In dentistry, amalgam is an alloy of mercury with various metals used for dental fillings. It commonly consists of mercury (50%), silver (~22-32% ), tin (~14%), copper (~8%), and other trace metals. Dental amalgams were first documented in a Tang Dynasty medical text written by Su Kung 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.
Recently however, its popularity has diminished somewhat. Concern for aesthetics, environmental pollution, health, and the availability of improved, reliable, composite materials have all contributed. In particular, concerns about the toxicity of mercury have made its use increasingly controversial. Due to a plan to phase out the use of mercury, Sweden, Norway and Denmark deliberated in 2008 on a ban of mercury dental amalgam, substituting it with composite fillings. Dentists in Denmark are no longer allowed to use mercury in fillings since April 1, 2008. The Swedish amalgam ban is for both environmental and health issues, according to the Swedish authorities.
History of use
There are, according to Geir Bjørklund, indications that dental amalgam was used in the first part of the T'ang Dynasty in China (618-907 A.D.), and in Germany by Strockerus in about 1528. Evidence of a dental amalgam first appears in the Tang Dynasty medical text Hsin Hsiu Pen Tsao written by Su Kung in 659, manufactured from tin and silver. Historical records hint that the use of amalgams may date even earlier in the Tang Dynasty. It was during the Ming Dynasty that the composition of an early dental amalgam was first published, and a text written by Liu Wen Taiin 1505 states that it consists of "100 shares of mercury, 45 shares of silver and 900 shares of tin." Ever since its introduction in the Western World in the 1830s, amalgam has been the subject of recurrent controversies because of its mercury content. Early amalgam was made by mixing mercury with the filings of silver coins.
In 1833 the Crawcour brothers, two Frenchmen, brought amalgam to the United States, and in 1844 it was reported that fifty percent of all dental restorations placed in upstate New York consisted of amalgam. However, at that point the use of dental amalgam was declared to be malpractice, and the American Society of Dental Surgeons (ASDS), the only US dental association at the time, forced all of its members to sign a pledge to abstain from using the mercury fillings. This was the beginning of what is known as the first dental amalgam war.
The dispute ended in 1856 with the disbanding of the old association. The American Dental Association was founded in its place in 1859, which has since then strongly defended dental amalgam from allegations of being too risky from the health standpoint.
The ratio of the mercury to the remaining metallic mixture in dental amalgam has not always been 50:50. It was as high as 66:33 in 1930. Relative ratios between the other metals used in dental amalgams has also been highly variable. Conventional (or gamma 2)-amalgams have 32% silver and 14% tin, and they are most susceptible to corrosion due to their low copper content. Non-gamma-2 dental amalgams have been developed that were, however, found to release higher levels of mercury vapor compared with traditional amalgams. Amalgam is the dental material that has the strongest tendency to create galvanic currents and high electric potentials as it ages. The rate of mercury release with the corrosion is accelerated when the amalgam filling is in contact with old restorations or coupled with gold artifacts present in the mouth.
Longevity and other advantages
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Amalgam is "tolerant to a wide range of clinical placement conditions and moderately tolerant to the presence of moisture during placement". In contrast, the techniques for composite resin placement are more sensitive to many factors and require "extreme care".
Mercury has properties of a bacteriostatic agent whereas certain methacrylate polymers (for example TEGMA, triethylene glycol methacrylate) composing the matrix of resin composites "encourages the growth of microorganisms".
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.
These are some of the reasons why amalgam has remained 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 likely to require replacement and composites were seven times as likely to require repair.
There are circumstances in which composite serves better than amalgam. For example, 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." For cosmetic purposes, composite is preferred when a restoration is required on an immediately visible portion of a tooth.
Controversy over the mercury component of dental amalgam dates back to its inception, when it was opposed by the United States dental establishment, but it became a prominent debate in the late 20th century, with consumer and regulatory pressure to eliminate it "at an all-time high". In a 2006 nationwide poll, 76% of Americans were unaware that mercury is the primary component in amalgam fillings, and this lack of informed consent was the most consistent issue raised in a recent U.S. Food and Drug Administration panel on the issue by panel members. Environmental concerns over external costs exist as well, as the use of dental amalgam is unregulated at the federal level in, for example, the United States. The WHO reports that in the United Kingdom mercury from amalgam accounts for 5% of total mercury emissions and that when combined with waste mercury from laboratory and medical devices, represents 53% of total mercury emissions. Separators may dramatically decrease the release of mercury into the public sewer system, where dental amalgams contribute one-third of the mercury waste. Although several states (NJ, NY, MI, etc.) require the installation of dental amalgam separators, they are not required by the United States government. As of 2008, the use of dental amalgam has been banned in Norway, Sweden and Denmark, and a committee of the US Food and Drug Administration (FDA) has refused to ratify assertions of safety.
In the 1990s, several governments evaluated the effects of dental amalgam and concluded that the most likely health effects would be due to hypersensitivity or allergy. Germany, Austria, and Canada recommended against placing amalgam in certain individuals such as pregnant women, children, those with renal dysfunction, and those with an allergy to metals. In 2004, the Life Sciences Research Office analyzed studies related to dental amalgam published after 1996. Concluding that mean urinary mercury concentration (μg of Hg/L in urine, HgU) was the most reliable estimate of mercury exposure, it found those with dental amalgam were unlikely to reach the levels where adverse effects are seen from occupational exposure (35 μg HgU). 95% of study participants had μg HgU below 4-5. Chewing gum, particularly for nicotine, along with more amalgam, seemed to pose the greatest risk of increasing exposure; one gum-chewer had 24.8 μg HgU. Studies have shown that the amount of mercury released during normal chewing is extremely low. However, from reviewing medical literature, the World Health Organization states mercury levels in biomarkers such as urine, blood, or hair do not represent levels in critical organs and tissues. Additionally, Gattineni et al. found that mercury levels do not correlate with the number or severity of symptoms. It concluded that there was not enough evidence to support or refute many of the other claims such as increased risk of autoimmune disorders, but stated that the broad and nonspecific illness attributed to dental amalgam is not supported by the data. Mutter in Germany, however, concludes that "removal of dental amalgam leads to permanent improvement of various chronic complaints in a relevant number of patients in various trials."
Under the Comments of the American Dental Association Before the Dental Products Panel of the Medical Devices Advisory Committee, the ADA supports the 2009 FDA ruling on dental amalgam.
After FDA’s deliberations and review of hundreds of scientific studies relating to the safety of dental amalgam, the FDA concluded that “clinical studies have not established a causal link between dental amalgam and adverse health effects in adults and children age six and older.” The FDA concluded that individuals age six and older are not at risk to mercury-associated health affects from mercury vapor exposure that come from dental amalgam.
On the controversy of dental amalgam toxicity, the ADA asserts the best scientific evidence supports the safety of dental amalgam. Clinical studies have not established an occasional connection between dental amalgam and adverse health effects in the general population.
The recent World Health Organization report reaffirms the safety and importance of maintaining the availability of dental amalgam. The comments of the ADA concluded that dental amalgam remains an excellent and valuable restorative material for both dentists and patients; other alternative tooth restorative materials haven’t been proven to be as effective as dental amalgam.
The comments of the ADA state that there is no scientific reason to revisit the 2009 FDA ruling; while high exposure to elemental mercury has been associated to adverse health effects, the mercury exposure in dental amalgam is not high enough to cause harm in patients. Dental amalgam is a safe restorative material which now have special controls on this device, imposed by the FDA to ensure the safety and effectiveness of dental amalgam. Also, in the FDA final regulation on dental amalgam in 2009, the FDA recommended the product labeling of dental amalgam. The suggested labeling included: a warning against the use of dental amalgam in patients with mercury allergy, a warning that dental professionals use appropriate ventilation when handling dental amalgam, and a statement discussion of scientific evidence on dental amalgam’s risks and benefits in order to make informed decisions amongst patient and professional dentists.
Metallurgy of amalgam
To fabricate an amalgam filling, the dentist triturates a silver-base alloy by mixing roughly equal parts (by mass) of shavings of the alloy with liquid mercury in a mortar and pestle or other grinding device until the shavings are thoroughly wetted. The silver alloy is typically 40-70% Ag, 0-30% Sn, 2-40% Cu and 0-2% Zn. The dentist packs the plastic amalgamation, before it ages, into the cavity. The amalgam expands ~0.1% over 6-8 hours by the slow aging reaction, γ-Ag3Sn + Hg → Ag2Hg3 + Sn7Hg. Non-metallic composite fillings are typically 1-5 μm silicate glass particles bonded by a UV-cured methacrylate resin.  
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