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Amalgam


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Amalgam


Dental amalgam is a mixture of metals, consisting of liquid (elemental) mercury and a powdered alloy composed of silver, tin, and copper. Approximately half (50%) of dental amalgam is elemental mercury by weight. The chemical properties of elemental mercury allow it to react with and bind together the silver/copper/tin alloy particles to form an amalgam.


Dental amalgam fillings are often referred to as "silver fillings" because of their silver-like appearance, although the use of this term in not recommended because the term does not correctly explain the materials in amalgam.


When placing dental amalgam, the dentist first drills the tooth to remove the decay and then shapes the tooth cavity for placement of the amalgam filling. Next, under appropriate safety conditions, the dentist mixes the encapsulated powdered alloy with the liquid mercury to form an amalgam putty. This softened amalgam putty is placed and shaped in the prepared cavity, where it rapidly hardens into a solid filling.


Deciding what filling material to use to treat dental decay is a choice that should be made by you and your dentist. As you consider your options, you should keep in mind the following information about dental amalgam fillings.


Approximately half of a dental amalgam filling is liquid mercury and the other half is a powdered alloy of silver, tin, and copper. Mercury is used to bind the alloy particles together into a strong, durable, and solid filling. Mercury's unique properties (it is a liquid at room temperature that bonds well with the alloy powder) make it an important component of dental amalgam that contributes to its durability.


No. There are several different chemical forms of mercury: elemental mercury, inorganic mercury, and methylmercury. The form of mercury associated with dental amalgam is elemental mercury, which releases mercury vapor. The form of mercury found in fish is methylmercury, a type of organic mercury. Mercury vapor is mainly absorbed by the lungs. Methylmercury is mainly absorbed through the digestive tract. The body processes these forms of mercury differently and has different levels of tolerance for mercury vapor and methylmercury. However, some recent studies show that inorganic mercury from dental amalgam and organic (methylated) mercury from seafood can transform into each other in the body, making it difficult to distinguish between their potential health effects.


Bioaccumulation refers to the build-up or steadily increasing concentration of a chemical in organs or tissues in the body. Mercury from dental amalgam and other sources (e.g., fish) is bioaccumulative. Although much of the mercury (either from dental amalgam or food) is eliminated, part of it can accumulate in the bodily fluids and tissues including the kidneys and brain. However, studies have not shown that increased mercury levels and bioaccumulation due to dental amalgam result in detectable damage to target organs. For more information about bioaccumulation, please see Mercury Overview.


If your fillings are in good condition and there is no decay beneath the filling, the FDA does not recommend that you have your amalgam fillings removed or replaced. Removing intact amalgam fillings results in unnecessary loss of healthy tooth structure and exposes you to a temporary increase in mercury vapor released during the removal process. Intact amalgam fillings in any individual, including the sensitive groups such as pregnant/nursing mothers and children, should not be removed for the purpose of preventing any disease or health condition, unless considered medically necessary by a health care professional. If you have a health condition (especially sensitivity/allergy to mercury, or neurological/kidney disease), you may discuss the need for removal and replacement with your dentist and/or physician.


Information reviewed by the FDA over the past two decades points to uncertainties about the effects of mercury exposure from dental amalgam, the acceptable exposure levels for mercury vapor (gas), the potential for mercury to collect in the body, and whether the degree of accumulation of mercury from dental amalgam results in adverse health outcomes. The majority of evidence shows exposure to mercury from dental amalgam does not lead to negative health effects in the general population. Exposure to mercury may pose a greater health risk in certain groups of people, who may be more susceptible to potential adverse effects generally associated with mercury. These high-risk populations include:


If you fall into any of the greater risk groups listed above, the FDA strongly encourages the use of non-amalgam restorations (fillings) such as composite resins and glass ionomer cements if your dentist thinks these materials are appropriate for your affected tooth's structure and location, and if you have no history of allergic reaction or hypersensitivity to these materials.


The FDA does not recommend anyone remove or replace existing amalgam fillings in good condition unless it is considered medically necessary by a health care professional (for example, documented hypersensitivity to the amalgam material). Removing intact amalgam fillings results in unnecessary loss of healthy tooth structure and a temporary increase in exposure due to additional mercury vapor released during the removal process.


Dental amalgams were first documented in a Tang Dynasty medical text written by Su Gong (苏恭) in 659, and appeared in Germany in 1528.[4][5] In the 1800s, amalgam became the dental restorative material of choice due to its low cost, ease of application, strength, and durability.[6]


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.[12] Amalgam restorations made from this balanced formula were reasonably successful and its longevity increased.[13] However, one disadvantage that remained was fracture at the tooth-amalgam interface commonly called marginal fracture.[13] Sn8Hg (γ2 phase) was considered to be responsible for this problem.[13][12] This phase has been shown to be the weakest phase in the set amalgam[14] and is subject to corrosion, particularly at the tooth-amalgam interface.[13][12]


In 1962 a new amalgam alloy, called Dispersalloy, 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 unicompositional 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.[13]


Dental amalgam is produced by mixing liquid mercury with an alloy made of silver, tin, and copper solid particles. Small quantities of zinc, mercury and other metals may be present in some alloys. This combination of solid particles is known as amalgam alloy.[13] The composition of the alloy particles are controlled by the ISO Standard (ISO 1559) for dental amalgam alloy in order to control properties of set amalgam such as corrosion and setting expansion. It is important to differentiate between dental amalgam and the amalgam alloy that is commercially produced and marketed as small filings, spheroid particles, or a combination of these, suitable for mixing with liquid mercury to produce the dental amalgam. Amalgam is used most commonly for direct, permanent, posterior restorations and for large foundation restorations, or cores, which are precursors to placing crowns.[12]


The reaction between mercury and alloy when mixed together is termed an amalgamation reaction.[15] It will result in the formation of a silver-grey workable mass which can be condensed into cavities.[15] After condensing, the dental amalgam is carved to generate the required anatomical features and then hardens with time. The standard composition of alloy prior to 1986 is referred to as conventional amalgam alloy. More recently (post-1986), there has been a change in the compositional standard of the alloy due to better understanding of structure-property relationships for the materials. Conventional amalgam alloy commonly consists of silver (65% ), tin (29%), copper (8%) and other trace metals; current amalgam alloy consists of silver (40%), tin (32%), copper (30%) and other metals.[12]


Amalgam is a mixture of two or more metals (alloy) with mercury which has been purified first by distillation to remove impurities. Currently,[when] major components of the alloy are silver, tin, and copper. The composition of the alloy powder is controlled by ISO standard for dental amalgam alloy (ISO 1559) to control the properties of amalgam.[12]


Creep or plastic deformation happens when subjected to intra-oral stresses such as chewing or grinding. Creep causes the amalgam to flow and protrudes from the margin of the cavity forming unsupported edges. "Ditch" is formed around the margins of the amalgam restoration after fracture due to amalgam creep at the occlusal margins. The γ2 phase of amalgam is primarily responsible for high values of creep.[12]


In spite of that, it is thought that corrosion actually offers a clinical advantage. The corrosion products will gather at the tooth-amalgam interface and fill the microgap (marginal gap) which helps to decrease microleakage. Even so, there are no reports of increased marginal leakage for the copper-enriched amalgams indicating that sufficient quantities of corrosion product are produced to seal the margins.[12] 59ce067264






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