Amalgam

Key points

  • Dental amalgam is a safe, affordable and durable restorative material.
  • Although amalgam remains an effective and inexpensive restorative option, environmental concerns regarding mercury have fueled legislative and regulatory actions in other countries to phase down amalgam use.
  • Considerations of post-placement sensitivity, longevity, esthetics, conditions under which the restoration is to be placed, and cost may all be factors in choosing a restorative material.
Introduction

For more than 150 years, dental amalgam has served as a safe, durable and affordable material in restorative dentistry.1 Dental amalgam is a mixture of metals: liquid (elemental) mercury and a powdered alloy mostly composed of silver, tin, and copper. The fact that its formulation includes mercury has raised potential health and environmental concerns.

Evidence suggests that amalgam fillings have a higher survivability rate and longevity than resin composite restorations,2-6 but aesthetic and environmental concerns have led to increasing usage of resin composite alternatives.7-9 Amalgam remains a safe, effective, and inexpensive dental restorative option. Environmental concerns have led to the international Minamata convention that was signed by the United States in 2013 and was entered into force in 2017.10 The 2017 U.S. Dental Effluent Guidelines (Amalgam Separator Rule), from the U.S. Environmental Protection Agency and supported by the ADA, have greatly reduced the already limited environmental impact from amalgam.  See the ADA Oral Health Topics page on Amalgam Separators and Waste Best Management for more information.

Material and composition

Amalgam is any alloy that contains mercury. Copper, silver, and tin are the major components in dental amalgam but it may also contain zinc, indium, gold, platinum, and palladium.2, 11, 12  Amalgamation is the name given to the process of mixing liquid mercury (approximately 42 to 50% by weight) with the other alloys to form a highly plastic material that hardens following precipitation.2 The American National Standards Institute/American Dental Association (ANSI/ADA) standard specifies only the use of precapsulated alloy and mercury, which is triturated by an amalgamator (i.e., a mechanical triturator), to minimize mercury vapor leakage.12

ANSI/ADA Standard No. 1 (ISO 1559) covers the specifications for the composition and physical properties requirements for precapsulated dental amalgam.12 The requirements for physical properties of dental amalgam are shown in Table 1, below.

Table 1. ADA/ANSI Standard No. 1 Physical Properties of Dental Amalgam12

Creep (%)

Dimensional Change (%)

Compressive Strength (MPa)

Maximum

Range

Min. after 1 hr.

Min. after 24 hrs.

1.0

-0.15 to 0.20

80

300

Silver and tin are the most common elements found in dental amalgam alloy, although amalgam alloys are sometimes referred to as either low-copper (i.e., less than ~5% Cu by weight) or high-copper alloys (i.e., 6% Cu or higher by weight).2, 8, 11  Copper reduces brittleness and when sufficient copper is present in the alloy (≥11.8% Cu by weight), little or no gamma-2 phase amalgam (γ2; Sn7-8Hg), which is the weaker phase and more susceptible to corrosion, will be formed.2, 11 A reduction in the amount of mercury in the reaction will also decrease formation of gamma-2 phase amalgam.13

Alloy powders to be mixed with liquid mercury are available as either lathe-cut (irregular particles) or spherical.2  Alloy powder may also be a mix of lathe-cut and spherical particles.2

Dental considerations

Placement considerations. Current ADA policy (see “ADA Policies on Amalgam as a Dental Restorative Material” below) calls for the elimination of bulk mercury and that dentists only use precapsulated amalgam alloy.14 The use of precapsulated amalgam formulations reduces waste and mercury vapor leakage and provides a more consistent mercury/alloy ratio,2, 12 and is consistent with both ADA policy and EPA regulations.  A very grainy mix indicates undertrituration, whereas overtrituration will result in a shinier, softer mixture that can stick to the wall of the capsule; overtriturated amalgam has a shorter working time and higher setting contraction.2

As part of its mercury hygiene guidance,15 the World Dental Federation (FDI) also recommends the use of precapsulated mercury/alloy as the preferred technique to avoid bulk mercury spills and to eliminate the mercury dispenser as a source of mercury vapor. FDI additionally recommends15 use of high-volume evacuation systems fitted with traps or filters when polishing or removing amalgam, as well as following best practices for amalgam waste (please see the ADA Oral Health Topic, “Amalgam Separators and Best Waste Management” for more information about amalgam waste). Phillips’ dental materials textbook also recommends the use of suction and water spray when amalgam is being ground.2  Use of a dental dam when placing or removing amalgam may have some utility in reducing mercury exposure, but the data on this are limited and any effect seen may be small in magnitude and of transient duration.16-18

Alloy composition.  It has been reported that high copper-containing alloys are more corrosion resistant because of the minimization of the formation of gamma-2 (ɣ2) phase amalgam.19  Amalgam alloys with zinc concentrations greater than 0.01% (m/m) can undergo excessive expansion when exposed to moisture during setting.12 Exposure to moisture can be reduced by the use of rubber dam isolation during the condensation, carving, and finishing steps. The inclusion of palladium in high-copper alloy improves corrosion resistance, compression resistance, lowers the creep, and contributes to the reduction of mercury vapor release during amalgam setting.20

Particle shape. Spherical-shaped particles usually exhibit a shorter working time, less mercury content, less dimensional change, and lower creep, as compared to irregular-shaped particles.2  However, amalgam containing spherical shaped particles reportedly releases higher levels of mercury vapor and has associated microleakage.2  The milling process for irregular-shaped particles may cause stresses that can cause changes in amalgam properties, such as amalgamation rate and dimensional changes during hardening.2  Amalgams made from a mixture of both spherical and lathe-cut particles combine the ease of condensation from lathe-cut particles with the lower amount of mercury necessary to triturate spherical particles.

Table 2.  Commercially Available Amalgam Products, Particle Type, and Copper Content

Product Name (Manufacturer)

Particle Type

Percent Copper

Dispersalloy (Dentsply)

Lathe-cut

12

Tytin (Kerr-Dental)

Spherical

5-10

Valiant (Ivoclar-Vivadent)

Spherical, palladium enriched

17-20

Megalloy EZ (Dentsply)

Spherical

16.5

Permite (SDI)

Spherical and lathe-cut

15.4

GS-80 (SDI)

Spherical and lathe-cut

28.7

Logic+ (SDI)

Spherical

11.8

GS-80 Spherical (SDI)

Spherical

15

Ultracaps+ (SDI)

Spherical and irregular

20

Ultracaps S (SDI)

Spherical and irregular

35

 

Biocompatibility and toxicity
A number of studies have attempted to link dental amalgam to adverse health effects, but literature reviews by national and international public health agencies and organizations continue to concur that amalgam is a safe and affordable restorative material. A 2015 review by the Scientific Committees of the European Commission on the safety of dental amalgam concluded that, while “reduction in the use of mercury in human activity would be beneficial…no increased risks on adverse systemic effects have been documented in the general population as a whole and it is considered that the current use of dental amalgam does not pose any risk of systemic disease.”21 A 2017 systematic review of the potential effect on autoimmunity from various forms of mercury found “no evidence to implicate a role for Hg0 [elemental mercury] exposure from dental amalgams in the development or perpetuation of autoimmune disease, apart from some suggestion of individual sensitivity.”22

Allergic reactions related to dental amalgam are experienced in less than 1% of the treated population, and usually consist of contact dermatitis, oral lichenoid lesions, gingivitis, and stomatitis; removal of the amalgam normally relieves these symptoms.2, 11, 23  Amalgam is associated with burning mouth syndrome less frequently than other dental materials.23, 24  Allergy to mercury is as common as allergies to any metal, although in vitro tests have demonstrated higher cytotoxicity levels in copper and zinc than mercury.11

Mercury appears in elemental, inorganic, and organic forms; the organic form is the primary health concern, as this form is the type commonly found in fish, as methyl mercury (MeHg).2, 25, 26  Elemental, or metallic, mercury (Hg0) is liquid at room temperature and is the form used in dental amalgam, as well as in thermometers and batteries.  Up to 80% of mercury vapor can be absorbed by the lungs and released into the circulatory system after which it accumulates in tissues.2, 27, 28 The renal system, the central nervous system, and the developing fetus are particularly vulnerable to mercury toxicity.2, 27, 28  Elemental mercury is poorly absorbed by the digestive system, but trace levels of mercury vapor may be released by amalgam restorations, particularly during chewing, although studies consistently show that mercury leakage from amalgam restorations is within safe limits established by the EPA and other public health organizations.29, 30 Further, scientific studies do not support the association between dental amalgams and effects on the renal and central nervous systems despite their vulnerability to mercury toxicity.2, 27, 28

Although the World Health Organization has estimated that 2 µg/kg body weight per day as the tolerable intake of total mercury per day,30 the U.S. EPA uses a more stringent estimate of 0.1 µg/kg/day.29 The EPA estimate translates to approximately 5.8 µg/day for a 130-pound person.  In terms of levels of inhaled mercury vapor, U.S. EPA and the Agency for Toxic Substances and Disease Registry have established 0.2 µg/m3 as the minimal risk level (MRL).31-33

Amalgam restorations release mercury vapor equivalent to approximately 0.2 to 0.4 µg/day for each amalgam-filled tooth surface.34, 35 These values are below the EPA established reference dose of 0.1 µg/kg/day25 (approximately 5.8 µg/day for a 130-pound person).

Overall, the data demonstrate that over time, mercury exposure in the U.S. has been on the decline and that exposure levels in the general population have been below levels of regulatory concern. 

The Canadian regulatory authorities published a health technology assessment (HTA) evaluating safety and effectiveness of dental amalgam in 2012.36 It compared amalgam and composite resins in permanent teeth in children and adults.  It concluded that both materials were clinically safe.  Cost effectiveness analysis found amalgam to be the less costly option.  It was suggested that the decision about which material to use should be made jointly with the patient and noted that development of cost-effective and safe amalgam substitutes remains an important goal of the research community.36

Occupational exposure and mercury hygiene

A 2015 study37 looked at occupational exposure of mercury among a convenience sample of dentists and found their levels to be similar to that of the U.S. general population. A 2016 study38 found increased inorganic and total blood mercury concentrations in the U.S. population with dental fillings (not specifically amalgam) during the 2003-2004 NHANES study. Whereas an increase in inorganic, total, and MeHg blood concentrations was observed during the 2010-2012 NHANES,38 the same study38 found a decrease in mercury blood levels for the period 2011-2012 (up to 0.99 µg/L total Hg) as compared to 2003-2004 (up to an average of 1.17 µg/L total Hg); and a 2017 study on NHANES data found a decrease in total and inorganic blood mercury levels in the U.S. population between 2005 and 2012.39 These reported values are well below the EPA reference dose of MeHg equivalent of 5.8 µg/L.29

As part of its mercury hygiene guidance,15 the World Dental Federation (FDI) recommends the use of precapsulated mercury/alloy as the preferred technique to avoid bulk mercury spills and to eliminate the mercury dispenser as a source of mercury vapor.  To avoid potential occupational exposure to mercury, FDI recommends 1) avoiding direct skin contact with mercury or freshly mixed amalgam and 2) avoiding exposure to potential sources of mercury vapor (e.g., during placement and condensation of amalgam, during polishing or removal of amalgam, or from malfunctioning or leaky equipment).15  FDI recommends use of high-volume evacuation systems fitted with traps or filters when polishing or removing amalgam as well as following best practices for amalgam waste (please see the ADA Oral Health Topic, “Amalgam Separators and Best Waste Management” for more information about amalgam waste). Phillips’ dental materials textbook also recommends the use of suction and water spray when amalgam is being ground.2  Use of a dental dam when placing or removing amalgam may have some utility in reducing mercury exposure, but the data on this are limited and any effect seen may be small in magnitude and of transient duration.16-18

FDI recommends cleaning amalgam contaminants from instruments before heat sterilization and avoiding heating mercury or amalgam or any equipment used with amalgam.15 Additionally, FDI recommends installation of impervious, easy-to-clean surfaces in the dental operatory, including continuous seamless-sheet flooring extending up the walls and working in well-ventilated areas, with fresh air exchanges and outside exhaust.15

Regulatory actions
FDA reclassification of dental amalgam

In 2009, with the support of the ADA, the FDA reclassified dental mercury from Class I (lowest risk) to Class II (special controls), along with amalgam alloy.  The 2009 reclassification included a literature review to determine the risks from mercury in dental amalgam and potential mitigation procedures. The review concluded that “there is insufficient evidence to support an association between exposure to mercury from dental amalgams and adverse health effects in humans, including sensitive subpopulations.”27

The Minamata Convention

By 2017, the Minamata Convention on Mercury was signed by over 128 countries including the United States, in order to address the environmental effect of mercury bioaccumulation.40, 41 The third Conference of the Parties (COP3) took place in November 2019, at which time it was agreed that a review of annex A (which is the section of the convention that covers dental amalgam) would take place as previously agreed to, which is no later than 2022.40 It was agreed that countries would report on the availability, technical and economic feasibility, and environmental and health risks and benefits of the non-mercury alternatives to dental amalgam no later than December 1, 2020.  An information document summarizing the measures and strategies taken by the parties will be prepared for COP4.42

EPA dental effluent guidelines

With ADA support, in June 2017, the U.S. EPA passed the final rule (40 CFR Part 441), requiring amalgam separators in dental operatories. The rule requires that dental offices that release wastewater into public water treatment systems and that place or remove amalgam install an ISO 11143-compliant amalgam separator by July 14, 2020, or if exempt, submit a one-time compliance report.  Visit our Oral Health Topic on Amalgam Separators and Waste Best Management for more information, or the Legal and Regulatory Amalgam page for in-depth insight and compliance information.

ADA policies on amalgam as a dental restorative material
References
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ADA resources

Reviewed by:

Research and Standards Subcommittee, ADA Council for Scientific Affairs

Last Updated: April 21, 2021

Prepared by:

Department of Scientific Information, Evidence Synthesis & Translation Research, ADA Science & Research Institute, LLC.


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