Fracture toughness determination of dental amalgams through microindentation

Marginal microhardness of corroded amalgams: a comparative in vitro study

One conventional and two high-Cu amalgams were tested for marginal microhardness after 2 months' corrosion in an 85 mM NaCl solution. Amalgams immersed in 200 mM phosphate buffer solution were used as controls. The microhardness tests were conducted on cross-sections of the amalgams 50 microns from the surface edges. The microstructure of the amalgams was studied in SEM and the amounts of Sn, Cu, Zn, Ag, and Hg dissolved in the solutions were analyzed with an atomic absorption spectrophotometer. For the amalgams immersed in the NaCl solution the depth of corrosion after 2 months was between 50 and 400 microns. The specimens immersed in the phosphate solution showed no signs of subsurface corrosion. The marginal microhardness of all the amalgams was reduced after corrosion in the NaCl solution. The greatest microhardness in both the uncorroded and corroded states was shown in the two high-Cu amalgams. The reduction in marginal microhardness after corrosion can probably be attributed mainly to degradation of the gamma-2 phase for the conventional amalgam and to degradation of the eta' phase for the two high-Cu amalgams.

Corrosion behavior and microhardness of three amalgams

The marginal microhardness of three different types of amalgam was tested after 2 months' immersion in an aqueous solution of NaCl (85 mM) and phosphates (Na2HPO4 100 mM and NaH2PO4 100 mM). Amalgams immersed in distilled water were used as controls. The microhardness tests were conducted at a distance of 50 micron from the margins and at the bulk of each specimen. The solutions were analyzed for Sn, Cu, Zn, Ag, and Hg by means of atomic absorption spectrophotometry (AAS). A statistically significant reduction in the marginal microhardness after immersion in the test solution was found for the conventional and the high-Cu single composition amalgam but not for the high-Cu blended amalgam. SEM-examination of cross-sections of the amalgams revealed small areas of subsurface grain boundary corrosion, no deeper than 10 micron for all the amalgams. The SEM-examination of the specimens and AAS analysis of the solutions indicated that the reduction in marginal microhardness was attributed mainly to corrosion of the Cu-rich phases for the high-Cu single composition amalgam and to corrosion of the gamma 2 phase for the conventional amalgam. The phosphates reduced the corrosion of the amalgams in the presence of NaCl. It is concluded that the marginal strength of dental amalgams in a corrosive environment is largely dependent upon their corrosion resistance.

Dental amalgam: reactor response

Dental amalgam continues to be an important restorative material; both fundamental and applied research should be encouraged. The criteria for replacement of amalgam restorations should be re-examined to determine if the integrity of the margins should continue to receive the dominant research interest.

In the studies of mechanical properties, first attempts have been made to apply fracture mechanics analysis to dental amalgam. The heterogeneous structure and time-dependent strain of dental amalgam will have to be considered in future studies. More attention should be given to the effects of zinc, which has been shown to improve resistance to marginal fracture.

The corrosion properties have been studied in depth, but there are some important unanswered questions. The pathways of corrosion attack in high-copper amalgams and the kinetics of attack in amalgams of different composition should be examined. Biocompatibility of dental amalgam has recently received renewed interest and will have to be examined in more detail. The clinical significance of the release of metallic elements from restorations is generally unknown.

Dental amalgam faces strong competition from non-metallic restorative materials. Future research should be more strongly focused on the critical issues. The involvement of scientists from related disciplines should be encouraged, and no effort should be made to restrict the talent pool by a centralization of resources.