be found at the grain boundaries of the major matrix phase.

Yield strength, hardness, and elastic modulus Several mechanical properties are important for good clinical performance of dental alloys (Fig. 3). The tensile yield strength must not be exceeded or permanent distortion of the restoration will occur. Yield strength is defined as the stress required to permanently deform an alloy a small, standardized amount (typically 0.1% or 0.2%). The most likely site for this type of failure is between pontics in a multipleunit fixed partial denture. Alloys with tensile yield strengths above 300 MPa are strong enough to resist permanent intraoral deformation in most clinical situ356

WATAHA

ations.24 The hardness of the alloy must be enough to resist occlusal forces but not wear opposing teeth. Generally, alloys with a Vickers hardness of less than 125 kg/mm2 are susceptible to wear and alloys that are harder than 340 kg/mm2 (hardness of enamel24) are at risk of wearing opposing teeth.23 The modulus of elasticity (elastic modulus) is a measure of the stiffness or rigidity of an alloy, since it corresponds to the amount of stress for unit elastic strain. The elastic moduli for prosthodontic alloys need to be high so that the prosthesis can resist flexure, especially in metal-ceramic restorations where any flexure will cause fracture of the porcelain. The elastic moduli of most gold- or palladium-based alloys range from 90 to 120 GPa (based on manufacturer alloy properties charts). Such elastic moduli are sufficient in most clinical situations but may not be adequate for long-span metal-ceramic restorations or removable partial dentures.10 In the latter cases, nickel- or cobalt-based alloys, which have a moduli of 180 to 230 GPa (Table IV), may be more appropriate.

Color The color of an alloy has been the focus of dentists and patients for many years. Historically, yellow-colored alloys have been associated with high gold content, high cost, high social value, and good clinical performance. Similarly, white- (silver-) colored alloys have been associated with corrosion, less social affluence, and low cost. However, in today’s alloy market, color is useless in making any judgments about composition, cost, or clinical performance. There are many examples of high-gold alloys that are white in color. In fact, any alloy that contains greater than 10 wt% palladium will be white, regardless of the gold content.25 Similarly, alloys exist that have a yellow color even with no gold present (Pd-In-Ag, for example).26 As described previously, the cost of alloys is not necessarily related to gold content but can be influenced by the cost of other non-yellow elements such as palladium, platinum, and silver. The clinical performance of alloys is related to almost every physical property except color. Thus, the dentist should never make a clinical judgment about an alloy based on its color. The patient’s association of social value with yellowcolored alloys may play a part in the clinical decision, but it should never be the sole basis for alloy selection.

Corrosion The corrosion of an alloy is of central importance to the success of a prosthesis. For metals and alloys, corrosion is always accompanied by a release of elements and a flow of current. Virtually every alloy known will corrode to some extent intraorally, but alloys vary significantly in this regard. Corrosion can lead to poor esthetics, compromise of physical properties, or VOLUME 87 NUMBER 4

WATAHA

increased biological irritation (Figs. 4 and 5).27 Corrosion can be measured either as the current flow or the amounts of released elements. Both methods are commonly used, but elemental release is probably more relevant to any adverse biological effects that corrosion might have. Corrosion is complex and impossible to predict based simply on the composition of the alloy. The presence of multiple phases or high percentages of non-noble elements does, however, increase the risk of corrosion.28,29 In dental metallurgy, 7 elements are recognized as noble: gold, platinum, palladium, iridium, rhodium, osmium, and ruthenium. Corrosion of alloys may be clinically visible if it is severe (Figs. 4 and 5), but more often the release of elements continues for months or years at low levels and is not visible to the eye.30 Corrosion is clearly related to biocompatibility, but the relationships between them are complex and difficult to predict.29 Currently, the only way to know the biological effects of an alloy is to test it for biocompatibility in vitro and in vivo.

THE JOURNAL OF PROSTHETIC DENTISTRY

Fig 4. Consequences of alloy corrosion. Severe corrosion in this specimen markedly compromised physical integrity of alloy and left unsightly and clinically unacceptable surface. (Photograph courtesy of Metalor Corporation, Neuchatel, Switzerland.)

Porcelain-bonding properties If an alloy will be used for a porcelain restoration, then several properties in addition to those mentioned previously are important for clinical performance. The color and thickness of the alloy oxide must be considered. High-gold alloys have a relatively light-colored oxide that is easier to mask with opaque porcelain, whereas most silver-, nickel-, and cobalt-based alloys have darker, gray oxides that require thicker layers of opaque porcelain to mask. If these oxides are not completely masked, they will impart a lower value to the porcelain shade.31 In general, research has shown that thicker oxides increase the risk of metal-ceramic bonding failure.32 Fracture tends to occur because oxides are brittle and weaker than either the porcelain or the alloy. Furthermore, stress from occlusal loads is often concentrated in the oxide layer. The thickest oxide layers occur in nickel- and cobalt-based alloys because these alloys contain elements that form oxides easily during the initial oxidation step (historically and incorrectly termed “degassing”) prior to firing of the opaque porcelain. Gold and palladium form oxides much more sparingly because of their noble character; alloys based on these metals therefore require the addition of tin, gallium, indium, or other trace elements to promote oxide formation. Even with such additions, oxides in gold- and palladium-based alloys are thinner.31 Caution must be exercised in reusing gold-based alloys for metal-ceramic restorations because the oxide-forming elements may be depleted from the first casting procedure. In that situation, the oxide layer would be inadequate for reliable bonding of the porcelain. The relative expansion between the metal and APRIL 2002

Fig 5. Consequences of alloy corrosion for surrounding tissues. When corrosion is severe, ions from alloy can be released in sufficient quantities to discolor adjacent tissues (seen here lingual to molar and premolar). Corrosion is ongoing in all alloys, but released ions do not normally reach sufficient quantities to be visible to eye. (Photograph taken by P. Colon, University of Paris VII, France, and provided courtesy of Metalor Corporation, Neuchatel, Switzerland)

ceramic is of prime importance in porcelain bonding (Fig. 6). Both alloys and porcelain expand when heated and contract when cooled. If porcelain and an alloy bond together at a high temperature (the sintering or firing temperature of the porcelain), then the relative contraction rates of these 2 materials will be important as the bi-material bond cools to room temperature. If the porcelain contracts less than the alloy as cooling progresses, then the porcelain will have residual com357

THE JOURNAL OF PROSTHETIC DENTISTRY

WATAHA

Table V. Solidus and liquidus temperatures of current common classes of prosthodontic alloys* ADA Classification

Solidus temperature (°C)

Liquidus temperature (°C)

High-noble High-noble High-noble Noble Noble Noble Noble Base-metal Base-metal Base-metal Base-metal

1060 1160 905 880 1145 1185 990 1160 1330 1250 1215

1140 1260 960 930 1270 1230 1045 1270 1390 1310 1300

Alloy type

Au-Pt Au-Pd Au-Cu-Ag-Pd Au-Cu-Ag-Pd Pd-Cu Pd-Ag Ag-Pd Ni-Cr-Be (Cr <20 wt%) Ni-Cr (Cr <20 wt%) Ni-Cr (Cr >20 wt%) Co-Cr

Fig 6. Forces that exist because of differences in coefficient of thermal expansion (CTE) between porcelain (P) and alloy (A) for metal-ceramic restorations. Before bonding, porcelain and alloy are same length at room temperature (25°C). When fired in porcelain oven, both materials expand (900°C expansion exaggerated for illustrative purposes). Bond between 2 materials occurs at high temperature, and then porcelain-metal couple is cooled to room temperature. If CTE of porcelain is greater than that of alloy (PCTE >> ACTE), then porcelain will try to contract more than alloy as couple cools. Because materials are bonded, however, porcelain will be placed in tension. Conversely, if CTE of alloy is greater than that of porcelain (ACTE >> PCTE), then alloy will shrink more and porcelain will be in residual compression at room temperature. Because porcelain is brittle, failure of restoration will be less likely when tensile stresses are avoided.

pressive stress at room temperature. If the porcelain contracts more than the alloy, then the porcelain will have residual tensile stress at room temperature. Because porcelain is a brittle material and thus subject to failure by crack propagation, it does not tolerate tensile stresses well. Thus, the metal-ceramic bond must minimize residual tensile stresses in the porcelain. It is best to select a porcelain with a coefficient of thermal expansion (and contraction) that is less than that of the alloy. Most dental alloys have coefficients of thermal expansion between 13.5 and 17.0 × 10–6/°C. Traditional ceramics have coefficients of 13.0-14.0 × 10–6/°C, but newer ceramics may vary substantially from this range.33 It is therefore critical to consult the alloy manufacturer when selecting a porcelain for a given alloy. It is important to note that the coefficient of thermal expansion for the porcelain cannot be too much smaller than the alloy, or the porcelain-metal bond will fail as a result of compressive stresses. Generally, a 0.5 × 10–6/°C difference in coefficients is desirable.34 Alloys for metal-ceramic restorations must have melting temperatures that are compatible with the 358

*Information adapted from manufacturer brochures (Metalor Dental USA, North Attleborough, Mass, Brochure 031405068.0698.3R; Williams, Amherst, NY, 1998 brochure; BEGO USA, Smithfield, RI, no brochure number available).

Table VI. Current American Dental Association definitions for alloy classification by composition36 Class

High-noble Noble Predominately base-metal

Composition

Au content ≥40 wt% Noble metal content ≥60 wt% Noble metal content ≥25 wt% Noble metal content <25 wt%

peak firing or sintering temperature of the porcelain. Because they are mixtures of elements, alloys have melting ranges rather than single melting point temperatures (Table V). Each alloy has a lower temperature (called the solidus temperature) at which melting begins and an upper temperature (called the liquidus temperature) at which the entire alloy is melted. During porcelain firing or soldering operations, it is critical to stay below the solidus temperature of the alloy. Most laboratory technicians recommend using a porcelain that sinters at least 50°C below the alloy solidus temperature (porcelain fuses from 870°C to 1370°C)35 to prevent distortion of the alloy substructure at high temperatures. If a restoration is to be soldered after the application of the porcelain, then the solder must have a liquidus temperature at least 50°C below that of the porcelain sintering temperature and the solidus temperature of the alloy.

ALLOYS AVAILABLE IN DENTISTRY TODAY Before 1975, alloys for full-cast restorations, metalceramic restorations, and removable partial denture frameworks were almost mutually exclusive. Today, there is significant overlap of alloys in these 3 areas. There are several reasons for this convergence. First, VOLUME 87 NUMBER 4

WATAHA

THE JOURNAL OF PROSTHETIC DENTISTRY

Table VII. Current American Dental Association definitions for alloys types by physical properties37 ADA type

Hardness

I II III

Soft Medium Hard

IV

Extra-hard

*In

Clinical use

Low stress, no occlusion, inlays Moderate stress, light occlusion, onlays and inlays High stress, full occlusal load, crowns, short-span fixed partial dentures Very high stress, thin veneer crowns, long-span fixed partial dentures, removable partial dentures

Yield strength* (MPa)

Elongation(%)

<140 140-200 201-340

18 18 12

>340

10

tension.

Table VIII. Approximate composition and properties of some current high-noble alloys* Subclass

Au-Pt Au-Pd Au-Cu-Ag-Pd

Approximate composition (major elements, wt%)

Elastic modulus (GPa)

Vickers hardness (kg/mm2)

Yield strength† (MPa, 0.2% offset)

CTE (×10–6/°C)

Au 85; Pt 12, Zn 1 (Ag)‡ Au 52; Pd 38; In 8.5 (Ag)‡ Au 72; Cu 10; Ag 14; Pd 3

65-96 105 100

165-210 280 210

360-580 385 450

14.5 14.3 NA

*All properties are in the hardened condition, when applicable. Information adapted from manufacturer brochures (Metalor Dental USA and Williams; see footnote to Table V). CTE = Coefficient of thermal expansion. †In tension mode. ‡Element in parentheses present in some formulations.

high-temperature casting (>1100°C) has become commonplace as the fabrication of cast prostheses has moved from the dental office to the dental laboratory, where high-temperature casting equipment is common and technicians are experienced in its use. Second, fluctuations in the price of gold led to the development of alloys with higher melting ranges for full cast restorations or prostheses. These newer alloys could often be used for full cast and metal-ceramic restorations. Third, an emphasis on low corrosion and the knowledge that dissimilar alloys may lead to corrosion has encouraged the use of fewer alloy types. For all of the reasons, it is no longer possible to categorize an alloy exclusively by its use. Today, alloys are classified with 2 separate criteria: composition and physical properties. Because alloys are no longer exclusively gold-based, a broader classification scheme was developed in 1984 by the ADA36 to accommodate new alloys (Table VI). This classification scheme, which identifies alloys as high-noble, noble, or predominately base-metal, is based on the amount of gold and other noble elements in each alloy. The underlying assumption is that alloys with higher noble metal content will corrode less. The percentages used as boundaries between categories are arbitrary, however, and the correlation between the categories and corrosion is not perfect by any measure.29 Thus, although high-noble alloys will generally corrode less intraorally than noble or base-metal alloys, there are numerous exceptions. The practitioner is therefore advised to know the corrosion properties of an alloy regardless of its ADA composition classification. This type of information is available from most APRIL 2002

reputable manufacturers. It is important to note that the type of alloy in Table VI is unrelated to any physical property, including strength and hardness. In addition to composition, the mechanical properties of alloys have been classified by yield strength and percent elongation (Table VII).23,37 As previously noted, percentage elongation is the amount of permanent deformation an alloy can endure before tensile fracture. The definitions of type I-IV alloys in Table VII are not substantially different from the system discussed previously. However, whereas the old system stipulated both composition and physical properties, alloy types in the new classification system indicate nothing about composition. The dentist or laboratory technologist must therefore evaluate 2 facets of an alloy: its compositional class (Table VI) and its physical properties (Table VII). Each of these classifications is independent of the other. To encourage dentists and laboratory technologists to keep track of alloys used in patients, the Identalloy system (Identalloy Council Inc, York, Pa.) has been developed. Identalloy certificates are available that specify an alloy’s composition, manufacturer, name, and ADA compositional class (as in Table VI). This information comes on a small sticker that can be affixed to the patient’s chart. These stickers are available from most manufacturers or laboratories. The remainder of this section surveys alloys that are available today for clinical use. Alloy subclasses (as designated by the author) within each ADA compositional class are presented. The reader should be aware that there is tremendous diversity among alloy composi359

THE JOURNAL OF PROSTHETIC DENTISTRY

WATAHA

Table IX. Approximate composition and properties of some current noble alloys* Subclass

Au-Cu-Ag-Pd Pd-Cu-Ga Pd-Ag Ag-Pd

Approximate composition (major elements, wt%)

Au 45; Cu 15; Ag 25; Pd 5 Pd 79; Cu 7; (Ga 6)‡ Pd 61; Ag 24 (Sn 8)‡ Ag 66; Pd 23 (Au 2)‡

Elastic modulus (GPa)

Vickers hardness (kg/mm2)

Yield strength† (MPa, 0.2% offset)

100 127 125 93

250 280 275 230

690 580 620 400

CTE (×10–6/°C)

NA 14.2 14.6 NA

CTE = Coefficient of thermal expansion. *All properties are in the hardened condition, when applicable. Information adapted from manufacturer brochures (Metalor Dental USA and Williams; see footnote to Table V). †In tension mode. ‡Element in parentheses present in some formulations.

Table X. Approximate composition and properties of some current base-metal alloys* Subclass

Approximate composition (major elements, wt%)

Ni-Cr-Be Ni-Cr (high Cr) Ni-Cr Co-Cr

Ni 77; Cr 13; Be 2; C 0.1 Ni 65; Cr 23; (no C)‡ Ni 69; Cr 16 (no C)‡ Co 56; Cr 25 (no C)‡

Elastic modulus (GPa)

192 205 159 159

Vickers hardness (kg/mm2)

350 180 350 390

Yield strength† (MPa, 0.2% offset)

825 330 310 310

CTE (×10–6/°C)

15.0 14.0 14.4 15.2

CTE = Coefficient of thermal expansion. *All properties are in the hardened condition, when applicable. Information adapted from manufacturer brochures (Metalor Dental USA, Williams, and BEGO USA; see footnote to Table V). †In tension mode. ‡Element in parentheses present in some formulations. Note that for removable partial denture alloys, added carbon increases yield strength, hardness, and elastic modulus by 20% to 30%.

tions, and it is not always possible to assign an alloy to one of the subclasses presented.

High-noble alloys High-noble alloys have, by definition, at least 40 wt% gold and 60 wt% noble elements in their composition (Table VI). There are 3 common subclasses in the high-noble class (Table VIII).38,39 Gold-platinum alloys are high-noble alloys that may be used for full cast or metal-ceramic applications. They may contain zinc or silver as hardeners and are often multiple-phase alloys. These alloys were initially developed as palladium-free alternatives in the early 1990s. Gold-palladium alloys also may be used for full cast or metal-ceramic restorations. These alloys may or may not contain silver but almost always contain tin, indium, or gallium as oxide-forming elements to promote porcelain adherence. Gold-palladium alloys are commonly selected for metal-ceramic restorations. Gold-copper-silver-palladium alloys are used exclusively for full cast restorations. The solidus temperatures of these alloys are too low for metal-ceramic applications (Table V), and the copper and silver content is often problematic during ceramic application, as discussed previously.5,6 The properties of high-noble alloys are generally favorable for manipulation and clinical service (Table VIII), but none of these alloys have a high elastic modulus value. Many but not all of these alloys are 360

single-phase; the gold-platinum-zinc systems are one exception, for example. When palladium or platinum contents are above 10 wt%, the solidus temperatures of the alloys are higher (Table V) and the alloys are white in color. The corrosion of these alloys is generally low but may be higher if multiple phases are present.40

Noble alloys Noble alloys have no stipulated gold content but must contain at least 25 wt% noble metal (Table VI). This is a very diverse group of alloys, with gold-, palladium-, and silver-based systems represented. There are 3 common subclasses of alloys in this class (Table IX).38,39 Gold-copper-silver-palladium alloys are a lower-gold variation of the high-noble gold-copper-silver-palladium alloys. Generally silver or copper is increased to compensate for gold content. These alloys are always single-phase. Palladium-copper alloys are used for full cast or metal-ceramic applications. These alloys commonly contain gallium, which lowers the liquidus temperature, can provide improved porcelain adherence, and contributes to strength.41,42 The presence of copper may place these alloys at risk for sagging during porcelain firing, but good laboratories can use them successfully in this regard.5,6 Palladiumcopper alloys are nearly always multiple-phase.43 Corrosion can vary widely depending on the specific composition and manipulation of the alloys.29 The VOLUME 87 NUMBER 4

WATAHA

elastic moduli of these alloys is the highest among the gold- and palladium-based systems. Palladium-silver (or silver-palladium) alloys are very diverse and range from systems with only 26 wt% palladium and more than 60 wt% silver to alloys with 60 to 70 wt% palladium and approximately 20 wt% silver. Because of this variation, the corrosion of this subclass is also variable, tending to be higher with high-silver compositions.44 These alloys are commonly multiple-phase and may be used for full-cast or metal-ceramic restorations if the silver greening effect can be controlled for the dental porcelain. Manufacturers often include 1 to 2 wt% gold in palladium-silver alloys, but the gold has little effect on their physical properties. As a group, the noble alloys have moderately high solidus temperatures, which reflect the higher palladium content. The exception is Au-Cu-Ag-Pd noble alloys, which have solidus temperatures that are too low for use in metal-ceramic restorations (Table V). The noble alloys may be yellow or white in color but are more often white, reflecting the high concentration of palladium in many formulations. Noble alloys are generally very strong (Table IX), with good hardness and moderate percentage elongation (10%-20%). The elastic moduli of palladium-copper-gallium and palladium-silver alloys are significantly higher than those of high-noble alloys (because of the former’s higher palladium content) but only about 60% of the elastic moduli of base-metal alloys (Table X). Corrosion of the noble alloys is variable; it depends on the microstructure and the presence of corrosion-prone microstructural phases such as silver and copper.29 Alloys with high hardness (>250 kg/mm2) may be difficult to cut, shape, and/or polish.

Base-metal alloys Base-metal alloys contain less than 25 wt% noble metal according to the ADA classification (Table VI), but in practice, most contain no noble metal. There are 3 subclasses of alloys in this category (Table X).38,39 Nickel-chromium alloys may be used for full cast and metal-ceramic restorations or for removable partial denture frameworks. These alloys generally contain ≥60% nickel and are always multiple-phase. Nickel-chromium alloys may contain >20 wt% chromium, <20% chromium with no beryllium, or <20% chromium with 1 to 2 wt% beryllium. The latter subgroup is very common in the United States, whereas the higher chromium/no beryllium alloys are more common in Europe and Japan. Beryllium is added to the alloys to reduce the liquidus temperature so that investing and casting are easier,17 but it also vastly increases the corrosion of these alloys.44 Alloys may or may not contain approximately 0.1 wt% carbon. If present, the carbon significantly hardens the alloy via the formation of carbides. Thus, for clinical situations in APRIL 2002

THE JOURNAL OF PROSTHETIC DENTISTRY

which high elastic modulus, hardness, or strength is desired, a carbon-containing alloy is preferable. The cobalt-chromium subgroup is not nearly as common in the United States as its nickel-based counterpart. These alloys contain at least 60 wt% cobalt and are always multiple-phase. The chromium content is generally at least 30 wt%, and carbon is often added to strengthen the alloy. Although these alloys can be used for full cast, metal-ceramic, or removable partial denture restorations, their most common use is for removable partial denture frameworks, especially in Europe and Japan. In the United States, cobalt-chromium alloys are used primarily as an alternative to nickel-based alloys for individuals who are allergic to nickel. The third subclass of base-metal alloys is titanium alloys (not shown in Table X). These alloys have been proposed for full-cast, metal-ceramic, and removable partial denture frameworks, but their use is not common at present because of the need for special casting machines and investments and the considerable dental laboratory expertise required for the casting process. The base-metal alloys (excluding the titaniumbased systems) generally have superior mechanical properties compared to noble or high-noble alloys (Table X). Their elastic moduli are nearly twice as high as those of other systems (Table IV), and their hardness may exceed 400 kg/mm2 if carbon is present. In addition, these alloy systems may be etched for resin bonding. These alloys also have several negative characteristics, however: markedly higher corrosion in acidic environments; difficult finishing and polishing; dark, thick oxides; risk of patient allergy; and difficult soldering. Furthermore, their liquidus temperatures are the highest among all prosthodontic alloys (Table V), making it harder to cast them and ensure appropriate marginal fit of restorations. The physical properties of titanium alloys vary significantly depending on the alloy.44-47

TRENDS FOR TOMORROW Although it is difficult to predict, several trends are likely for prosthodontic alloys. The trend toward “metal-free” dentistry and associated use of all-ceramic restorations has received much promotion in recent years. Although all-ceramic restorations are clearly advantageous in some clinical applications and can provide excellent esthetics, they currently are not a viable replacement for the metal-ceramic restoration. The vast majority of tooth-colored restorations are still metal-ceramic48; these restorations have proven, longterm clinical records that are not available for any all-ceramic system. All-ceramic systems require the removal of significantly more tooth structure and are susceptible to fracture, especially in posterior teeth or in fixed partial denture applications.49-51 If properly 361

THE JOURNAL OF PROSTHETIC DENTISTRY

constructed by a qualified laboratory technologist, the traditional metal-ceramic restoration can yield excellent esthetic results. Finally, the claims of superior biocompatibility of all-ceramic materials are often not proven but assumed based on tests with traditional ceramic materials. A relatively recent development has been the use of a sintered metal composite as a metallic substructure for metal-ceramic restorations.52 These composites consist of a sintered high-noble alloy sponge infiltrated with an almost pure-gold alloy. The result is a composite between 2 gold alloys that is not cast, but fired onto a special refractory die. The porcelain does not bond to an oxide layer in these systems but presumably bonds mechanically to a micro-rough gold surface. Any stress concentrations at the metal-ceramic interface are presumably relieved by the excellent ductility of the gold. The esthetics of these metal-ceramic restorations are good because the yellow color of the metal is more like that of dentin than other alloys. Several companies make gold composite systems. These systems are interesting, but published long-term clinical, basic science, and biocompatibility data on them are not readily available. Furthermore, other claims for these alloy systems, such as a reduced number of periodontal pathogens around restorations, have yet to be substantiated by thorough clinical research. Gold composite systems do show promise as an alternative to cast metal substructures for single units. Periodic controversies about the biological safety of metals leached from alloys will probably continue. Most manufacturers are already aware of elemental release and have taken steps to reduce it through new alloy formulations. However, every material will release some mass intraorally, and questions about the biological safety of these released elements will continue to arise. Some questions will be based on irrational premises or premises designed to profit the promoter. A number of clinical assessments exist to test the compatibility of the patient with metals (and other substances). These types of tests are clinically unproven and may be completely unreliable. Other questions, such as those about allergic or chronic low-dose effects, are more reasonable and will deserve further investigation. The practitioner must always try to decide whether questions about biological safety are founded in fact or hyperbole. Because the patient may be strongly influenced by questionable sources of information, the practitioner must try to focus on quality data from reputable sources of information such as the Food and Drug Administration, the manufacturer, literature in peer-reviewed and respected scientific journals, and academicians with expertise in the area. Convincing patients of the irrationality of their fears may be a formidable task. In the end, practitioners must balance the need to satisfy patients with 362

WATAHA

their commitment to sound, proven clinical treatments that are based on published evidence.

GUIDELINES FOR THE SELECTION OF ALLOYS Choosing an alloy for prosthodontic restorations is a formidable task. Although there is no proven formula for selection, practitioners may find the following guidelines helpful.

Develop an understanding of alloys 1. Avoid selecting an alloy based on its color unless all other factors are equal. 2. Know the complete composition of alloys, and avoid elements to which the patient is allergic. Know the alloys that the laboratory uses; specify a specific alloy in the laboratory prescription. 3. When possible, use single-phase alloys over multiple-phase alloys. 4. Keep track of alloys used in the patient with the Identalloy system or something similar. At minimum, the name of the alloy and the manufacturer should be recorded.

Use clinically proven products from quality manufacturers 5. Use alloys from companies that research and manufacture their own alloys. These companies will be able to provide the most accurate information, the best service, and the best answers when problems arise. 6. Use alloys that have been tested for elemental release and corrosion and that have the lowest possible release of elements. 7. Use a dental laboratory that is knowledgeable about its alloys and willing to discuss issues about them. Be comfortable with the alloys that the laboratory uses.

Develop a clinical philosophy 8. Focus on the long-term clinical performance and long-term costs of restorations rather than on shortterm costs. 9. Consider the clinical situation (esthetics, occlusion, space, and systemic allergy) when selecting an alloy. Select the alloy that meets the needs of the patient. Avoid a “one size fits all” approach. 10. Remember that the practitioner is ultimately responsible for the safety and efficacy of any restoration. I thank the Academy of Prosthodontics for their invitation to speak at the annual meeting, the Journal of Prosthetic Dentistry and its reviewers for their invaluable help during the review process, Dr Steve Nelson for his consultation, Petra E. Lockwood for her selfless contributions to research in this area, and the Medical College of Georgia Biocompatibility Program for its support. VOLUME 87 NUMBER 4

WATAHA

REFERENCES 1. Craig RG. Restorative dental materials. 10th ed. St. Louis: Mosby; 1997. p. 383. 2. Flinn RA, Trojan PK. Engineering materials and their applications. 3rd ed. Boston: Houghton Mifflin; 1986. p. 21-54. 3. Craig RG. Restorative dental materials. 7th ed. St. Louis: Mosby; 1985. p. 360-1. 4. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 552. 5. Anusavice KJ. Phillips’ science of dental materials. 10th ed. Philadelphia: WB Saunders; 1996. p. 439. 6. Craig RG. Restorative dental materials. 8th ed. St. Louis: Mosby; 1989. p. 502. 7. Craig RG. Restorative dental materials. 7th ed. St. Louis: Mosby; 1985. p. 451. 8. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 453. 9. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 481. 10. Moffa JP, Lugassy AA, Guckes AD, Gettleman L. An evaluation of nonprecious alloys for use with porcelain veneers. Part 1. Physical properties. J Prosthet Dent 1973;30:424-31. 11. Flinn RA, Trojan PK. Engineering materials and their applications. 3rd ed. Boston: Houghton Mifflin; 1986. p. 71. 12. Aberer W, Holub H, Strohal R, Slavicek R. Palladium in dental alloys— the dermatologists’ responsibility to warn? Contact Dermatitis 1993;28:163-5. 13. Wataha JC, Hanks CT. Biological effects of palladium and risk of using palladium in dental casting alloys. J Oral Rehabil 1996;23:309-20. 14. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 516-43. 15. Phillips RW. Science of dental materials. 7th ed. Philadelphia: WB Saunders; 1973. p. 250-3. 16. Nielsen JP, Tuccillo JJ. Grain size in cast alloys. J Dent Res 1966;45:9649. 17. Brantley WA, Cai Z, Carr AB, Mitchell JC. Metallurgical structures of ascast and heat-treated high-palladium dental alloys. Cells Mater 1993;3:103-14. 18. Phillips RW. Science of dental materials. 7th ed. Philadelphia: WB Saunders; 1973. p. 384. 19. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 483. 20. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 170-3. 21. Wataha JC, Craig RG, Hanks CT. The release of elements of dental casting alloys into cell-culture medium. J Dent Res 1991;70:1014-8. 22. Bumgardner JD, Lucas LC. Corrosion and cell culture evaluations of nickel-chromium dental casting alloys. J Appl Biomater 1994;5:203-13. 23. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 481-5. 24. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 105. 25. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 453. 26. Malhotra ML. New generation of palladium-indium-silver dental casting alloys: a review. Trends Tech Contemp Dent Lab 1992;9:65-8. 27. Johansson BI, Lemons JE, Hao SQ. Corrosion of dental copper, nickel, and gold alloys in artificial saliva and saline solutions. Dent Mater 1989;5:324-8. 28. Wataha JC, Lockwood PE. Release of elements from dental casting alloys into cell-culture medium over 10 months. Dent Mater 1998;14:158-63. 29. Wataha JC. Biocompatibility of dental casting alloys: a review. J Prosthet Dent 2000;83:223-34. 30. Wataha JC, Lockwood PE, Nelson SK, Bouillaguet S. Long-term cytotoxicity of dental casting alloys. Int J Prosthodont 1999;12:242-8. 31. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 578. 32. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 580. 33. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 579.

APRIL 2002

THE JOURNAL OF PROSTHETIC DENTISTRY

34. Fairhurst CW, Anusavice KJ, Hashinger DT, Ringle RD, Twiggs SW. Thermal expansion of dental alloys and porcelains. J Biomed Mater Res 1980;14:435-46. 35. Anusavice KJ. Phillips’ science of dental materials. 10th ed. Philadelphia: WB Saunders; 1996. p. 622. 36. Classification system for cast alloys. Council on Dental Materials, Instruments, and Equipment. J Am Dent Assoc 1984;109:766. 37. Revised ANSI/ADA specification no. 5 for dental casting alloys. Council on Dental Materials, Instruments, and Equipment. J Am Dent Assoc 1989;118:379. 38. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 461. 39. Anusavice KJ. Phillips’ science of dental materials. 10th ed. Philadelphia: WB Saunders; 1996. p. 428. 40. Craig RG, Hanks CT. Reaction of fibroblasts to various dental casting alloys. J Oral Pathol 1988;17:341-7. 41. Papazoglou E, Brantley WA, Carr AB, Johnston WM. Porcelain adherence to high-palladium alloys. J Prosthet Dent 1993;70:386-94. 42. Wu Q, Brantley WA, Mitchell JC, Vermilyea SG, Xiao J, Guo W. Heattreatment behavior of high-palladium dental alloys. Cells Mater 1997;7:161-74. 43. Vermilyea SG, Cai Z, Brantley WA, Mitchell JC. Metallurgical structure and microhardness of four new palladium-based alloys. J Prosthodont 1996;5:288-94. 44. Wataha JC, Lockwood PE, Khajotia SS, Turner R. Effect of pH on element release from dental casting alloys. J Prosthet Dent 1998;80:691-8. 45. Wataha JC. Materials for endosseous dental implants. J Oral Rehabil 1996;23:79-90. 46. Craig RG, Powers JM. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002. p. 488-91. 47. Kononen M, Kivilahti J. Fusing of dental ceramics to titanium. J Dent Res 2001;80:848-54. 48. Giordano RA. Dental ceramic restorative systems. Compendium 1996;17:779-82, 784-6 passim; quiz 794. 49. Anusavice KJ, Zhang NZ. Chemical durability of Dicor and lithia-based glass-ceramics. Dent Mater 1997;13:13-9. 50. Anusavice KJ. Recent developments in restorative dental ceramics. J Am Dent Assoc 1993;124:72-4, 76-8, 80-4. 51. Deany IL. Recent advances in ceramics for dentistry. Crit Rev Oral Biol Med 1996;7:134-43. 52. Nathanson D, Shoher I. Initial evaluations of a gold composite alloy restorative system (Captek); Captek: an advanced gold composite alloy coping. Laboratory Digest 1998;Spring:4-6. 53. Craig RG. Restorative dental materials. 10th ed. St. Louis: Mosby; 1997. p. 385. 54. Craig RG. Restorative dental materials. 7th ed. St. Louis: Mosby; 1985. p. 361. 55. Craig RG. Restorative dental materials. 7th ed. St. Louis: Mosby; 1985. p. 452. 56. Craig RG. Restorative dental materials. 7th ed. St. Louis: Mosby; 1985. p. 365, 389, 452. Reprint requests to: DR JOHN C. WATAHA DEPARTMENT OF ORAL REHABILITATION SCHOOL OF DENTISTRY MEDICAL COLLEGE OF GEORGIA 1120 15TH ST AUGUSTA, GA 30912-1260 FAX: (706)721-8349 E-MAIL: [email protected] Copyright © 2002 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2002/$35.00 + 0. 10/1/123817

doi:10.1067/mpr.2002.123817

363