Hg emission from dental amalgam as related to the amount of Sn in the Ag-Hg (gamma 1) phase

Increased mercury emissions from modern dental amalgams

All types of dental amalgams contain mercury, which partly is emitted as mercury vapor. All types of dental amalgams corrode after being placed in the oral cavity. Modern high copper amalgams exhibit two new traits of increased instability. Firstly, when subjected to wear/polishing, droplets rich in mercury are formed on the surface, showing that mercury is not being strongly bonded to the base or alloy metals. Secondly, high copper amalgams emit substantially larger amounts of mercury vapor than the low copper amalgams used before the 1970s. High copper amalgams has been developed with focus on mechanical strength and corrosion resistance, but has been sub-optimized in other aspects, resulting in increased instability and higher emission of mercury vapor. This has not been presented to policy makers and scientists. Both low and high copper amalgams undergo a transformation process for several years after placement, resulting in a substantial reduction in mercury content, but there exist no limit for maximum allowed emission of mercury from dental amalgams. These modern high copper amalgams are nowadays totally dominating the European, US and other markets, resulting in significant emissions of mercury, not considered when judging their suitability for dental restoration.

Effect of ball milling on structures and properties of dispersed-type dental amalgam

OBJECTIVES

The purpose of the present study was to investigate the effect of ball milling on the initial mercury vapor release rate and mechanical properties such as compressive strength, diametral tensile strength and creep value, of the dispersed-type dental amalgam, and comparison was made with respect to two commercial amalgam alloys.

METHODS

Ball milling was employed to modify the configuration of the originally spherical-shaped Ag-Cu-Pd dispersant alloy particles. Improvement in mechanical properties while maintaining a low early-stage mercury vapor release rate of the amalgam is attempted.

RESULTS

The experimental results show that the amalgam (AmB10) which was made from Ag-Cu-Pd dispersant alloy particles that were ball-milled for 10 min and heat-treated at 300 °C for 2 days exhibited a low initial mercury vapor release rate of 69 pg/mm(2)/s, which was comparable with that of commercial amalgam alloy Tytin (68 pg/mm(2)/s), and was lower than that of Dispersalloy (73 pg/mm(2)/s). As for mechanical properties, amalgam AmB10 exhibited the highest 1h compressive strength (228 MPa), which was higher than that of commercial amalgam alloy Dispersalloy by 72%; while its 24h diametral tensile strength was also the highest (177 MPa), and was higher than that of Dispersalloy by 55%. Furthermore, the creep value of the amalgams made from Ag-Cu-Pd alloy particles with 10 min ball-milling and heat treatment at 300 °C for 2 days was measured to be 0.12%, which was about 20% that of Dispersalloy.

SIGNIFICANCE

It is found that ball milling of the dispersant Ag-Cu-Pd alloy particles for 10 min was able to modify the configuration of the alloy particles into irregular-shapes. Subsequently, heat treatment at 300 °C significantly lowered the initial mercury vapor release rate, increased its 1h compressive strength and 1h diametral tensile strength, and lowered its creep value.

Phase transitions of silver and silver?palladium alloys immersed in mercury

A series of Ag-Hg and Ag-Pd-Hg phases have been prepared by immersion in mercury and subsequent heat treatment of pure Ag and a series of Ag-Pd alloys. Phase transitions, along with the changes involved in morphology and chemistry, are investigated. Experimental results indicate that when mercury-immersed Ag discs are heat-treated at 90 degrees C for 12 h, large (>50 microm) crystals with Ag/Hg atomic ratios close to those of stoichiometric AgHg(2) form on the Ag surface. Heat treatment for 36 h causes these crystals to disintegrate into smaller (majority <10 microm) Ag(2)Hg(3) (gamma(1)) crystals. After 60 h, the stoichiometric gamma(1) crystals further breakdown into even smaller stoichiometric AgHg(beta(1)) crystals. The oft-referenced beta(1) phase Ag(1.1)Hg(0.9) is not supported in the present study. Addition of Pd to Ag has a dramatic effect on the morphology of the gamma(1) phase. With more addition of Pd, large-faceted and elongated gamma(1) crystals tend to grow in the midst of smaller matrix gamma(1) crystals. All three different Pd-containing crystals have (Ag + Pd)/Hg ratios very close to those of the stoichiometric gamma(1) phase, indicating that added Pd tends to replace Ag, rather than Hg, in the gamma(1) phase.

Initial mercury evaporation from experimental Ag–Sn–Cu amalgams containing Pd

This study examined the Hg evaporation during setting from experimental Ag-Sn-Cu alloy powders with and without Pd. Four series of alloy powders were fabricated to examine the effect on the Hg evaporation of the alloy compositions (all percentages in this report are weight percents): Pd (0-1.5), Cu (9.0-14.0), Ag (57.0-63.7), and Sn (24.9-29.5). These variations in composition produced alloy powders with gamma-Ag3Sn to beta-AgSn ratios varying from 0.0 to 23.9. The total amounts of Hg released from 10 min after trituration were measured from cylindrical specimens (4 x 8mm; n = 4 ) at 37 degrees C using a Hg vapor analyzer. The results were compared to those from commercial alloys (one high-Cu and one low-Cu alloy). All amalgams made from alloys containing 1.5% Pd exhibited lower Hg vapor release than any other amalgams, with the exception of the low-Cu amalgam. The results clearly showed that the alloy formulation affected the mercury evaporation behavior during setting of the resultant amalgams. A small addition of Pd to the alloy can produce amalgams with 50-60% less Hg vapor release during setting than a leading commercial high-Cu amalgam, Tytin.

Evaluation of the long-term corrosion behavior of dental amalgams: influence of palladium addition and particle morphology

OBJECTIVES

The purpose of this investigation was to evaluate the long-term corrosion behavior of experimental amalgams as a function of particle morphology and palladium content.

METHODS

Samples of four experimental high copper amalgams were prepared according to ADA specifications. Two of them had the same chemical composition but one had lathe cut particles (LCP) and the other had spherical particles (SP). The two others had spherical powders with an addition of 0.5 wt% of palladium (SP 0.5) and 1 wt% of palladium(SP 1) for the other. Corrosion resistance was evaluated by electrochemical techniques in Ringer's solution in a thermostated cell at 37 degrees C for samples aged 5, 8, 12, 16 months and 10 years. Potentiokinetic curves were drawn and the potential and the current density corresponding to the first anodic peak were registered.

RESULTS

For all the amalgam samples the corrosion behavior improves over the 10-year period. SP samples exhibit a better behavior than LCP. Palladium addition improves corrosion behavior as compared to samples without palladium. No real difference is found regarding the amount of palladium between 0.5 and 1%. The potentials progress from a range between 0 and 20 mV/SCE to a range of 60-80 after 10 years. The stabilization of the potential begins after only 16 months. Except for the LCP, all the values converge to the same level of 80 mV/SCE.

SIGNIFICANCE

The addition of no more than 0.5 wt% Palladium in a high copper amalgam powder improves the corrosion behavior of the amalgam up to a period of 10 years. The potential of the first anodic peak increases for each amalgam, probably in relation to the evolution of the structure of the material. Clinically, it is of interest to consider the good electrochemical behavior of older restorations when contemplating the repair or replacement of such fillings. At the same time, galvanic current can occur when a new amalgam restoration is placed in contact with an old one even if the same amalgam is used. In this situation, the new filling will be anodic and its degradation will be accelerated. The evaluation of the corrosion behavior of dental amalgams has to take into account the age of the samples.

Effects of palladium addition on emission of mercury vapor from dental amalgam

OBJECTIVES

The purpose of this investigation was to test the hypothesis that palladium causes a reduction in mercury emission when added to dental amalgam during condensation.

METHODS

Mercury vapor release was measured in a closed bottle system and an Intraoral Flow device(IOF). Conventional amalgam restorations were modified by addition of various palladium pellets. 1.57 mm diameter palladium pellets with different porosities were fabricated. These pellets were then placed in amalgam restoration using typical condensation and carving procedures. The samples were stored in a closed bottle and mercury measurements were taken from the bottles at 30 min, 1, 3, 5, 24 and 48 h and 7 days after trituration using a Jerome 411 Mercury Vapor Analyzer (Arizona Instrument Corp., Jerome, AZ). The palladium pellets identified as the most effective in mercury vapor reduction were further tested in an IOF device. Data were analyzed by two-way ANOVA followed by Tukey HSD pairwise analysis for significant findings (alpha = 0.05).

RESULTS

The palladium containing amalgams when tested in the closed bottle system yielded significantly lower (p < 0.05) mercury vapor release than the controls. Pellets fabricated with the highest porosity yielded the greatest reduction in overall mercury vapor release. In the IOF device the overall amount of mercury vapor released from the palladium containing amalgams was also significantly less than the control (p < 0.05).

SIGNIFICANCE

Mercury vapor emission from dental amalgam was greatly reduced by adding palladium pellets to amalgam during condensation. These techniques require only slight modifications of the standard operative procedures.

Vaporization of Hg from Hg-in amalgams during setting and after abrasion

OBJECTIVE

The aim of this study was to determine if Hg vaporization during setting and after abrasion of amalgams could be reduced by adding indium to Hg prior to trituration.

METHODS AND MATERIALS

Hg-In alloys (0,5,10,15 wt% In) were triturated with commercial amalgam alloys (Tytin, Kerr; Artalloy, Degussa; Sybraloy, Kerr) and condensed into cylinders (4 x 8 mm). In one experiment, Hg release during setting was measured in air (37 degrees C) with a Jerome 431 Hg analyzer (n = 4). In a second experiment, amalgams aged two months were uniformly abraded on wet #600 SiC, blotted dry, and Hg release was measured in air (22 degrees C) for 30 min with a Jerome 411 Hg analyzer (n = 6). Total Hg was determined by integration (ng/mm2). Results were compared by ANOVA and Tukey's test (alpha = 0.05).

RESULTS

Indium reduced Hg release from amalgams during setting. Amalgams ceased Hg release within 5 h. Indium did not reduce Hg release from abraded, set amalgams except Artalloy w/15% In.

SIGNIFICANCE

Coupled with our previous studies, this work shows that 5-15 wt% indium can be added to effectively reduce Hg release during setting, but not after abrasion of set amalgams.

The effect of tin in the Ag-Hg phase of dental amalgam on dissolution of mercury

OBJECTIVES

The aim of this study was to determine the relationship between the tin content in the gamma 1 phase of dental amalgam and the kinetics of mercury dissolution.

METHODS

The tested materials were the same eleven commercial amalgams used by Mahler et al. (1994) to study mercury evaporation, which contained from 0.13% to 2.49% Sn in the gamma 1. In one set of tests, specimens were stabilized in air before exposure to synthetic saliva for three 24 h sampling periods. In another set of tests, they were exposed for 2 h immediately after wet-grinding. Solutions were analyzed for mercury by atomic absorption spectrophotometry. The results were analyzed by ANOVA and Turkey tests (p < 0.05).

RESULTS

For air-stabilized specimens, the 24 h Hg loss was independent of the tin content in gamma 1 for 0.92 to 2.49% Sn. The loss was higher for amalgams containing 0.13% and 0.32% Sn. For abraded specimens, the 2 h Hg loss decreased with increasing tin content in gamma 1. It is proposed that tin oxide growth is initially controlled by diffusion of tin to the gamma 1/tin oxide interface, the rate of which increases with increasing Sn content in gamma 1. Later the growth becomes controlled by the electric field across the oxide. Mercury release rate, controlled by diffusion through the oxide, is inversely proportional to the oxide thickness and initially lower for high Sn content in gamma 1. For a steady state oxide thickness, tin diffusion and tin content in gamma 1 no longer affect mercury dissolution. When tin oxide film dissolves, a minimum tin content is required to maintain a barrier against mercury dissolution.

SIGNIFICANCE

The results show the importance of rapid formation of a surface oxide to minimize mercury dissolution. The theoretical analysis provides a quantitative model for explanation of the effects of tin content and time.

Effect of addition of palladium on properties of Ag2Hg3 (gamma 1) phase

The effect of palladium addition on the microstructure, compressive strength, creep rate and mercury release rate of Ag2Hg3 (gamma 1) phase was evaluated. Experimental results indicated that fairly pure gamma 1 phase could be fabricated using the present trituration method. The heat treatment of gamma 1 at 90 degrees C increased porosity level, increased dimensional shrinkage, increased mercury vapour release and enhanced the formation of beta1 phase. Addition of palladium in gamma 1 slowed down the amalgamation reaction, largely suppressed the phase transition to beta1 and caused a slight shift in open circuit potential toward the anodic direction. Although the overall anodic polarization profiles did not show a significant effect of palladium, scanning electron microscopy revealed morphological differences between pure and palladium-containing gamma 1. Addition of palladium in gamma 1 also increased compressive strength, increased creep resistance, and largely reduced both mercury vapour and ion release rates. Considering overall performance, the optimal palladium content in gamma 1 seems to be in the range between 0.50 and 0.75 wt%.

Dissolution of mercury from dental amalgam at different pH values

Dissolution of mercury from dental amalgam has been shown to be diminished by the formation of a tin oxide film on the surface of the mercury-rich gamma 1 phase (Marek, 1990b). Since tin oxides dissolve at low pH values (Deltombe et al., 1974), acidic conditions in the oral cavity may cause an increase in the mercury release. The purpose of this study was to determine the effect of acidity in the range of pH 1 to pH 8 on the rate of mercury dissolution in synthetic saliva from tin-free and tin-containing gamma 1 phase and two commercial dental amalgams. The tested hypothesis was that pH affects mercury dissolution only when a protective oxide film dissolves in an acidic environment. After exposures of the specimens for 2 hr or 24 hr in sealed glass bottles, the solutions were analyzed by flameless atomic absorption spectrophotometry for mercury and silver. The results have shown pH-independent mercury dissolution in the range of pH 3 to 8, and a much faster dissolution at pH 1. At all pH values, more mercury dissolved from the tin-free phase than from the tin-containing phase, and the rate of dissolution was lowest for the dental amalgams. The results were affected by the length of the test exposure. The pH independence in a wide range of pH values has been attributed to the atomic mechanism of mercury dissolution. The low rate of mercury dissolution from specimens containing tin has been explained by the formation of a barrier tin oxide film, which dissolved only at the lowest pH. Dissolution of silver at low pH values is believed to have accelerated dissolution of mercury from the tin-free gamma 1 phase. Variation of the dissolution rate with concentration of the dissolved species and kinetics of oxide film dissolution caused the effect of the exposure period.

Surface characterization of amalgam made with Hg-In liquid alloy

When amalgam was triturated with Hg-In liquid alloys instead of pure mercury, the resultant amalgams released a significantly smaller amount of mercury vapor during setting. To understand the mechanisms responsible for the drastic decrease in mercury evaporation from the In-containing amalgam, we used Auger Electron Spectroscopy to examine surface oxide films on amalgams made with Hg-10 wt% In or pure mercury. The surface of the In-containing amalgam was rapidly covered with both indium and tin oxide films. Greater amounts of oxygen were found on the gamma 1 Ag-Hg matrix in the In-containing amalgam than in the amalgam without indium. The rapid formation of the oxide film contributes to a reduction in the mercury release from the In-containing amalgam by forming an effective barrier to evaporation.

Mercury vaporization from amalgams with varied alloy compositions

The fact that mercury is released from dental amalgam restorations after abrasion provides a source of continued controversy over the safe use of this material. Studies have shown that the amount and rate of mercury release vary for different amalgam products. The objective of this study was to determine how alloy composition affects mercury vaporization from experimental amalgams with similar alloy particle size and shape and percent residual mercury. An hypothesis to be tested was that mercury release is dependent upon the concentration of tin in the silver-mercury matrix phase of the amalgam. Seven spherical amalgam alloys (two low-copper and five high-copper) were made by a dental manufacturer (Tokuriki Honten, Japan). Trituration conditions were adjusted so that all set amalgams had the same residual Hg (47.3%). ADA-type amalgam cylinders were aged for 14 days at 37 degrees C, then lightly wet-abraded on #600 silicon carbide, dried, and placed into a tube through which air was blown at a rate of 750 mL/min. Mercury vaporization was monitored with a gold film analyzer (Jerome 411) for 30 min. Total Hg release was determined by integration. We analyzed polished specimens via electron microprobe to determine composition, volume fraction of silver-mercury matrix (gamma 1), and amount of tin in the gamma 1. The results showed a strong negative correlation (r2 = 0.941) between the log of total mercury released and the amount of tin in the gamma 1. The effect of alloy composition, specifically the presence or absence of zinc in the amalgam, could not be definitively determined.(ABSTRACT TRUNCATED AT 250 WORDS)