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.
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 (AD 618–907), 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 two natives of England, Edward Crawcour and his nephew Moses Crawcour (incorrectly referred to as "the Crawcour brothers"), 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 (ADA) 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 gamma2)-amalgams have 32% silver and 14% tin, and they are most susceptible to corrosion due to their low copper content. Non-gamma2 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.
In 2008, Sweden, Norway and Denmark deliberated a ban of mercury dental amalgam in favor of composite fillings. Since 1 April 2008, dentists in Denmark are forbidden to use mercury in fillings, except for molar masticating surface fillings in permanent (adult) teeth. The Swedish amalgam ban is for both environmental and health issues, according to the Swedish authorities.
Amalgam's setting reaction
Amalgam has basically 2 types of powders/alloys as Low copper amalgam and High copper amalgam. But first we will see various phases during setting reaction,
γ=Ag3Sn (powder form so strongest phase) γ1=Ag2Hg (Noble phase, main constituent of set amalgam) γ2=Sn7–8Hg (weakest phase, causes tarnish and corrosion so undesirable) β=AgSn η=Cu6Sn5 ε=Cu3Sn
[Note: Properties of hardened amalgam depends upon proportion of reaction phases. If more unconsumed Ag3Sn is present, the stronger the amalgam. The gamma2 phase is weakest component and is least stable to corrosion.]
1) Low copper alloy
(Alloy powder) + Hg → Alloy-Hg + (unreacted powder particles).
i.e. (Ag3Sn+AgSn) + Hg >Ag2Hg3+Sn7–8Hg3 + (Ag3Sn+AgSn).
i.e. (γ+β) + Hg → γ1+γ2 + (unreacted powder particles).
2) High Copper alloy
In High copper alloy, high copper is added to improve mechanical properties, resistance to corrosion and marginal integrity. It has 2 types:
- Admixed alloy [2 Powders with different contents are mixed with mercury]
- Uni/single-composition alloy [1 powder is made available instead of 2 (that's why uni) to mix with mercury]
Admixed alloy setting reaction
Made by mixing 1/3 high copper particles and 2/3 silver–tin alloy (low copper lathe cut particles). Here copper avoids Sn to react with Hg avoiding gamma2 phase (weakest phase), forming Cu6Sn5 (eta phase).
2/3 silver tin alloy + 1/3 Eutectic alloy+Hg → Alloy-Hg + halo of Cu6Sn5+Unreacted particles of silver tin powder + unreacted particles of eutectic alloy.
i.e. 2/3 (Ag3Sn+AgSn) + 1/3 (AgCu)+Hg → Ag2Hg3 + Cu6Sn5+unreacted (Ag3Sn+AgSn)+unreacted (AgCu).
i.e. 2/3 (γ+β) + 1/3 eutectic alloy+Hg → γ1 + η + unreacted (γ+β)+unreacted eutectic alloy.
The main role of adding eutectic alloy of copper is to avoid Sn to not react with Hg and to react with Cu. To avoid formation of Sn7–8Hg or gamma2 phase, which is responsible for tarnish and corrosion, thus reducing the same property. There is new phase formed because of binding of Cu and Sn called η (eta) phase, which then benefits by reacting with unreacted particles of AgSn to form gamma1 phase, thus creating a cascade.
Uni/single composition alloy
Here, only 1 powder is added with mercury having all the contents of admixed alloy powders, which uses 2 powders, thus the name is derived. In addition indium/palladium in smaller amounts is added.
Ag-Sn-Cu + Hg → Cu6Sn5 + Ag2Hg3+ unreacted powder.
i.e. (γ β ε) + Hg → η + γ1 + (γ β ε)
The difference in eta phase of admixed alloy and unicomposition alloy is that in unicomposition alloy Cu6Sn5 crystals are much larger and rod-shaped than those in admixed alloy. Copper is added in unicomposition also, which causes removal of gamma2 phase. Gamma2 phase causes tarnish and corrosion. Gamma2 phase also causes amalgam to go in permanent deformation and if you send this amalgam in higher loading like occlusion then this phenomenon of deformation is called "creep". So, in short, Cu reduces the amount of creep by avoiding formation of gamma2 phase. Gamma2 phase starts corroding from its surface of restoration and not from center of restoration so it is like going on penetrating the center of amalgam from the surface, which is why it is called "Penetration type of corrosion". As we now know, low copper undergoes more penetration type of corrosion indirectly due to its low copper content and directly due to its high tin content, which makes it more porous and spongy day by day, thus reducing its mechanical strength as compare to high copper amalgam.
Amalgam vs. polymer resins
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". 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.
Dental amalgam toxicity
The greatest toxicity concern with dental amalgam is the potential for mercury poisoning when used as the dental material in a dental filling. Major health and professional organizations say that amalgam is safe. However, critics argue that it has toxic effects that make it unsafe, both for the patient and perhaps even more so for the dental professional manipulating it during a restoration. A study by the Life Sciences Research Office found that studies on mercury vapor and dental amalgam "provided insufficient information to enable definitive conclusions." They identified several "research gaps", including: "well-controlled studies using standardized measures that evaluate whether low level [mercury vapor exposures] produce neurotoxic and/or neuropsychological effect", studies on "co-exposure to HgO and methylmercury", studies on "in utero exposure to HgO", "occupational studies on [pregnant workers] with well-defined HgO exposure", studies on the absorption of Hg2+ by the "human neonatal gut from breast milk", studies on "whether dental professionals have increased incidences of kidney disease, emotional instability, erethrism, pulmonary dysfunction, or other characteristics of occupational HgO exposure", studies on whether there exist "potential gender differences" or "genetic basis for sensitivity to mercury exposure." Some dentists recommend removing amalgam fillings for health and cosmetic reasons, however removal also involves exposure to mercury vapor released during the removal process. Amalgams also contribute to mercury toxicity in the environment.
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
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