Dental Amalgam: Material Properties and its Use in Clinical Dentistry

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.

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

Alloy Powder Composition Type Ag Sn Cu Zn Other Low copper 63-72 26-28 2-7 0-2 — High-Cu admixed lathe-cut 40-70 26-30 12-30 High-Cu admixed spherical 40-65 0-30 20-40 0-1 Pd High-Cu unicomp- ositional spherical 40-60 22-30 13-30 0-5 In, 0-1 Pd compositions in weight percent

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

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

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

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

Silver-Gold Phase Diagram The silver-gold binary system is an example of a continuous solid solution. Silver and gold are completely miscible in each other in the solid state. Nearly all pairs of metals are miscible in the solid state, but only a few are continuously miscible across the entire range of composition in the solid state.

Silver-Tin Phase Diagram The silver-tin binary system contains an area of solid solution in which tin is soluble in silver up to around 9 atomic percent tin. Above that level of tin, additional phases precipitate out as intermetallic compounds. Of particular dental interest in this diagram is the composition of 25 atomic percent tin, 75 atomic percent silver, which is the gamma phase, Ag3Sn. This phase constitutes the main phase in the unreacted amalgam particles.

Silver-Mercury Phase Diagram The silver-mercury system also exhibits a solid solubility region, in which mercury forms a solid solution with silver up to about 35 atomic percent mercury. Above that composition, various intermetallic compounds form. The principal phase that is of interest in dentistry is the gamma phase, Ag2Hg3, which is usually referred to as “gamma-1” to distinguish it from the gamma phase in the silver-tin system.

Reaction: Low-Copper Alloys Ag3Sn() + Hg  Ag2Hg3(1) + Sn7-8Hg(2) + Ag3Sn() Low-copper alloys were used from the late 1800’s till the 1970’s, but are rarely used today. The reaction of low-copper alloys with mercury produces a corrosion-prone phase, Sn7-8Hg, which is the gamma phase of the mercury-tin system. To distinguish it from the gamma phases of the silver-tin and silver-mercury systems, the Sn7-8Hg gamma phase is usually referred to as the gamma-2 phase.

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) High copper alloys were first developed in the early 1960’s by D.B.K Innes and W.V. Youdelis (J Canad Dent Assoc 1963;29:587-593). The principal advantage of high-copper alloys is their elimination of the gamma-2 phase by the reactions shown in this slide.

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.) Some amalgam opponents have referred to dental amalgam as a “solid emulsion,” with the implication that the mercury is somehow soaked into the particles of silver-tin alloy powder without reaction or the formation of mercury intermetallic compounds. The quote from Dr. Laurier L. Schramm, University of Calgary, is a personal communication to Dr. J. R. Mackert on February 19, 2003.

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 Benjamin DJ. The effect of gases and vapours on mercury evaporation. Mater Res Bull 1984;19:443-450.

Université Laval 3.7-m LMT The 3.7-m liquid mirror telescope (LMT) at Université Laval. The reflecting surface is liquid mercury. Visit http://wood.phy.ulaval.ca/ for more information about liquid mirror telescopes.

This graph shows the measured mercury vapor levels in the air around a liquid mirror telescope at the Université Laval. After the mercury layer is spun out to form a paraboloid reflecting surface, the formation of a mercury oxide film on the surface of the mercury layer dramatically reduces the mercury vaporization. Graph courtesy of Ermanno F. Borra, Département de physique, Université Laval, Québec, Canada. http://wood.phy.ulaval.ca/ Courtesy of E.F. Borra, Université Laval

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 Baseline vaporization rate is based upon a per-surface weighted average of 0.278 ng·min–1 for six studies cited in Mackert and Berglund, Crit Rev Oral Biol Med 1997;8:410-436. Data on 24-h vaporization rate and average area of amalgam surface (0.171 cm²) is from Berglund A. J Dent Res 1990;69:1646-1651.

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 Daily absorbed dose estimates from Mackert and Berglund, Crit Rev Oral Biol Med 1997;8:410-436. EPA data from Clarkson TW, Cranmer J, Sivulka DJ, Smith R. Mercury Health Effects Update. Health Issue Assessment. Research Triangle Park NC: US Environmental Protection Agency, 1984, pp. 1-1-7-22.

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 The Jerome mercury vapor analyzer, and any similar such instrument, is designed to measure mercury vapor concentrations in a volume of air that is large (such as a room) compared to the sample volume (250 mL). When this condition is not met, corrections must be applied to the meter reading to determine the amount of mercury that would actually be inhaled by the individual.

When a Jerome Model 401 mercury vapor analyzer is used to measure mercury vapor in room air, it draws a programmed volume of 250 mL of air in through its sampling tube in 20 seconds. In this schematic, the Jerome instrument is used to sample the mercury vapor concentration in a room with a mercury vapor level of 32 µg/m³.

As the instrument draws the 250 mL sample into the chamber, the sample is passed across a gold film that adsorbs and detects the amount of mercury. A 250 mL air sample from a room with a mercury vapor concentration of 32 µg/m3 would contain 8 ng of mercury. This amount (8 ng) is then divided by the programmed sample volume (250 mL), and the resultant concentration is displayed on the instrument meter (32 µg/m3).

If a human being were to enter this room, the initial concentration of mercury in the lungs is 0 µg/m³. A single breath is about 500 mL and is inhaled in about 2.5 seconds (12 breaths per minute—5 s per breath—2.5 s inhaling, 2.5 s exhaling).

When the person inhales, the mercury vapor concentration of the air inhaled is always 32 µg/m³, regardless of the amount of air inhaled or the rate at which the air is inhaled.

In contrast, consider the person whose amalgam restorations emit mercury vapor at a rate of 0.4 ng/s (or 1 ng every 2.5 sec). Because the Jerome instrument requires 20 seconds to “inhale” a single 250 mL sample of air, there will be 8 ng of mercury in that sample. In contrast, there will be only 1 ng in a single breath inhaled by the person in 2.5 sec. Not only is the duration of sampling 8 times longer for the Jerome instrument than for the inhalation time of the human, but the volumes are also unequal (250 mL for the Jerome instrument and 500 mL for a single breath in the human).

When the amount of mercury inhaled or sampled is divided by the volume of the sample, the reading of the Jerome instrument is seen to be 16 times higher than concentration of the air actually inhaled. It is significant that measurements of actual tracheal Hg concentrations during inhalation through the mouth and through the nose ranged from less than 1 µg/m³ to 6 µg/m³ for subjects with from 8 to 54 amalgam surfaces during inhalation through the mouth and less than 1 µg/m³ during inhalation through the nose (Langworth et al., Swed Dent J 12:71-72).