Mercury vapor emission from 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.

Release of mercury from dental amalgam fillings in pregnant rats and distribution of mercury in maternal and fetal tissues

Mercury vapor released from a single amalgam restoration in pregnant rats and mercury concentrations in maternal and fetal rat tissues were studied. Dental treatment was given on day 2 of pregnancy. Mercury concentration in air sample drawn from the metabolism chamber with the rat was measured serially for 24 h on days 2, 8 and 15 of pregnancy. An average mercury concentration in the air samples from the rats given amalgam restorations was 678.6+/-167.5 ng/day on day 2. The average mercury concentration in the air samples tended to decline as time elapsed but a marked amount (423.2+/-121.5 ng/day) was observed even on day 15. The amount of mercury in the air samples increased 7--20-fold after chewing. The placement of the single amalgam restoration (3.8--5.5 mg in weight) increased the levels of mercury approximately three to 6 times in the maternal brain, liver, lung, placenta and 20 times in the kidneys. The highest mercury concentration among fetal organs was found in the liver, followed by the kidneys and brain. Mercury concentrations in maternal organs and fetal liver were significantly higher than those of the controls, and concentrations in maternal whole blood, erythrocytes and plasma, and in fetal whole blood were also significantly higher. Mercury concentrations in the fetal brain, liver, kidneys and whole blood were lower than those of the maternal tissues.

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%.

Exposure to mercury vapor during setting, removing, and polishing amalgam restorations

The level of mercury vapors in the oral cavity was determined by analyzing 242 samples of intra-oral air before and after removing, setting, and polishing dental amalgam. The analysis was effected using atomic absorption spectrometry in cold vapors with the detection limit of 6 micrograms/m3. A score of 1 to 3 was assigned to each restoration to adjust the differences of the amalgam sizes. The present study showed that the vapor was released during any procedure: removing, setting, and polishing amalgam. The mean levels were between 85 and 326 micrograms/m3. However, before the restorative procedures, no mercury vapors had been detected in the oral cavity. This investigation has also demonstrated a significant direct correlation between Hg vapor concentrations in intra-oral air and the sizes of amalgam restorations during each procedure: removing, setting, and polishing. The statistic analysis showed no significant difference of mercury vapors in the oral cavity between the use or nonuse of water coolant during the amalgam polishing.

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.

A study of mercury in the hair of dentists and dental-related professionals in 1985 and subcohort comparison of 1972 and 1985 mercury hair levels

Dentists may be exposed to mercury used in dental restorations. An epidemiologic study was conducted in central New York in 1985 comparing the concentration of mercury in the hair of 85 dentists to that of 23 dental-related health professionals (mainly orthodontists and oral surgeons) who do not use mercury. Mercury in the hair of dentists was nominally higher (2.98 +/- 0.57 ppm, mean +/- SE) than that of the control group (1.84 +/- 0.20 ppm) but not significantly so. There was a significant positive linear trend in the concentration of mercury in the dentists' hair with the number of years they had been in practice. Those dentists who wore masks when removing old mercury containing fillings showed significantly (p = 0.03) lower mercury concentrations in hair (1.7 +/- 0.26 ppm, mean +/- SE) than those who did not (3.65 +/- 0.85 ppm). Mercury concentration in the hair of 23 of the above dentists analyzed in 1972 was 16.77 +/- 4.00 ppm (mean +/- SE), significantly higher (p = 0.008) than that of the same dentists in 1985 (5.27 +/- 1.85 ppm). The reduction is believed due to the currently used pre-prepared disposable or reusable capsules containing the amalgam versus formerly mixing the ingredients manually. This may also account for the lack of a significant difference in mercury in the hair of the dentists versus that of the control group in 1985. The statistical relationships between mercury concentrations in hair of the two groups and reported health effects, as well as other factors including fish consumption, number of restorations per day, office floor covering, and urban or rural domicile, are discussed.

Oral lichenoid reactions related to mercury sensitivity

Lichen planus is a common disorder of unknown aetiology. It has been proposed that in some cases it represents a form of allergic reaction to the metals contained in dental amalgam, particularly mercury. Twenty-nine consecutive dentate patients who had lichen planus of the oral mucosa were patch-tested to the range of metals contained in dental amalgam. Ten out of 29 (34%) showed an allergic reaction to mercury and all of these patients had amalgams greater than 5 years old. The amalgams were poorly contoured and had corroded, resulting in continued release of mercury ions. Six patients had their amalgams replaced with composite or glass ionomer materials resulting in resolution of ulcerated lesions. In a follow-up of 3-24 months, one patient had a recurrence of ulcerated areas and another, despite resolution of the oral lesions had persistent discomfort.

Microstructure of amalgam surfaces

The polished amalgam surface was smoother than the burnished and the carved surfaces. The burnished surface was smoother than the carved surface. The granular structures on the burnished amalgam surface were found to be of tin-mercury alloy. Polishing appears to be the best finishing technique for amalgam restorations, and there is no substitute for it.

Quantitative measure of mercury vapor emission from setting dental amalgam

The rate of mercury vapor emission from setting dental amalgam was measured by use of an ultraviolet light absorption technique under nondiffusion-controlled conditions. An average of 0.3 X 10-6 gm mercury per square centimeter was emitted at the experimental temperature. At 37 C, this amount was estimated to be 1.2 × 10 -6 gm/ cm2.

Effect of retainer margin and distribution of mercury on dental amalgem mercury vaporization patterns

The results of this study showed that mercury vaporization patterns were not related directly to specimen postset mercury distribution. During the setting process, amalgam specimen retainer configurations affected these patterns significantly.