Dental Amalgam: Material Properties and its Use in Clinical Dentistry

1. Dental Amalgam:Material Properties and itsUse in Clinical Dentistry J. Rodway Mackert, Jr., DMD, PhDMedical College of GeorgiaAugusta, Georgia

2. What is Amalgam? a·mal·gam: any alloy of mercury with another metal or other metals [silver amalgam is used as a dental filling]† †Source: Webster’s New World Dictionary of the American Language, Guralnik DB, Ed., New York: World Publishing Co., 1972.

3. Types of Dental Amalgam • Copper amalgam (no longer used) • Conventional (low-copper) amalgam: formulation standardized in 1890’s • High copper amalgam: first developed in the 1960’s

4. Alloy Powder Composition

5. Alloy Powder: Dispersalloy® Mixing proportions: 50% alloy, 50% mercury

6. Alloy Powder: Tytin® Mixing proportions: 57.5% alloy, 42.5% mercury

7. Amalgam Capsules • Contain (in separate compartments): • powdered amalgam alloy • liquid mercury • Some are manually activated, others self-activated • Pestle usually included

8. Amalgamator (Triturator) • Speeds vary upward from 3000 rpm • Times vary from 5–20 seconds • Mix powder and liquid components to achieve a pliable mass • Reaction begins after components are mixed

9. Silver-Gold Phase Diagram

10. Silver-Tin Phase Diagram

11. Silver-Mercury Phase Diagram

12. Reaction: Low-Copper Alloys Ag3Sn() + Hg Ag2Hg3(1) + Sn7-8Hg(2) + Ag3Sn()

13. Reaction: High-Copper Alloys Ag3Sn() + Ag-Cu + Hg Ag2Hg3(1) + Sn7-8Hg(2) + Ag3Sn() + Ag-Cu and Sn7-8Hg(2) + Ag-Cu Cu6Sn5() + Ag2Hg3(1)

14. Composition of Set Amalgam • Matrix of 1 (Ag2Hg3) and  (Cu6Sn5) phases, with embedded particles of unreacted  (Ag3Sn) and Ag-Cu phases • Not a “solid emulsion” (“...it is not correct to refer to an aggregate of intermetallic compounds, all of which are solid at room temperature, as a solid emulsion.” —Laurier Schramm, author of Dictionary of Colloid and Interface Science.)

15. Properties of Mercury • Only liquid metal at room temperature • Evaporation rate • Theoretical maximum is 57.9 µg·cm2·s–1 from pristine, oxide-free surface into a vacuum (= 57,900 ng·cm2·s–1) • Measured rate in vacuum is ~40 µg·cm2·s–1 (= ~40,000 ng·cm2·s–1) • Oxidation of Hg lowers rate by factor of 1000

16. Université Laval 3.7-m LMT

17. Courtesy of E.F. Borra, Université Laval

18. Hg Evaporation from Amalgam • Baseline Hg vaporization rate (unstimulated) from amalgam in humans is 0.027 ng·cm2·s–1 • Average vaporization rate (over 24-h period) in humans is 0.048 ng·cm2·s–1 • The evaporation rate of Hg from amalgam is 800–1500 times lower than from oxidized mercury

19. Mercury Dose from Amalgam • Person with average number of fillings (7) would absorb ~1.6 µg/day • Person with a moderately high number of fillings (13) would absorb ~3 µg/day • According to EPA, absorbed dose of mercury from food, water, and air is 5.7 µg/day

20. Jerome Mercury Vapor Analyzer • Used by many anti-amalgam dentists to measure intra-oral mercury vapor • Differences in sampling volumes and flow rates must be taken into account, or gross errors will result