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Amalgam is a commonly used dental restorative material used for dental fillings which has been used for over 150 years. It contains a mixture of mercury with at least one other metal. Amalgam has been the restoration of choice for many years due to its advantages over other restorative materials, such as low cost, ease of application, strength, durability, and bacteriostatic effects. Factors which have led to recent decline in use are partly due to a lingering, but controversial concern about the detrimental health effects of the mercury content. Furthermore the metallic colour does not blend very well with tooth structure and there are cosmetic concerns, especially when used on front teeth which can be addressed using alternative dental materials. Research into relatively new alternative materials which possess better aesthetic properties, and which attempt to bridge the gap of amalgams history of superior strength and longevity continues to decrease their viability. There are also environmental concerns about mercury emissions from waste amalgam and cremation of deceased individuals.

Contents 1 Composition 1.1 Gamma 2 phase amalgams 1.2 Galvanic shock 2 History of use 3 Modern use as a dental restoration 3.1 Longevity 4 Dental amalgam toxicity controversy 5 See also 5.1 unrelated to dentistry 6 References

Composition

Currently, many dental amalgams are composed of 43% to 54% mercury; the remaining powder is made up of mainly silver (~20-35%)¹ and some tin, copper (~10%), and zinc (~2%).

Gamma 2 phase amalgams

After widespread adoption and wildly varying standards, the multitude of formulas for making amalgams were standardised into the gamma-2-phase amalgam formula in 1895.

The gamma-2-phase amalgams contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:

• > 65% silver (Ag)
• < 29% tin (Sn)
• < 6% copper (Cu)
• < 2% zinc (Zn)
• < 3% mercury (Hg)

The resulting amalgam is composed of the gamma phase (the silver-tin eutectic Ag₃Sn, which reacts with mercury, yielding the gamma-1 phase (Ag₂Hg₃) and gamma-2 phase (Sn_7-8Hg). The gamma phase is prone to corrosion and its mechanical strength is low. The alloy tends to undergo crevice corrosion and form local galvanic cells.

Around 1970, the ingredients changed to the new non-gamma-2 form, with lower manufacturing cost, greater mechanical strength, and better corrosion resistance. The reduced-gamma-2 amalgams (sometimes referred to as "high-copper" amalgams) contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:

• > 40% silver (Ag)
• < 32% tin (Sn)
• < 30% copper (Cu)
• < 2% zinc (Zn)
• < 3% mercury (Hg)

The amalgam alloy is strengthened by presence of Ag-Cu particles. The gamma-2 phase reacts with the Ag-Cu particles to form eta phase Cu₆Sn₅ and gamma-1 phase.

The possible difference in toxicology between the two has not been studied conclusively.

Galvanic shock

When aluminium foil makes contact with some amalgam fillings, saliva in the mouth can act as an electrolyte. This can generate small electrical currents which are felt through the nerves in the tooth as (often extremely painful) electrical "jolts" or shocks.

History of use

The earliest instance of amalgam use as a dental restorative material is not well established, but it has been reported that a silver paste was used to restore a tooth in as early as 659 A.D. in China.² Prior to amalgam, dentists restored teeth using filling material such as stone chips, resin, cork, turpentine, gum, lead, and gold leaf, among other metals.^{citation needed} The renowned physician Ambroise Paré (1510 – 1590) had used lead or cork to fill teeth. In 1603, a German named Tobias Dorn Kreilius described a process for creating an amalgam filling by dissolving copper sulfide with strong acids, adding mercury, bringing to a boil, and then pouring onto the teeth.³ In France D'Arcet's Mineral Cement was popular, but it had to be boiled into a liquid before being poured on patients' teeth. Louis Regnart added mercury to the mixture, lowering the temperature required significantly, and for this became known as the "Father of Amalgam".^4:45 Amalgam was placed by Auguste Taveau in France as early as 1826,⁵ although he had developed it in 1816. Gold, platina, silver, tin, lead, and alloys of these substances were highly preferred to amalgam around the 1840s. Amalgam was controversial; one of the first dental textbooks by the leading US dental researcher of the time called it "the most pernicious material that has ever been employed for filling teeth". Although the textbook admitted that amalgam had certain benefits, the book strongly discouraged its use, stating that "the mercury is the mischievous ingredient".⁶

The Crawcour brothers, two Frenchmen, brought amalgam to the United States in 1833,² and in 1844 it was reported that fifty percent of all dental restorations placed in upstate New York consisted of amalgam.⁷ Prior to the amalgam introduced by them, the two main options were having teeth completely removed, or a long appointment before having hot gold hammered into the tooth. The Crawcour bothers avoided calling attention to the mercury in their "silver" fillings, calling them "mineral succedaneum" or "royal mineral succedaneum", which the public associated with gold. In 1843, the American Society of Dental Surgeons (ASDS), the only US dental association at the time, declared the use of dental amalgam to be malpractice and forced all of its members to sign a pledge to abstain from using it. ⁸ This was the beginning of what are known as the amalgam wars.⁹ The ASDS ran the Crawcour brothers out of the country.¹⁰ The position against amalgam led to the decline of the ASDS, as dental amalgam was much cheaper than gold, easier to apply, and less painful, as it was not boiled. In 1850, the ASDS rescinded its anti-amalgam resolution, and in 1856 it disbanded. The American Dental Association was founded a few years later in 1859. It placed its focus on the mechanical aspects of dentistry; until 1917 a high-school diploma was not needed to enter dental school.¹¹

Over the next fifty years, many different metal combinations were 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 seventy years.⁵

In 1959, Dr. Wilmer Eames suggested a modification to the mercury-to-amalgam ratio, recommending it be dropped from 8:5 to 1:1.¹² The standard formula was again changed in 1963, when a superior amalgam consisting of a high-copper dispersion alloy was introduced.¹³ Although it was initially believed that this superiority was due to dispersion strengthening of the amalgam, it was later discovered that the improved strength of the amalgam was 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.¹⁴ 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.

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Modern use as a dental restoration

Amalgam is an "excellent and versatile restorative material"¹⁵ and is used in dentistry for 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;¹⁶. According to one study from 1989, most amalgam restorations serve for 10 to 12 years, whereas resin-based composites serve for about half that time.¹⁷ However, this difference has decreased with continual development of composite resins physical properties.¹⁸

Amalgam is typically compared to resin-based composites because many applications are similar and many physical properties and costs are comparable.

Longevity

Reasons for amalgams historically superior longevity include: Techniques for composite placement are more sensitive to many factors and require "extreme care"¹⁹ and "considerably greater number of exacting steps"¹⁸. 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.²⁰

Longevity is also effected by the join at the tooth-restoration margin. Amalgam has properties of a bacteriostatic agent. TEGMA (constituting many resin-based composites), actually "encourages the growth of microorganisms."¹⁸. This leads to increased decay underneath resin-based composites "requires almost immediate removal, whereas those underneath amalgam restorations progress much more slowly."¹⁸

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.²¹ Polymerization shrinkage, the shrinkage that occurs during the composite curing process, has been implicated as the primary reason for postoperative marginal leakage.²²²³

It is for 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 teeth²⁴²⁰ and composites in permanent teeth.²⁵²⁰ Compomers were seven times as likely to require replacement and composites were seven times as likely to require repair.²⁰

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,²⁶ as well as in "enamel sites beyond the height of contour."²⁷

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."²⁸

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Dental amalgam toxicity controversy

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Main article: Dental amalgam controversy

Controversy over the mercury component of dental amalgam dates back to its inception, when it was vigorously opposed by the dental establishment, but it has become a prominent debate in the late 20th century, with the pressure to eliminate it at an all-time high.²⁹ Many people are unaware of the mercury in fillings,³⁰ and this lack of informed consent was the most consistent issue raised in a recent FDA panel on the issue by panel members.²⁹ 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.³¹ The WHO reports that 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.³² Separators may dramatically decrease the release of mercury into the public sewer system, where dental amalgams contribute one-third of the mercury waste,³² but they are not required in the United States.³³ As of 2008, the use of dental amalgam has been restricted in Sweden, Norway and Finland, and a committee of the US Food and Drug Administration (FDA) has refused to ratify assertions of safety.

Scientists agree that mercury amalgam fillings expose the bearers to a daily dose of mercury, but the level and effects of the chronic exposure are disputed. 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. 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.³⁴ 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."³⁵

In recent years evidence of serious toxic effects, for instance 25 studies of 5821 patients reviewed in "Effects of Amalgam Removal on Health", by Mats Hanson, plus a later study by Wojcik, Godfrey, Christie, Haley (2006).^{citation needed} The American Dental Association Council on Scientific Affairs has concluded that both amalgam and composite materials are considered safe and effective for tooth restoration,³⁶ and a study has stated that amalgam fillings pose no personal health risk, and that replacement by non-amalgam fillings is not indicated.³⁷ Recent randomized clinical trials have found no evidence of neurological harm or deleterious renal effects associated with use of amalgam in children after examining a period of 5–7 years following treatment.^{38 39} Both these trials were published in the same issue of the JAMA. Also published in the same journal was an editorial by Prof. Herbert Needleman noting these two articles, explicitly advising against using them as evidence of dental amalgam safety. He says:

“It is predictable that some outside interests will expand the modest conclusions of these studies to assert that use of mercury amalgam in dentistry is risk free. This conclusion would be unfortunate and unscientific. The conclusions that can be extrapolated from these 2 studies are constrained by several factors.”⁴⁰

The health problems usually focused upon include chronic illnesses, oral lesions, birth defects, mental disorders, autoimmune disorders, neurodegenerative diseases, erethism, and multiple sclerosis. There is strong evidence that a certain percentage of lichenoid lesions are caused by amalgam fillings.⁴¹

See also

• Dental composite
• Dentistry
• Mercury poisoning

unrelated to dentistry

• Sodium amalgam
• Mercury-aluminum amalgam

References

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