Dental amalgam is a liquid mercury and metal alloy mixture used in dentistry to fill cavities caused by tooth decay.  Dental amalgams were first documented in a Tang Dynasty medical text written in 659, and appeared in Germany in 1528.In the 1800s, amalgam became the dental restorative material of choice due to its low cost, ease of application, strength, and durability.

Amalgam has been used for many years for restorations, commonly known as fillings. Prior to 1900 many compositions were tried but few were successful when placed in the oral environment. Around 1900, small amounts of copper and occasionally zinc were added. Zinc acts as a scavenger because it prevents oxidation of the other metals in the alloy during the manufacturing process. Zinc accomplishes this by combining readily with oxygen to form zinc oxide. Amalgam restorations made from this balanced formula were reasonably successful and its longevity increased. However, one disadvantage that remained was fracture at the tooth-amalgam interface commonly called marginal fracture. Sn8Hg (γ2 phase) was considered to be responsible for this problem. This phase has been shown to be the weakest phase in the set amalgam and is subject to corrosion, particularly at the tooth-amalgam interface.

In 1962 a new amalgam alloy, called Disperse alloy, was introduced with the addition of a spherical silver-copper eutectic particle to the traditional lathe-cut Ag3Sn particle in a ratio of 1:2. The mixture of these two types of particles is known as admix alloy. This alloy strengthened the set amalgam and reduced the γ2 phase (Sn8Hg). The increased copper in the silver-copper eutectic reacted preferentially with tin so that Sn8Hg could not form. Early results from the clinical use of this new amalgam showed an improvement in marginal integrity. About 10 years later, another alloy, called Tytin, was introduced by adding significant amount of Cu3Sn together with Ag3Sn, in the form of a uni compositional spherical particle to eliminate the γ2 phase. Both of these relatively new alloys raised the copper content from 5%, present in the older balanced composition alloy, to about 13% for the newer alloys Dental amalgam does not by itself bond to tooth structure. This was recognized as a shortcoming by early practitioners such as Baldwin. He recommended that the prepared cavity be coated with zinc phosphate cement just prior to filling with amalgam, in order to improve the seal and retention. The practice did not become universally accepted and eventually fell into disuse. Until the 1980s, most amalgam restorations placed worldwide were done without adhesives, although in the 1970s a polycarboxylate-based adhesive liner was formulated specifically for this purpose In the mid-1980s the first reports of the use of resins to bond amalgam to etched tooth structure, much like is done for composite resins, appeared in the literature. Since then, a number of papers have been published on laboratory as well as clinical studies of the technique. For large cavity restorations, features such as pins, slots, holes and grooves can be used for the retention of large amalgam restorations, but they do not reinforce the amalgam or increase its strength.

There is no current scientific evidence to justify the extra cost and effort associated with the use of adhesively bonded amalgam restorations in comparison with non-bonded amalgam restorations. In view of the lack of evidence on the additional benefit of adhesively bonding amalgam compared with non-bonded amalgam, it is important that clinicians are mindful of the additional costs that may be incurred.

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Sarah eve

Editorial Assistant

Journal of Oral Hygiene and Health