David B. Mahler

Modulus of elasticity in bending of composites and amalgams.

In this study, the modulus of elasticity in bending (a measure of rigidity) and transverse strength (a measure of edge strength) were determined for composites and amalgams. the results were as follows. The modulus of elasticity of microfilled composites is approximately one-third that of the macrofiller-containing composites. Composites containing a relatively coarse-sized macrofiller have a higher elastic modulus than those containing a finer macrofiller. The transverse strength of microfilled composites is approximately one half that of the macrofiller-containing composite, which, in turn, is close in value to the transverse strength of amalgam. High-copper amalgams are substantially more rigid than the traditional low-copper amalgams, but do not show large differences among themselves. The transverse strength among various amalgams does not appear to be related to amalgam type but does correlate to final Hg content. The modulus of elasticity of composites most likely to be used in posterior teeth is approximately one third of the modulus of elasticity of amalgam; that is, amalgams are three times more rigid.

Assessing Composite Resin Wear in Primary Molars: Four-year Findings

Many investigators have reported minimal wear of resin restorations in primary molars. Until recently, quantitative wear assessments have not been reported. The purpose of this investigation was (1) to present and contrast data from two 48-month clinical trials wherein quantitative wear assessments were used to evaluate wear of resin restorations in primary molars, and (2) to compare these results with those using the USPHS method of wear assessment. The data were collected from two different clinical trials. One was conducted at the University of California in San Francisco by Tonn and Ryge (TR Trial). The other was conducted at the University of North Carolina at Chapel Hill by Oldenburg, Vann, and Dilley (OVD Trial). The OVD Trial had a sample size of 45 patients with 106 restorations; the TR Trial had 44 patients with 96 restorations. The restorations in both trials were Class I and II restorations in occlusion. They were placed with the experimental light-cured posterior composite resin F-70, later marketed as Ful-Fill®. At baseline, six, 12, 24, 36, and 48 months, restorations were evaluated by two evaluators trained in USPHS criteria as well as by a quantitative method of wear assessment, the standard cast technique as described by Leinfelder. Interstudy comparisons were made by Chi-square tests (USPHS evaluations) and ANOVA (quantitative assessments). The results showed that resin restorations in primary molars exhibited progressive wear with time in service. The rate and quantity of wear were very similar to those seen in permanent posterior teeth over 36 months. Findings agree with previous reports that the USPHS evaluation criteria are insensitive in detecting early wear in primary molars.

Hg Emission from Dental Amalgam as Related to the Amount of Sn in the Ag-Hg (γ1) Phase

Previous studies have shown that: (1) different dental amalgams emit different amounts of Hg vapor after surface films have been removed by abrasion (Boyer, 1988); (2) Sn oxide may be the predominant factor in these surface films that inhibit Hg vapor loss (Okabe et al., 1989); and (3) Sn in the Ag-Hg (γ1) phase may influence the vapor pressure of γ 1 (Reynolds, 1974). The purpose of this study was to explore the relationship between Hg vapor emission and the Sn content of the Ag-Hg (γ1) phase in dental amalgam. Eleven commercial amalgams were selected whose γ 1 phases contain different amounts of Sn. Amalgam specimens were ground on 600-grit carborundum paper and immediately placed into an apparatus designed around a gold film Hg vapor detector. Hg vapor loss in air over a 30-minute period was determined, and the log10 Hg loss/volume fraction γ 1 was plotted vs. the Sn content of the γ1 phase for the 11 test amalgams. A linear regression of these data, showing that of the higher the Sn content in γ1, the lower the Hg vapor loss, produced a highly significant R2 = 0.94 (p < 0.001). To examine for differences in vapor pressure among alloys, we eliminated the oxidation effect by conducting these same tests in an argon atmosphere. Although the Hg loss was significantly greater in argon than in air, the same differences among alloys were observed. Thus, from the standpoint of both vapor pressure and oxidation, the amount of Sn in the γ1 phase of dental amalgam has a significant and specific influence on the potential for Hg vapor emission.

Factors influencing the sealing behavior of retrograde amalgam fillings

In this study, the sealing behavior of apical (retrograde) amalgam fillings was evaluated with particular regard to the nature of the amalgam and the manipulative procedures used. With the use of an in vitro air pressure test, four high-copper alloys and one traditional composition low-copper alloy were selected for determination of their microleakage characteristics. Measurements were made on both normal and water-contaminated mixes, and the effect of mix plasticity was evaluated. In addition, both setting and postsetting dimensional changes of these alloys were determined. The results showed that amalgams differ in their ability to provide a complete seal and that low plasticity mixes result in greater microleakage. All zinc-containing alloys that were contaminated with water exhibited a postsetting or delayed expansion. However, only the low-copper alloy exhibited a delayed expansion of a magnitude that might induce root fracture.

Effect of Pd on the Clinical Performance of Amalgam

The purpose of this study was to determine the effect of a small amount of Pd on the clinical performance of a high-copper spherical-particle dental amalgam. Two identical alloys, with and without 0.5% Pd, were investigated. The marginal fracture, surface texture, and surface luster of 193 clinical restorations were evaluated at one year, those of 182 at two years, and those of 144 at three years of service. In vitro corrosion tests of anodic polarization and constant potential coulometry were also conducted. Over the three-year period, the results showed no differences in marginal fracture or surface texture between the two alloys. However, the alloy with Pd showed a significant superiority in surface luster over this time period. Both corrosion tests showed less electrochemical activity on the part of the alloy containing Pd.

Sn in the Ag-Hg Phase of Dental Amalgam

In a recent study, Sarkar and Eyer (1986) studied an amalgam which was formed using a unique procedure. Based on their results, they concluded that the solubility of Sn in the Ag-Hg (γ1) phase of dental amalgam was virtually nil (<0.25 wt%). This finding is contrary to the published results of other investigations. The purpose of the present study was to clarify these disparate findings. A low-copper dental amalgam was examined by electron probe microanalysis. This amalgam was selected because it exhibits unusually large γ1 grains (10 μm). Because the influence of the electron-analyzing beam can be completely contained within these large grains, grain boundaries or peripheral phases would not be excited, and a valid analysis of the γ 1 grains themselves could be made. The analyses showed significant Sn contents of 2.0-3.0 wt%. Analyses of areas which contained grain boundaries showed slight or no differences in Sn content compared with that for the grains alone. Furthermore, secondary electron scans of this dental amalgam failed to reveal the intergranular precipitates of Sn-Hg observed by Sarkar and Eyer (1986) in their amalgam. These different results can be explained by the fact that the unique amalgam investigated by Sarkar and Eyer does not exhibit the same diffusional patterns as does dental amalgam, and that the composition of the γ1 in the two amalgams is not the same. The final conclusion is that the Ag-Hg (γ1) phase in dental amalgam does indeed contain a significant amount of Sn.

Effect of Time at 37°C on the Creep and Metallurgical Characteristics of Amalgam

When aged at 37°C for six months, dental amalgam exhibits a marked decrease in the property of creep. The objective of this study was to investigate the relationship between this decrease in creep and selected metallurgical characteristics. The formation of β1 (Ag-Hg), the grain size of γ 1 (Ag-Hg), and the composition of γ1 were chosen for investigation. Creep was determined according to ADA Specification test No. 1, β1 was measured by x-ray diffraction, and γ 1 grain size and γ1 composition were determined by electron-probe microanalysis. The results showed that the decrease in creep was related to β1 formation and not to changes in either γ1 grain size or γ1 composition.

A comparison of methods for evaluating the marginal fracture of amalgam restorations

In this study, two methods for evaluating the marginal fracture of clinical amalgam restorations were compared. These were (1) matching photographs of the occlusal surfaces of restored teeth to a reference set of photographs; and (2) matching gypsum models of the same restored teeth to a reference set of gypsum models. Amalgam restorations made from 3 alloys were assessed at polish and at 1-, 2- and 3-year recalls. For each assessment period, both evaluation methods showed differences among the 3 alloys and ranked the alloys in the same order. Although the marginal fracture values for the photograph method were slightly higher than those for the model method, the correpondence between the values for each method was almost perfect (regression R2=0.988, α<0.001). Thus, in defining the 3-dimensional character of marginal fracture, the evaluation method using photographs was found to be equivalent to the evaluation method using models.

Influence of liquid films on mercury vapor loss from dental amalgam

OBJECTIVES The aim of this study was to determine the effect of aqueous liquid films on Hg emission from of dental amalgam. METHODS Amalgam specimens (4 mm dia. x 4 mm long) made from ten alloys were uniformly abraded on wet ASTM 600 grit SiC paper, quickly dried and covered by liquid films of a thickness that approximated the thickness of saliva films on tooth surfaces in vivo. Distilled water and two formulations of artificial saliva were tested. After abrasion and film placement, the specimens were immediately inserted in a plastic tube which was then closed. After 30 min, the Hg vapor that had collected in the tube was measured using a Jerome 411Hg analyzer. Hg emission from abraded surfaces exposed to air was also measured and used for comparative purposes. The test results were compared using ANOVA and Tukeys contrast test (p=0.05). RESULTS Hg emission from abraded surfaces under liquid films was one to two orders of magnitude less than Hg emission from abraded surfaces exposed to air. SIGNIFICANCE In vitro measurement of Hg vapor emission from abraded surfaces exposed to air should not be used to estimate directly the Hg vapor release from dental amalgam restorations in vivo.

Standardizing Evaluations of the Clinical Marginal Fracture of Amalgam

In this study, we investigated both inter-examiner and intra-examiner differences in the evaluation of the marginal fracture characteristics of dental amalgam restorations. Evaluations were made by matching intra-oral photographs of the occlusal surfaces of clinical restorations to a reference set of clinical photographs illustrating increasing amounts of marginal fracture. Four amalgam alloys and five examiners provided the data base. Differences in marginal fracture indices were found among examiners and between evaluation times for the same examiners. However, these differences were reduced significantly after application of a standardization procedure.