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 prompted legislative and regulatory action in the U.S. and other countries pertaining to amalgam.
  • 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.10 Environmental concerns led to the international Minamata Convention that was signed by the United States in 2013 and was entered into force in 2017.11 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, ”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, mercury, gold, platinum, and palladium.2, 12, 13  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.13

ANSI/ADA Standard No. 1 (ISO 1559) covers the specifications for the composition and physical properties requirements for precapsulated dental amalgam.13 The requirements for physical properties of dental amalgam are shown in Table 1.

Table 1. ADA/ANSI Standard No. 1 Physical Properties of Dental Amalgam13

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, 12 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, 12 Reducing the amount of mercury in the reaction will also decrease formation of gamma-2 phase amalgam.14

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

Dental Considerations

Placement considerations. Current ADA policy strongly recommends that dentists use precapsulated amalgam alloy.15 The use of precapsulated amalgam formulations reduces waste and mercury vapor leakage, provides a more consistent mercury/alloy ratio,2, 13 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

A 2023 systematic review, in a report from the ADA Council on Scientific Affairs, compared the effect of different direct restorative materials for treating cavitated caries lesions on anterior and posterior primary and permanent teeth.10 One conclusion presented in the review is that, for Class I and II restorations combined on permanent teeth, low certainty evidence suggests that both amalgam and macrofilled resin composite were more effective than hybrid resin composite across most patient-important outcomes (e.g., restoration longevity, patient satisfaction, others).10

As part of its mercury hygiene guidance,16 the FDI World Dental Federation 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 recommends16 use of high-volume evacuation systems fitted with traps or filters when polishing or removing amalgam, as well as following best management practices for amalgam waste (please see the ADA Oral Health Topic, “Amalgam Separators and Waste Best Management,” for more information). 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.17-19

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.20 Amalgam alloys with zinc concentrations greater than 0.01% (m/m) can undergo excessive expansion when exposed to moisture during setting.13 Exposure to moisture can be reduced by using rubber dam isolation during the condensation, carving, and finishing steps. The inclusion of palladium in high-copper alloy improves corrosion resistance and compression resistance, lowers the creep, and contributes to the reduction of mercury vapor release during amalgam setting.21

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.

Biocompatibility and toxicity

Various 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 effective restoration material. A 2021 systematic review concluded that dental amalgam restorations were not associated with any increase in systemic diseases in children or adults, when compared with individuals who had received resin-based composites.22 Another recent systematic review found insufficient evidence to confirm that dental amalgam can cause nephrotoxicity when compared with resin composite restorations.23

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.”24 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.”25

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, 12, 26 Amalgam is associated with burning mouth syndrome less frequently than other dental materials.26, 27 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.12

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 methylmercury (MeHg).2, 28 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, 29, 30 The renal system, the central nervous system, and the developing fetus are particularly vulnerable to mercury toxicity.2, 29, 30 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.31, 32 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, 23, 30, 33

Although the World Health Organization has estimated that 2 µg/kg body weight per day as the tolerable intake of total mercury per day,32 the U.S. EPA uses a more stringent estimate of 0.1 µg/kg/day.31 The EPA estimate translates to approximately 5.8 µg/day for a 130-pound person. In terms of levels of inhaled mercury vapor, the Agency for Toxic Substances and Disease Registry has established a chronic-duration (≥365 days) provisional minimal risk level (MRL) of 0.3 μg Hg/m3 for elemental mercury.34

Amalgam restorations release mercury vapor equivalent to approximately 0.2 to 0.4 µg/day for each amalgam-filled tooth surface,35, 36 which is below the EPA established reference dose of 0.1 µg/kg/day (about 5.8 µg/day for a 130-pound person).31

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 2018.37 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.37

Occupational Exposure and Mercury Hygiene

A 2015 study38 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 study39 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, the same study39 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 2012.40 These reported values are well below the EPA reference dose of MeHg equivalent of 5.8 µg/L.31

In its mercury hygiene guidance,16 the 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).16 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 page, “Amalgam Separators and Waste Best 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.17-19

FDI recommends cleaning amalgam contaminants from instruments before heat sterilization and avoiding heating mercury or amalgam or any equipment used with amalgam.16 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.16

Regulatory actions

FDA Reclassification of Dental Amalgam

In 2009, 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.”29

In 2020, the FDA recommended that amalgam restorations be avoided in the following categories of people:41

  • Pregnant women and their developing fetuses;
  • Women who are planning to become pregnant;
  • Nursing women and their newborns and infants;
  • Children, especially those younger than six years of age;
  • People with pre-existing neurological disease such as multiple sclerosis, Alzheimer’s disease or Parkinson’s disease
  • People with impaired kidney function; and
  • People with known heightened sensitivity (allergy) to mercury or other components of dental amalgam.

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.42, 43 The third Conference of the Parties (COP3) to the Minamata Convention took place in November 2019, and 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, by no later than 2022.42

In preparation for the fourth Conference of the Parties (COP4) meeting, the ADA and the International Association for Dental Research (IADR) developed a statement supporting the Minamata Convention on Mercury’s treaty to phase down the use of amalgam.44 An appendix within the ADA/IADR statement presents the following as a take-home message:

“In the short-term, alternatives to dental amalgam are still less than optimal based on clinical, economic or practical reasons. Therefore, it is important to ensure the access of dental amalgam as a restorative option until investments in research can deliver an alternative restorative material that addresses current shortcomings.”44

A summary report of the COP4 meeting is also available online.45

EPA Dental Effluent Guidelines

With ADA support, in June 2017, the U.S. EPA passed the final rule (40 CFR Part 441) requiring the use of 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.  A flowchart of amalgam separator compliance is available here (login required).  Visit our Oral Health Topic on ”Amalgam Separators and Waste Best Management” for more information, or the ADA Center for Professional Success for in-depth insight and compliance information.

ADA policies on amalgam as a dental restorative material
References
  1. Kingston G. The rise and fall of mercury amalgam. Prim Dent J 2013;2(3):74-5.
  2. Anusavice KJ, Shen C, Rawls HR. Phillips' Science of Dental Materials. St. Louis, MO: Elsevier/Saunders; 2013.
  3. Hurst D. Amalgam or composite fillings--which material lasts longer? Evid Based Dent 2014;15(2):50-1.
  4. Rasines Alcaraz MG, Veitz-Keenan A, Sahrmann P, et al. Direct composite resin fillings versus amalgam fillings for permanent or adult posterior teeth. Cochrane Database Syst Rev 2014(3):CD005620.
  5. Moraschini V, Fai CK, Alto RM, Dos Santos GO. Amalgam and resin composite longevity of posterior restorations: A systematic review and meta-analysis. J Dent 2015;43(9):1043-50.
  6. Alhareky M, Tavares M. Amalgam vs composite restoration, survival, and secondary caries. J Evid Based Dent Pract 2016;16(2):107-9.
  7. American Dental Association Council on Scientific Affairs. Direct and indirect restorative materials. J Am Dent Assoc 2003;134(4):463-72.
  8. Bharti R, Wadhwani KK, Tikku AP, Chandra A. Dental amalgam: an update. J Conserv Dent 2010;13(4):204-8.
  9. Bayne SC. Beginnings of the dental composite revolution. J Am Dent Assoc 2013;144 Spec No:42S-46S.
  10. Pilcher L, Pahlke S, Urquhart O, et al. Direct materials for restoring caries lesions: Systematic review and meta-analysis—a report of the American Dental Association Council on Scientific Affairs. J Am Dent Assoc; 2023 Jan 5.
  11. Araujo MWB, Lipman RD, Platt JA. Amalgam: Impact on oral health and the environment must be supported by science. J Am Dent Assoc 2019;150(10):813-15.
  12. Sakaguchi RL, Ferracane J, Powers JM. Craig's Restorative Dental Materials. St Louis, MO: Elsevier; 2018.
  13. American National Standards Institute/American Dental Association (ANSI/ADA). ANSI/ADA Standard No. 1 --- Alloy for Dental Amalgam; 2003 (last reviewed and reaffirmed 2013).
  14. Mitchell RJ, Okabe T. Setting reactions in dental amalgam. Part 1. Phases and microstructures between one hour and one week. Crit Rev Oral Biol Med 1996;7(1):12-22.
  15. American Dental Association. ADA Current Policies, 1954-2020. 2022.  https://www.ada.org/about/governance/current-policies  Accessed June 12, 2023.
  16. FDI World Dental Federation. Mercury Hygiene Guidance (adopted 1998, revised 2007). https://preprod.fdiworlddental.org/mercury-hygiene-guidance Accessed June 12, 2023.
  17. Kremers L, Halbach S, Willruth H, et al. Effect of rubber dam on mercury exposure during amalgam removal. Eur J Oral Sci 1999;107(3):202-7.
  18. Berglund A, Molin M. Mercury levels in plasma and urine after removal of all amalgam restorations: the effect of using rubber dams. Dent Mater 1997;13(5):297-304.
  19. Imbery TA, Carrico CK. Dental dam utilization by dentists in an intramural faculty practice. Clin Exp Dent Res 2019;5(4):365-76.
  20. Okabe T, Mitchell RJ. Setting reactions in dental amalgam. Part 2. The kinetics of amalgamation. Crit Rev Oral Biol Med 1996;7(1):23-35.
  21. Chern Lin JH, Chen FY, Chiang HJ, Ju CP. Effect of ball milling on structures and properties of dispersed-type dental amalgam. Dent Mater 2011;27(4):e65-79.
  22. Gallusi G, Libonati A, Piro M, et al. Is dental amalgam a higher risk factor rather than resin-based restorations for systemic conditions? A systematic review. Materials 2021;14(8):1980.
  23. Patini R, Spagnuolo G, Guglielmi F, et al. Clinical effects of mercury in conservative dentistry: a systematic review, meta-analysis, and trial sequential analysis of randomized controlled trials. Int J Dent 2020;2020:8857238.
  24. European Commission Scientific Committee on Emerging Newly Identified Health Risks. The safety of dental amalgam and alternative dental restoration materials for patients and users (adopted 4/29/15).  2015. http://ec.europa.eu/health/scientific_committees/emerging/docs/scenihr_o_046.pdf. Accessed June 12, 2023.
  25. Crowe W, Allsopp PJ, Watson GE, et al. Mercury as an environmental stimulus in the development of autoimmunity - a systematic review. Autoimmun Rev 2017;16(1):72-80.
  26. Syed M, Chopra R, Sachdev V. Allergic reactions to dental materials-a systematic review. J Clin Diagn Res 2015;9(10):ZE04-9.
  27. Marino R, Capaccio P, Pignataro L, Spadari F. Burning mouth syndrome: the role of contact hypersensitivity. Oral Dis 2009;15(4):255-8.
  28. Centers for Disease Control and Prevention. Mercury Factsheet.  2017. https://www.cdc.gov/biomonitoring/Mercury_FactSheet.html Accessed June 12, 2023.
  29. United States Food and Drug Administration. White Paper: FDA Update/Review of Potential Adverse Health Risks Associated with Exposure to Mercury in Dental Amalgam.  2009. https://www.fda.gov/medical-devices/dental-amalgam-fillings/white-paper-fda-updatereview-potential-adverse-health-risks-associated-exposure-mercury-dental  Accessed June 12, 2023.
  30. Carocci A, Rovito N, Sinicropi MS, Genchi G. Mercury toxicity and neurodegenerative effects. Rev Environ Contam Toxicol 2014;229:1-18.
  31. United States Environmental Protection Agency. Integrated Risk Information System (IRIS): Methylmercury.  2001. https://cfpub.epa.gov/ncea/iris/iris_documents/documents/subst/0073_summary.pdf  Accessed June 12, 2023.
  32. World Health Organization. Exposure to Mercury: A Major Public Health Concern. Geneva; 2007.
  33. United States Food and Drug Administration. D. Dental devices: classification of dental amalgam, reclassification of dental mercury, designation of special controls for dental amalgam, mercury, and amalgam alloy. Final rule. Fed Regist 2009;74(148):38685-714.
  34. Agency for Toxic Substances and Disease Registry (ATSDR). 2022. Toxicological Profile for Mercury (Draft for Public Comment). Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service. https://wwwn.cdc.gov/TSP/ToxProfiles/ToxProfiles.aspx?id=115&tid=24 Accessed June 12, 2023.
  35. Richardson GM, Wilson R, Allard D, et al. Mercury exposure and risks from dental amalgam in the US population, post-2000. Sci Total Environ 2011;409(20):4257-68.
  36. Rodriguez-Farre E, Testai E, Bruzell E, et al. The safety of dental amalgam and alternative dental restoration materials for patients and users. Regul Toxicol Pharmacol 2016;79:108-09.
  37. Khangura SD, Seal K, Esfandiari S, et al. CADTH Health Technology Assessments. Composite Resin Versus Amalgam for Dental Restorations: A Health Technology Assessment (CADTH Health Technology Assessment). Ottawa (ON); CADTH: 2018:  Available from: https://cadth.ca/sites/default/files/pdf/feedback/HT0021-Dental-Amalgam.pdf  Accessed June 12, 2023.
  38. Anglen J, Gruninger SE, Chou HN, et al. Occupational mercury exposure in association with prevalence of multiple sclerosis and tremor among US dentists. J Am Dent Assoc 2015;146(9):659-68 e1.
  39. Yin L, Yu K, Lin S, Song X, Yu X. Associations of blood mercury, inorganic mercury, methyl mercury and bisphenol A with dental surface restorations in the U.S. population, NHANES 2003-2004 and 2010-2012. Ecotoxicol Environ Saf 2016;134P1:213-25.
  40. Jain RB. Trends in and factors affecting the observed levels of urinary inorganic and total blood mercury among US children, adolescents, adults, and senior citizens over 2005-2012. Environ Toxicol Pharmacol 2017;56:268-81.
  41. United States Food and Drug Administration. Information for patients about dental amalgam fillings. https://www.fda.gov/medical-devices/dental-amalgam-fillings/information-patients-about-dental-amalgam-fillings  Accessed June 15, 2023.
  42. United Nations Treaty Collection. Minimata Convention on Mercury.  2013. "https://treaties.un.org/doc/Treaties/2013/10/20131010%2011-16%20AM/CTC-XXVII-17.pdf". Accessed June 12, 2023.
  43. Fisher J, Varenne B, Narvaez D, Vickers C. The Minamata Convention and the phase down of dental amalgam. Bulletin of the World Health Organization 2018;96:436-38.
  44. American Dental Association; International Association for Dental Research. Conference of Parties 4 statement (dental amalgam).  April 2020. https://mercuryconvention.org/sites/default/files/documents/submission_from_organization/ADA_and_IADR_DentalAmalgam.pdf  Accessed June 13, 2023.
  45. United Nations Environment Programme. UNEP/MC/COP.4/28/Add.1 - Report of the Conference of the Parties to the Minamata Convention on Mercury on the work of its fourth meeting - In-person segment.  2022. https://mercuryconvention.org/en/documents/report-conference-parties-minamata-convention-mercury-work-its-fourth-meeting-person  Accessed June 12, 2023.
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Last Updated: June 21, 2023

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