Expansion of contaminated amalgams assessed by photoelastic resin

Objective: Eight amalgam alloys, 6 high-copper and 2 low-copper, were assessed for expansion via photoelastic resin. Method and materials: The photoelastic resin sheet was sectioned into 20 x 25-mm blocks, and two 4 x 8-mm holes were drilled into the resin. The amalgam alloys were hand condensed into the prepared holes. Test conditions for each alloy were (1) uncontaminated, (2) contaminated with 5 µL of Ringer's solution, and (3) contaminated with 5 µL of cell culture medium. Results: Contaminated high-copper amalgam alloys may exhibit expansion to varying degrees but do not show the classic delayed expansion. One spherical, nonzinc high-copper alloy showed no expansion, whether it was uncontaminated or contaminated, and 3 high-copper alloys with different amounts of zinc (0% to 1%) showed slight expansion when contaminated. One low-copper alloy displayed delayed expansion; within 17 days after initial expansion in the photoelastic resin, it had exceeded the greatest stress observed in any contaminated, high-copper zinc-containing alloy at 3 months. Conclusion: Contamination of dental amalgam is to be avoided, but a zinc-containing high-copper amalgam will not exhibit classic delayed expansion even if contaminated.