Amalgam - Dental amalgam
Posted by John Doe at Dental Assistant on March 6, 2010.
Categories: Dental Materials
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An amalgam is an alloy of mercury and one or more other metals. Dental amalgam is produced by mixing liquid mercury with solid particles of an alloy of silver, tin, copper, and sometimes zinc, palladium, indium, and selenium. This combination of solid metals is known as the amalgam alloy. It is important to differentiate between dental amalgam and the amalgam alloy that is commercially produced and marketed as small filings, spheroid particles, or a combination of these, suitable for mixing with liquid mercury to produce the dental amalgam.
Once the amalgam is freshly mixed with liquid mercury, it has the plasticity that permits it to be conveniently packed or condensed into a prepared tooth cavity. After condensing, the dental amalgam is carved to generate the required anatomical features. Amalgam is used most commonly for direct, permanent, posterior restorations and for large foundation restorations, or cores, which are precursors to placing crowns. Dental amalgam restorations are reasonably easy to insert, are not overly technique sensitive, maintain anatomical form, have reasonably adequate resistance to fracture, prevent marginal leakage after a period of time in the mouth, can be used in stress bearing areas, and have a relatively long service life.
The principal disadvantage of dental amalgam is that the silver color does not match tooth structure. In addition, they are somewhat brittle; are subject to corrosion and galvanic action; may demonstrate a degree of marginal breakdown; and do not help retain weakened tooth structure.
Finally, there are regulatory concerns about amalgam being disposed in the wastewater. In summary, dental amalgam is a highly successful material clinically and is very cost effective, but alternatives such as cast gold and esthetic restorative materials are now very competitive in terms of frequency of use. Many argue, however, that the use of amalgam must be strongly supported given its large public health benefit in the United States and many other countries.
In this chapter, the composition and morphology of the different dental amalgams are presented, followed by a discussion of low- and high-copper amalgams, the chemical reactions occurring during amalgamation, and the resultant micro structures. Various physical and mechanical properties are covered in the next section, as well as the factors related to the manipulation of amalgam. Finally, biological effects of amalgam and mercury are presented.
DENTAL AMALGAM ALLOYS
COMPOSITION AND MORPHOLOGY
ANSI/AD A Specification No. 1 for amalgam alloy (ISO 1559) includes a requirement for composition. This specification does not state precisely what the composition of alloys shall be; rather, it permits some variation in composition. The chemical composition must consist essentially of silver and tin. Copper, zinc, gold, palladium, indium, selenium, or mercury may be included in lesser amounts. Metals such as palladium, gold, and indium in smaller quantities and copper in larger quantities have been included to alter the corrosion resistance and certain mechanical properties of the finished amalgam mass. These and other elements may be included, provided the manufacturer submits the alloy's composition and adequate clinical and biological data to the Ada's Council on Scientific Affairs to show that the alloy is safe to use as directed.
| Approximate Composition of High Copper Amalgam Alloys | |||||||
|---|---|---|---|---|---|---|---|
| Element (wt%) | |||||||
| Alloy | Particle Shape | Ag | Sn | Cu | Zn | In | Pd |
| Low copper | Irregular or spherical | 63-70 | 26-28 | 2-5 | 0-2 | 0 | 0 |
| High copper | |||||||
| Admixed regular | Irregular | 40-70 | 26-30 | 2-3 | 0-2 | 0 | 0 |
| Spherical | 40-65 | 0-30 | 20-40 | 0-1 | 0 | 0-1 | |
| Admixed uni composition | Irregular | 52-53 | 17-18 | 29-30 | 0 | 0 | 0.3 |
| Spherical | 52-53 | 17-18 | 29-30 | 0 | 0 | 0.3 | |
| Uni compositional | Spherical | 40-60 | 22-30 | 13-30 | 0 | 0-5 | 0-1 |
Alloys with more than 0.01% zinc are classified as zinc containing, and those with less than 0.01% as non-zinc alloys. Zinc has been included in amalgam alloys as an aid in manufacturing by helping to produce clean, sound castings of the ingots. However, improved manufacturing procedures have resulted in the elimination of zinc in most alloys. Recent studies have shown, however, that small amounts of zinc in high-copper dental amalgams improve clinical performance, presumably by reducing brittleness.
The approximate composition of commercial amalgam alloys is shown in Table 11-1, along with the shape of the particles. The alloys are broadly classified as low-copper (5% or less copper) and high-copper alloys (13% to 30% copper). Particles are irregularly shaped; micro-spheres of various sizes; or a combination of the two. Scanning electron micrographs of the particles are presented in Fig. 11-1. The low-copper alloys are either irregular or spherical. Both morphologies contain silver and tin in a ratio approximating the inter metallic compound Ag3Sn. High-copper alloys contain either all spherical particles of the same composition (uni compositional) or a mixture of irregular and spherical particles of different or the same composition (admixed).
Fig. 11-1 Scanning electron microsraphs. A, Lathe-cut; B, spherical; and C, admixed amalgam alloys.
When the particles have different compositions, the admixed alloys are made by mixing particles of silver and tin with particles of silver and copper. The silver-tin particle is usually irregular, whereas the silver-copper particle is usually spherical in shape. The composition of the silver-tin particles in most commercial alloys is the same as that of the low-copper alloys. Different manufacturers, however, have somewhat different compositions for the silver-copper particle. The compositional ranges of the spherical silver-copper particles are shown in Table 11-1. The admixed regular alloy contains 33% to 60% spherical particles that have a composition close to the eutectic composition of Ag3Cu2 (see Fig. 6-9); the balance are irregular particles.
Like the admixed alloy, the uni compositional alloys have higher copper contents than the conventional lathe-cut or spherical low-copper alloys, but all the particles are spherical, as seen in Fig. 11-1. The silver content of the uni compositional alloys varies from 40% to 60%, copper content from 13% to 30%, and tin content varies only slightly.
A high-copper admixed alloy is also available, in which both spherical and irregular particles have the same composition and the copper content is between 29% and 30%. High-copper alloys are less commonly supplied as uni compositional, irregular particles. The lathe-cut, high-copper alloys contain more than 23% copper.
Interest has increased in admixed amalgams containing 10% to 15% indium (In) in the mercury. The addition of In to Hg decreases the amount of Hg needed, decreases the Hg vapor during and after setting, and increases the wetting. These amalgams have low creep and lower early-compressive strengths, but higher final strengths than comparable amalgams without indium. It is proposed that the lower levels of Hg vapor are due to oxides of In formed at the surface or the lower amount of Hg used in the mix.
It is estimated that more than 90% of the dental amalgams currently placed are high-copper alloys. Of the high-copper alloys, admixed are used more often than spherical types, and fewer irregularly shaped or lathe-cut types are selected. A high-copper alloy is selected to obtain a restoration with high early strength, low creep, good corrosion resistance, and good resistance to marginal fracture.
In general, alloy composition; particle size, shape, and distribution; and heat treatment control the characteristic properties of the amalgam.
PRODUCTION
Irregular Particles To produce lathe-cut alloys, the metal ingredients are heated and protected from oxidation until melted, then poured into a mold to form an ingot. The ingot is cooled relatively slowly, leading to the formation of mainly Ag3Sn (y) and some Cu3Sn (e), Cu6Sn5 (yf) and Ag4Sn (|3). After the ingot is completely cooled, it is heated for various periods of time (often 6 to 8 hours) at 400° C to produce a more homogeneous distribution of Ag3Sn. The ingot is then reduced to filings by being cut on a lathe and ball milled. The particles are passed through a fine sieve and then ball milled to form the proper particle size. The particles are typically 60 to 120 [im in length, 10 to 70 u.m in width, and 10 to 35 (Xm in thickness. Most products are labeled as fine-cut. The particle size and shape of lathe-cut amalgam alloys are shown in Fig. 11-1, A.
In general, freshly cut alloys amalgamate and set more promptly than aged particles, and some aging of the alloy is desirable to improve the shelf life of the product. The aging is related to relief of stress in the particles produced during the cutting of the ingot. The alloy particles are aged by subjecting them to a controlled temperature of 60° to 100° C for 1 to 6 hours. Irregularly shaped high-copper particles are made by spraying the molten alloy into water under high pressure.
Spherical Particles Spherical particles of low- or high-copper alloys are produced when all the desired elements are melted together. In
the molten stage the metallic ingredients form the desired alloy. The liquid alloy is then sprayed, under high pressure of an inert gas, through a fine crack in a crucible into a large chamber. Depending on the difference in surface energy of the molten alloy and the gas used in the spraying process, the shape of the sprayed particles may be spherical or somewhat irregular, as shown in Fig. 11-1, B. The size of the spheres varies from 2 to 43 Hm.
SILVER-TIN ALLOY
Because two of the principal ingredients in the amalgam alloy are silver and tin, it is appropriate to consider the binary system and the equilibrium phase diagram for these two metals, as shown in Fig. 11-2.
Weight Percentage Tin.
The most important feature in this diagram concerning the silver-tin alloy is that, when an alloy containing approximately 27% tin is slowly cooled below a temperature of 480° C, an inter-metallic compound (Ag3Sn) known also as the gamma (y) phase is produced. This Ag3Sn compound is an important ingredient in the silver amalgam alloy and combines with mercury to produce a dental amalgam of desired mechanical properties and handling. This silver-tin compound is formed only over a narrow composition range. The silver content for such an alloy would be approximately 73%. Practically, the tin content is held between 26% and 30%o, and the remainder of the alloy consists of silver, copper, and zinc. If the concentration of tin is less than 26%, the beta one (pj) phase, which is a solid solution of silver and mercury, forms. In one product, 5% tin is replaced by 5% indium, whereas another product contains less than 1% palladium. Adding this small amount of palladium enhances the mechanical properties and corrosion resistance. The replacement of silver by an equal amount of copper produces a copper-tin compound (Cu3Sn).
In general, larger (>30%) or smaller (<26%) quantities of tin in the alloy are detrimental to the final properties of the amalgam. The reason for this unfavorable shift in properties is generally considered related to the fact that the amount of Ag3Sn is reduced as the percentage of tin is altered beyond the indicated limits. This is the basis for the rather narrow limits of the alloy compositions of current products with acceptable properties.
Silver-tin amalgam alloys compounded to produce largely Ag3Sn react favorably with mercury to produce only slight dimensional setting changes when properly manipulated. The strength of the amalgam mass is greater from the Ag3Sn compound than from an excess of tin. In addition, the setting time is shortened by increasing silver content. Creep resistance is also superior when an alloy of Ag3Sn is used rather than one with higher tin content.
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