Amalgam

Overview
An amalgam is an alloy of mercury with another metal. Most metals are soluble in mercury, but some (such as iron) are not. Amalgam also may be a solution of metal-like ion complexes, such as ammonium. Amalgams are commonly used in dental fillings.

History of use
The earliest history of amalgam being used as a a dental restorative material is not very well established, but it has been reported that a silver paste had been used to restore a tooth in as early as 659 A.D. in China. In modern times, amalgam was placed by Auguste Taveau in France as early as 1826, although he had developed it in 1816.

The Crawcour brothers, two Frenchmen, brought amalgam to the United States in 1833, and in 1844 it was reported that 50% of all dental restorations placed in upstate New York consisted of amalgam. Prior to this, dentists had been restoring teeth using filling material such as stone chips, resin, cork, turpentine, gum, lead and gold leaf. The renowned physician Ambroise Paré (1510 – 1590) had used lead or cork to fill teeth.

Over the next 50 years, many different metal combinations had been tried, including the use of, among other things, platinum, cadmium, antimony and bismuth. In 1895, G. V. Black published a dental amalgam formula that provided for the most clinically acceptable performance, and his recipe remained unchanged for virtually 70 years.

In 1959, Dr. Wilmer Eames suggested a modification to the mercury-to-alloy ratio, recommending it be dropped from 8:5 to 1:1. The standard formula was again changed in 1963, when a superior amalgam consisting of a high-copper dispersion alloy was introduced. Although it was intially believed that this superiority was due to dispersion strengthening of the allow, it was later discovered that the improved strength of the amalgam was, in fact, a result of the additional copper forming a copper-tin phase that was less susceptible to corrosion than the tin-mercury phase in the earlier amalgam. This union of tin-mercury, now known as the gamma-2 phase, contributes to failure and is ideally allowed to rise during condensation of the amalgam while it is being placed in a tooth, to subsequently be removed when the amalgam is carved to achieve proper occlusal anatomy and functional occlusion.

Modern use as a dental restoration
Amalgam is an "excellent and versatile restorative material" and is used in dentistry because of a number of reasons. It is inexpensive and relatively easy to use and manipulate during placement; it remains soft for a short time so it can be packed to fill any irregular volume, and then forms a hard compound. Amalgam possesses greater longevity than other direct restorative materials, such as composite; on average, most amalgam restorations serve for 10 to 12 years, whereas resin-based composites serve for about half that time. However, because of recent improvements in composite material science and a better understanding of the technique-sensitivity of placement, the length of composite survival has increased substantially.

The cost benefit analysis of the use of amalgam is often compared to that of resin-based composites because it would be the latter material that would generally be used as an alternative should the amalgam analysis prove unfavorable. There are many reasons why amalgam enjoys greater overall longevity than that of resin-based composites. Among these reasons are that composites are technique sensitive and require "extreme care" and "considerably greater number of exacting steps" in their proper placement. On the other hand, amalgam is "tolerant to a wide range of clinical placement conditions and moderately tolerant to the presence of moisture during placement.

Another important issue is the environment at the tooth-restoration margin. Whereas the elemental composition of amalgam serves as a bacteriostatic agent, TEGMA, the basic constituents in many resin-based composites, actually "encourages the growth of microorganisms." Because of this, recurrent marginal decay underneath resin-based composites "requires almost immediate removal, whereas those underneath amalgam restorations progress much more slowly."

Recurrent marginal decay is a very important factor in restoration failure, but more so in composite restorations. In the Casa Pia study in Portugal (1986-1989), 1,748 posterior restorations were placed and 177 (10.1%) of them failed during the course of the study. Recurrent marginal decay was the main reason for failure in both amalgam and composite restorations, accounting for 66% (32/48) and 88% (113/129), respectively. Polymerization shrinkage, the shrinkage that occurs during the composite curing process, has been implicated as the primary reason for postoperative marginal leakage.

It is because of these reasons and more that amalgam has been substantiated as a superior restorative material over resin-base composites. The New England Children's Amalgam Trial (NECAT), a randomized controlled trial, yielded results "consistent with previous reports suggesting that the longevity of amalgam is higher than that of resin-based compomer in primary teeth and composites in permanent teeth.   Compomers were seven times as likly to require replacement and composites were seven times as likely to require repair.

There are circumstances in which composite serves better than amalgam; when amalgam is not indicated, or when a more conservative preparation would be beneficial, composite is the recommended restorative material. These situations would include small occlusal restorations, in which amalgam would require the removal of more sound tooth structure, as well as in "enamel sites beyond the height of contour."

Removal and replacement of amalgam restorations has traditionally been considered when "ditching" is present on the edges of the restoration. Ditching is "a deficiency of amalgam along the margin, preventing the margin of the cavity preparation from being flush... An area of ditching is also commonly referred to as a submarginal area and it requires removing tooth structure or replacing the amalgam to correct the situation."

Dental amalgam controversy
Dental amalgam is a source of low-level exposure to mercury, and concerns have been raised about whether this poses a health hazard. Despite considerable investigation, no scientific evidence links it as a cause of clinically significant toxic effects, except for the rare local hypersensitivity reaction. The American Dental Association Council on Scientific Affairs concluded that both amalgam and composite materials are safe and effective for tooth restoration, and The National Institutes of Health has stated that amalgam fillings pose no personal health risk, and that replacement by non-amalgam fillings is not indicated. Recent random clinical trials have also established that amalgams are safe, finding no evidence of neurological harm or deleterious renal effects associated with their use in children after examining a period of 5–7 years following treatment. However, these studies did not address longer-term effects. The preparation of amalgam could pose a potential health hazard to dental workers who work with mercury compounds in relatively high concentrations. Because mercury is a regulated waste in some countries, its disposal can be costly.