1932: Classification of Gold-Based Casting Alloys

The dental materials group at the National Bureau of Standards (now National Institute of Standards and Technology) evaluated the properties of alloys being used and classified them by their Vickers hardness number (VHN): type I (soft, VHN, 50 to 90), type II (medium, VHN, 90 to 120), type III (hard, VHN, 120 to 150), and type IV (extra hard, VHN 150 and above).

1933: Cobalt-Chromium and Nickel-Chromium Alloys

The advantages of cobalt-chromium and nickel-chromium alloys are their lighter weight, greater stiffness (elastic modulus), greater strength, and reduced cost. For these reasons, they have largely replaced gold-based alloys for making removable partial dentures. The rapidly fluctuating escalating price of gold between 1980 and 2012 has made them logical alternatives to gold alloys for FDPs.

1959: Porcelain-Fused-to-Metal Prostheses

It is possible to attain a bond between porcelain and gold alloys containing both platinum and palladium. This type of prosthesis is also called a metal-ceramic prosthesis; the mechanism of bonding is discussed in Chapter 18, “Dental Ceramics.”

1971: the End of the Bretton Woods System

The Bretton Woods system, implemented after World War II, was an international monetary framework of fixed exchange rates between gold and currency. The system ended on August 15, 1971, a step that completely abandoned the gold standard. Gold then became a commodity freely traded on the open market. As a result, the price of gold rose steadily over the next 9 years. The price of gold skyrocketed again between 2006 and 2011, approaching $1900 per ounce in August 2011.

In response to the ever-increasing price of gold, new dental alloys were introduced by replacing gold partially or entirely with a less expensive noble metal or adapting removable partial denture alloys for crown and bridge casting applications.

1976: the Medical and Dental Devices Act

This law placed the U.S. dental industry under the auspices of the U.S. Food and Drug Administration (FDA). Dental alloys for prosthetics were classified as passive implants. All materials on the market prior to 1976 were automatically “grandfathered” as acceptable for market distribution. Manufacturers were required to have a quality system in place, but no product standards were established.

1996: the European Medical Devices Directive

The European Union (EU) established that any imports of dental devices required a CE (Certification-Expert) mark. To sell products in the EU, a company must be compliant with an International Organization for Standardization (ISO) standard (ISO 9000) and meet the requirements of the European Medical Device Directive. This performance standard extended beyond the FDA requirements, since no existing products were grandfathered and safety had to be demonstrated. Information and data on the development process were also required. Again, no specific product standards were established.

1998: the Clean Air Acts

The price of palladium increased to a record high of $1000 per troy ounce in 2000 because of the increased demand for palladium-containing catalytic converters. At the same time during that decade, the price of gold was trading below $300 per troy ounce and gold-based dental alloys were less expensive than palladium-containing alloys. The poor economy of 2001 decreased the demand for palladium and its price fell to a level comparable to that of gold, in the range of $300 to $350 in 2002. In 2012, the price of palladium was less than half that of gold.

Alloy Classification by Noble Metal Content

In 1984, the American Dental Association (ADA) proposed a simple classification for dental casting alloys based on the content of noble metals. Three categories were described: high noble (HN), noble (N), and predominantly base metal (PB). This classification is presented in Table 16-1. Noble metals comprise a group of seven metals that are resistant to corrosion and tarnish in the mouth. In order of increasing melting temperature, they include gold, palladium, platinum, rhodium, ruthenium, iridium, and osmium. Only gold, palladium, and platinum, which have the lowest melting temperatures of the seven noble metals, are currently of major importance in dental casting alloys. The noble metals and silver are sometimes called precious metals, referring to their high economic values, but the term precious is not synonymous with noble. Silver is reactive in the oral cavity and is not considered a noble metal.

As described in Chapter 5, the IdentAlloy program was established by manufacturers to provide documentation of certified alloys. Under the program, each alloy has a certificate that lists its manufacturer, alloy name, composition, and ADA classification. Some insurance companies use it as well to determine the cost of crown and bridge treatment. This system lacks the potential to discriminate among alloys within a given category, which may have quite different mechanical properties.

Alloy Classification by Mechanical Properties

Over the past few decades base metal alloys have been developed to the point where they are superior to high noble and noble alloys in several respects, such as low cost and low density, excellent strength, high stiffness, and stable oxide formation (which is required for bonding to porcelain). The classification described in ADA specifications and ISO standards has changed over time.

ADA Specification No. 5 formerly classified gold alloys as types 1 through 4, depending on the content of gold, palladium, and platinum. The content of noble metals by weight ranges from 83% (type 1) to 75% (type 4). Both the current ADA Specification No. 5 (1997) and ISO Standard 1562 (2004) have classified four types of casting alloys using similar minimal yield strength and percent elongation values for each type of alloy. The only difference is that the ISO standard is specific for casting gold alloys whereas ADA Specification No. 5 covers all alloys that pass the tests for toxicity and tarnish. Table 16-2 lists the classification and mechanical properties described in ISO 1562.

The more recent ISO 22674 standard classifies metallic materials for fixed and removable restorations and appliances into six types according to their mechanical properties without referring to the composition of the alloys (Table 16-3). Although the properties for heat-treated specimens are not given in the table, it is assumed that the specimens are bench-cooled.

Alloy Classification by Principal Elements

Alloys may be classified based on the principal or most abundant element (e.g., a palladium-based alloy), or they may be named based on the two or three most important elements (e.g., Pd-Ag, Co-Cr, or Ni-Cr-Be alloys). When an alloy is identified according to the elements it contains, the components are listed in declining order of composition, with the largest constituent first followed by the second largest constituent. An exception to this rule is the identification of certain alloys by elements that significantly affect physical properties, which represent potential biocompatibility concerns, or both. For example, Ni-Cr-Mo-Be alloys are often designated as Ni-Cr-Be alloys because of the contributions of beryllium to the control of castability and surface oxidation at high temperatures and the relative toxicity potential of beryllium compared with other metals. Alloy groups shown in Table 16-4 are identified by principal elements.

Biocompatibility

The alloy must tolerate oral fluids and not release any harmful products into the oral environment. When components of the alloy are released in the oral environment, they can cause a toxic or allergic reaction. Toxic materials are eliminated by national or international regulations as well as sound business practices. Allergic reactions, however, are peculiar to the individual patient, and the practicing dentist has an obligation, morally and legally, to minimize this risk.

Tarnish and Corrosion Resistance

As previously discussed in Chapter 3, corrosion is the physical dissolution of a material in the oral environment and tarnish is a thin film of a surface deposit that is adherent to the metal surface. Corrosion resistance is derived from the use of noble metals that do not react in the oral environment (e.g., gold and palladium) or by the ability of one or more of the metallic elements to form an adherent passivating surface film, which inhibits any subsurface reaction (e.g., chromium and titanium).

Thermal Properties

The melting range of the casting alloys must be low enough to form smooth surfaces with the mold wall of the casting investment (Chapter 10). To achieve an accurate fit of cast prostheses, oversized dies for waxing and controlled mold expansion are needed to compensate for casting shrinkage of the alloy and provide space for the luting cement. For metal-ceramic prostheses, the alloys must have closely matching thermal expansion coefficients to be compatible with given porcelains, and they must tolerate high processing temperatures without deforming via a creep process.

Strength Requirements

The alloy must have sufficient strength for the intended application. Alloys for bridgework require higher strength than alloys for single crowns. Alloys for metal-ceramic prostheses are finished in thin sections and require sufficient stiffness to prevent excessive elastic deflection from functional forces, especially when they are used for long-span frameworks.

Fabrication of Cast Prostheses and Frameworks

The molten alloy should flow freely into the most intricate regions of the investment mold, without any appreciable interaction with the investment material, and wet the mold surface without forming porosity within the surface or subsurface regions of the alloy. This property is also termed castability, which is measured by percent completion of a cast mesh screen pattern or other castability patterns. The selection of an investment material suitable for the metal to be cast is critical.

Cutting, grinding, finishing, and polishing are necessary steps in obtaining a prosthesis with a satisfactory surface finish. The hardness of an alloy is a good primary indicator of the likely difficulty of cutting and grinding it.

Porcelain Bonding

To achieve a sound chemical bond to ceramic veneering materials, the alloy must be able to form a thin adherent oxide, preferably one that is light in color so that it does not interfere with the esthetic potential of the ceramic.

Economic Considerations

For the dental laboratory owner who must guarantee the cost of prosthetic work for a certain period of time, the cost of fabricating prostheses must be adjusted periodically to reflect the fluctuating prices of casting metals, mostly those of high noble and noble metal alloys.

Elastic Modulus

This property represents a proportional constant between stress and strain during the elastic deformation of a material. One characteristic of a material with high elastic modulus is its rigidity or stiffness. For a dental prosthesis, it is equivalent to its flexure resistance. For long-span FDPs, resistantce to flexure is important. When such a prosthesis flexes during loading of the pontic, the mesiodistal bending moment exerted on the abutment teeth can act as a dislodging force, lifting the mesial and distal aspects of the prosthesis. Furthermore, a flexing bridge can induce lateral forces on the abutment teeth, resulting in the loosening of teeth. For a metal-ceramic prosthesis, the overlying brittle porcelain will fail catastrophically when the metal substructure flexes beyond the flexural strength limit of the ceramic. Elastic modulus is also important for the major connectors of removable partial dentures, which must have enough rigidity to prevent flexure during placement and function of the prosthesis. Resistance to flexure also allows clasps to fit into areas of minimal undercuts and still provide adequate retention.

The elastic moduli of base metal alloys, excluding titanium alloys, are up to twice as high as those for some popular noble metal alloys. This is not considered a major disadvantage for noble metal alloys if proper geometries are employed for the framework connectors. The deflection of a cantilever beam is inversely proportional to t³E, where t is the beam thickness in the plane of bending and E is the elastic modulus. Calculations show that increasing the thickness of the gold alloy connector by 26% yields the same resistance to bending stress as a base metal alloy connector. For a circular cross section, the diameter must increase by 19% to achieve the same degree of stiffness.

Yield Strength

The yield strength, proportional limit, and elastic limit all are essentially measures of the same property under tension (Chapter 4). It is important to note the difference between these three properties. Yield strength is defined as the amount of stress needed to cause 0.2% plastic deformation of the material, which is why it is also called 0.2% offset yield strength. Proportional limit is the elastic stress above which the linear relationship between stress and strain no longer exists on the stress-strain plot. The elastic limit is the greatest stress to which the alloy can be subjected, such that it will return to its original dimensions when the forces are removed. Both yield strength and proportional limit can easily be located on the stress-strain curve, but elastic limit cannot be identified precisely.

Yield strength reflects the capacity of a cast prosthesis to withstand mechanical stresses without permanent deformation. Ideally, the alloys should have a high yield strength, so that a great deal of stress must be applied before a permanent change in dimensions occurs. Generally, alloys with tensile yield strengths above 300 MPa function satisfactorily in the mouth.

Ductility

This mechanical property represents the amount of plastic deformation under tensile stress that an alloy can undergo before it fractures. When this tensile strain is converted to a percent of the original length of the specimen, the property is called percent elongation. If the force applied is in compressive mode, the property is called malleability. A reasonable amount of ductility and malleability are essential if the clinical application requires some plastic deformation of the as-cast structure, as is needed for clasp and margin adjustment and for burnishing. Some base metal alloys have a ductility higher than that of age-hardened Au-Cu noble metal alloys. High ductility means that the amount of deformation that one can produce by adjusting a prosthesis or by burnishing a cast metal margin plastically is higher for the alloy. In order to burnish an alloy, one must exert a high enough stress that is sufficient to exceed its yield strength. Therefore, if the alloy with high ductility also has a high yield strength, the alloy may not achieve the full benefit of high ductility.

Hardness

Hardness is a measure of the resistance of the surface to indentation by an object in the shape of a spherical or a diamond-shaped point. Hardness of the metal should be high enough to resist scratching and abrasion and also to maintain the smoothness of the prosthesis in the oral environment. A hard restoration surface can also cause excessive wear of the opposing dentition or restoration(s) and requires more energy in grinding and polishing of the restorations.

An important requirement of inlay and crown and bridge alloys is that they must be easily burnished by the dentist. One measure of burnishability is the percent elongation divided by the hardness or yield strength. Some researchers believe that the burnishability of alloys can be compared by dividing the elongation (ductility) by the product of yield strength and hardness. This would indicate that the gold alloy would be easier to burnish. The extremely high hardness of most base metal alloys renders them difficult to cut, grind, and polish. From a clinician’s point of view, the lower hardness and greater ductility of most noble alloys are major advantages compared with those of base metal alloys.

Fatigue RESISTANCE

This phenomenon occurs when a material is subjected to repeated loading and unloading below its elastic limit. Most fractures of prostheses and restorations develop progressively over many stress cycles. When the load is above a certain threshold, it initiates cracks from surface flaws of the material. Eventually a crack propagates to a critical size, and sudden fracture occurs.

Fatigue behavior is often determined experimentally by subjecting a material to a cyclic stress between two values and determining the number of cycles required to produce failure. When a removable partial denture is inserted and removed daily, the clasps are strained elastically as they slide over the undercuts of abutment teeth. A comparison of number of constant-deflection cycles that caused fracture of cast clasps made of cobalt-chromium alloy, commercially pure CP Ti, Ti-6Al-4V alloy, and type 4 gold alloy after a number of constant-deflection cycles that caused fracture of cast clasps has shown that cobalt-chromium alloys exhibit the highest fatigue resistance, followed by the type 4 gold alloy, Ti-6Al-4V alloy, and CP Ti. Any casting procedure that produces porosity or that produces carbides in the microstructure of a clasp arm will reduce fatigue resistance because the former represents internal flaws and the latter reduces the elongation of the casting.

Other expressions of fatigue fracture resistance include fatigue strength and endurance limit. Fatigue strength (S_Nf) is defined as the stress at which failure occurs after N_f cycles. Endurance limit is the maximum stress that can be maintained without failure over an infinite number of cycles.

High Noble and Noble Alloys

For prosthetic dental applications, it is necessary to incorporate various elements in gold to produce alloys with suitable properties. Platinum increases the hardness and elasticity of gold and raises the melting temperature of the alloy. When palladium became expensive in the late 1990s, platinum was used in place of palladium. Copper in a sufficient quantity relative to the gold content renders the alloy heat-treatable (Chapter 5). Silver forms solid solutions with gold and palladium and is a common component in this group of alloys. In gold-copper (Au-Cu) alloys, silver is effective in neutralizing the reddish color of copper. In the case of palladium-based alloys, silver is needed to develop the white color of the alloys. Palladium has a good range of solubility with several metals, such as gold, silver, and copper, and an ability to impart good mechanical properties. It has excellent tarnish/corrosion resistance and is relatively biocompatible. It has also been incorporated in small quantities (about 1.5% by weight) in gold alloys to improve resistance to tarnish and corrosion without a significant loss of gold color. Zinc acts as an oxygen scavenger during melting and casting of noble and high noble alloys. Indium can be used in place of zinc and, when added in greater amounts (18% to 30% by weight), it can promote a gold-like color in Pd-Ag-based alloys. Iridium or ruthenium is added in small quantities as a grain refiner, since smaller grains improve yield strength.

The compositions of selected high noble and noble alloys are given in Table 16-5. Their physical and mechanical properties are shown in Table 16-6. Since some noble metals contain no gold and rely on palladium for corrosion resistance, these alloys are discussed in two categories: Au-based alloys and Ag-Pd alloys.

Predominantly Base Metals

Base metal alloys generally comprise the group of cast metals that rely on chromium for corrosion resistance. Chromium on the surface of the alloy rapidly oxidizes to form a thin layer of chromium oxide, which prevents the diffusion of oxygen into the underlying metals and improves its corrosion resistance. Chromium also strengthens the alloy by solution hardening. Since the introduction of cobalt-chromium alloys as cast dental appliances in 1928 and subsequent development of nickel-chromium and cobalt-nickel-chromium alloys, base metal alloys have gained widespread acceptance as the predominant choice for the fabrication of removable partial denture frameworks. Because of the high cost of noble metals, these base metals have been adapted also for dual applications such as the production of all-metal and metal-ceramic prostheses.

Currently there are two main groups of base metal dental alloys: nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr). The Ni-Cr alloys can be further divided into those with and without beryllium, which improves castability and promotes the formation of a stable metal oxide for porcelain bonding. The majority of Ni-Cr alloys are for small castings such as crowns and FDPs, and Co-Cr alloys are primarily used for casting removable partial dentures in which high elastic modulus and yield strength are needed. Some Ni-Cr alloys, which are used for partial denture frameworks, are formulated for their relative ease of finishing and polishing compared with Co-Cr alloys, which are used for crowns and FDPs in spite of their low ductility. Table 16-7 lists compositions and properties of selected base alloys for all metal and metal–ceramic applications. Molybdenum increases corrosion resistance and strength and decreases the thermal expansion coefficient of base metal alloys. The latter is beneficial for porcelain bonding and minimizes the risk of porcelain cracking or fracture. Base metal alloys for partial frameworks are discussed later.