1-5. DENTAL AMALGAM
a. History of Amalgam. Dr. G. V. Black investigated the properties of amalgams and their possible use for dentistry about 1895. His studies showed the effects of chemical composition and physical structure on the properties of amalgam restorations. Due largely to the work done by Dr. Black and the National Bureau of Standards, and other researchers, amalgam is now used more than any other filling material for the restoration of posterior teeth.
(1) Alloy. An alloy is a solid mixture of two or more metals. It is possible to produce a material in which the desirable properties of each constituent are retained or even enhanced, while the less desirable properties are reduced or eliminated. With few exceptions, the metals used in dentistry are in fact alloys.
(2) Amalgam. When one of the metals in an alloy mixture is mercury, an amalgam is formed. A dental amalgam is a combination of mercury with a specially prepared silver alloy, which is used as a restorative material.
(3) Mercury. Mercury is a silver-white, poisonous, metallic element that is liquid at room temperature (symbol Hg).
c. Composition and Effects of Amalgam.
(1) Combining desirable properties. Each metal incorporated into a dental silver alloy has specific properties when combined with mercury. Some properties are desirable and some are undesirable. An acceptable alloy is balanced. The combined effects of the properties of its ingredients should provide the most satisfactory restorative material.
(2) Quantity of mercury. Too little mercury in the mix results in a grainy, weak, readily tarnished, and corroded amalgam. Too much mercury will cause excessive expansion and weakened amalgam.
(3) Standards and requirements. Like other restorative materials, amalgam must meet the standards and requirements set by the National Bureau of Standards and the American Dental Association's (ADA) Specification #1 for alloy used in amalgam.
(4) Composition of the alloy. The ADA specification states that the composition of the alloy must include a minimum of 65 percent silver, a maximum of 29 percent tin, a maximum of 6 to 13 percent copper, and a maximum of two percent zinc by weight. See figure 1-1.
(5) Correct proportion important. Immediately prior to use, the silver alloy is mixed with pure and uncontaminated mercury. (Mercury, although an indispensable ingredient, imparts undesirable properties to the amalgam if added in incorrect proportions.) There are some alloys that are completely zinc free. They can therefore be used more successfully in a moisture-contaminated environment.
Figure 1-1. Approximate composition of an acceptable amalgam alloy.
(6) Properties of the finished product. Each element composing amalgam imparts certain properties to the finished product. Table 1-1 summarizes these properties. Silver imparts strength, durability, and color, gives the alloy desirable setting expansion, decreases flow, and accelerates (decreases) the setting time. Tin makes the amalgam easier to work, controls excessive setting expansion, and increases both flow and setting time. Copper increases hardness, contributes to setting expansion, reduces flow, and decreases setting time. Zinc increases workability, and unites with oxygen and other "impurities" to produce a clean amalgam.
Table 1-1. Effects on properties of an amalgam restoration imparted by ingredients.
d. Physical Properties of Amalgam. The most important physical properties of amalgam are flow and creep, dimensional change, and strength.
(1) Flow and creep. Flow and creep are characteristics that deal with an amalgam undergoing deformation when stressed. The lower the creep value of an amalgam, the better the marginal integrity of the restoration. Alloys with high copper content usually have lower creep values than the conventional silver-tin alloys.
(2) Dimensional change. An amalgam can expand or contract depending upon its usage. Dimensional change can be minimized by proper usage of alloy and mercury.
(3) Compression strength. Sufficient strength to resist fracture is an important requirement for any restorative material. At a 50 percent mercury content, the compression strength is approximately 52,000 pounds per square inch (psi). In comparison, the compressive strength of dentin and enamel is 30,000 psi and 100,000 psi, respectively. The strength of an amalgam is determined primarily by the composition of the alloy, the amount of residual mercury remaining after condensation, and the degree of porosity in the amalgam restoration.
|David L. Heiserman, Editor||
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Revised: October 24, 2010