Quantitative measure of mercury vapor emission from setting dental amalgam

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.

Effect of heat treatment on structure and properties of dispersed-type dental amalgam

Effect of heat treatment of Ag-Cu-Pd dispersant particles on the structure, mechanical properties and mercury vapor release rate of an Ag-Cu-Sn/Ag-Cu-Pd-based dental amalgam has been investigated. Experimental results indicate that crystallinity of dispersant Ag-Cu-Pd alloy increases with increasing HTT, with most notable increase occurring between 100 and 200 degrees C. Increasing HTT of Ag-Cu-Pd alloy does not change much of the mercury/alloy ratio for amalgamation, but largely reduces working/setting time of the amalgam. The Ag-Cu-Pd particles in 7 d-aged amalgam are comprised primarily of an outer Sn/Cu/Pd-rich zone and an inner Ag/Cu/Pd-rich zone with eutectic-type morphology and chemical distribution. The annealing-enhanced Pd segregation effect is most significantly observed in the amalgam derived from 300 degrees C-annealed Ag-Cu-Pd dispersant. This amalgam also has the highest compressive strength, highest DTS, and lowest creep rate. Higher annealing temperature causes mechanical property of the amalgam to deteriorate. The initial mercury vapor release rates of amalgams derived from 100, 200 and 300 degrees C-annealed Ag-Cu-Pd dispersant are significantly lower than that derived from 400 degrees C-annealed dispersant.

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%.

The level of mercury in human dental plaque and interaction in vitro between biofilms of Streptococcus mutans and dental amalgam

Mercury levels (micrograms/mg dry weight) in dental plaque from amalgam and enamel surfaces in human subjects with amalgam restorations were (range, mean, SD) 0.5-1.31, 0.72, 0.34 and 0.01-0.54, 0.2, 0.19, respectively. The levels of mercury in plaque from amalgam surfaces were significantly higher than those from plaque on enamel (p < 0.001). No mercury was detected in plaque from subjects without amalgam restorations. The mean level of mercury in a 24-hour collection of plaque was 2 micrograms (median, 1.8 micrograms), an amount close to those calculated by other workers (1.2-1.7 micrograms) for the amount of mercury liberated in the mouth from amalgam restorations in 24 h. Freshly prepared amalgam liberated relatively large amounts of mercury into culture broth in the first 24 h of exposure; subsequently, the levels declined except in the presence of Streptococcus mutans. In vitro, biofilms of Streptococcus mutans facilitated the release of mercury from freshly prepared amalgam, in what appeared to be a cyclical fashion. Amalgam aged for two years did not release mercury, even when supporting the growth of an S. mutans biofilm. The resistance of aged amalgam was attributed to the presence of a passive tarnish layer. The mercury released by the biofilm had an effect on the composition of the biofilm. The biofilms on fresh amalgam had significantly lower levels of carbohydrate (p < 0.001-p < 0.01) and protein (p < 0.001-p < 0.02) than did biofilms on aged amalgam and on control stainless steel wires.

Effectiveness of oxide films in reducing mercury release from amalgams

The release of mercury from four freshly-triturated amalgams into air, argon, and moist air environments was quantitated at three different temperatures. Although a measurable amount of mercury was released from dental amalgam, the evaporation rate was immediately reduced by several phenomena, the most important being the formation of an oxide film on the surface. This hypothesis was supported by the fact that release rates were elevated in an inert argon environment, but declined dramatically once air was introduced and oxidation could occur. The further amalgamation of "free" mercury, as well as the presence of water vapor, also contributed to the reduced release rates observed during aging. Mercury release was reduced to a negligible level within three to four hours after trituration. Only one of the amalgams, Tytin, demonstrated a thermal dependence for mercury release in air. The results of this study suggest that the exposure to mercury vapor from a freshly placed amalgam restoration would be negligible in consideration of the normal estimated daily intake from all other sources.

Elevated T cell subpopulations in dental students

The absolute numbers of circulating white cells and lymphocyte subpopulations were studied in 25 final-year dental students and compared with a control group of 28 medical students. The total lymphocyte count, total T cell numbers (CD3), T helper/inducer (CD4), and T suppressor/cytotoxic (CD8) numbers were significantly elevated in the dental students as compared with the control group. There was no significant difference in the T helper/inducer to T suppressor/cytotoxic cell ratios or the circulating B cell (CD21) and natural killer cell (CD16) numbers between the study and control groups. Patch testing to mercury and mercuric compounds in both the study and control groups showed no evidence of cutaneous hypersensitivity to mercury. The reason for the observed elevations in T cell subpopulations in dental students is not clear. However, one possible explanation is the dental student's occupational exposure to mercury. Further work is underway to examine this possible relationship and it is suggested that dental personnel take adequate measures to reduce their exposure to mercury until the results of these studies are available.

The release of mercury from dental amalgam: the mechanism and in vitro testing

Dissolution of mercury from the Ag-Hg matrix phase of dental amalgam is distilled water and synthetic saliva, and the mercury evaporation from the solutions, were studied in vitro. The main objectives of the study were to evaluate the factors that affect the results of the mercury release tests, and to consider the possible mechanisms of the release in vivo. Specimens were exposed to the liquids in open or closed bottles, and the changes in the mercury concentration were determined by cold-vapor Atomic Absorption Spectrophotometry. Concentration vs. time tests showed the dissolution rate decreasing with time. Tests involving sequences of short and long exposures with solution changes showed higher average rates for short-term dissolution into the fresh solution than for the longer preceding exposures. The differences were attributed to a stifling effect of the concentration of elemental mercury on the dissolution. It is believed that mercury dissolved mainly in the elemental form and that a continuous increase in the concentration was made possible by oxidation in the solution. In open cells, some of the mercury was lost by evaporation. The analysis showed that the results of mercury dissolution tests depend on many test variables, such as time, solution volume, oxidation and evaporation conditions, etc. Evaporation, dissolution, and evaporation/dissolution mechanisms of the mercury release in vivo were considered. It was concluded that the dissolution/evaporation model best described the mercury release from dental amalgam restorations.

The effect of dental amalgams on mercury levels in expired air

The expired air of a group of 48 persons, 40 with and eight without dental amalgam restorations, was analyzed for its mercury content before and after chewing. Expired air samples were collected in polyethylene bags, and a known quantity of each was pumped into the mercury detector for measurement. The results showed that examined subjects with dental amalgams had higher pre-chewing mercury levels in their expired air than those without amalgams. After chewing, these levels were increased an average of 15.6-fold in the former and remained unchanged in the latter group. It was concluded that in situ dental amalgams can increase the level of mercury in expired air.