Time-dependent corrosion resistance of new dental amalgam restorations

Corrosion resistance of dental amalgam restorations increases following placement due to surface oxide growth. However, the rate of increase is unknown, despite its importance to alloy component release, including mercury. Objectives: The purpose of this study was to determine the rate of corrosion resistance increase by measuring the corrosion potential of amalgam restorations that were up to 24mo old. Corrosion potential was used as an indicator of corrosion resistance, where a positive increase in its value for newly placed amalgam indicates oxide growth and, consequently, increasing corrosion resistance. Methods: The corrosion potentials of 271 amalgam restorations, ages 4min to 24mo, were measured in a convenience sample of 55 human subjects. To eliminate the effect of galvanic polarization on the measured potentials, selected amalgams had no occlusal or interproximal contacts with other metallic restorations. Potentials were measured using a high impedance voltmeter and a Ag/AgCl (3M KCl) micro-reference electrode. Subjects had not eaten, brushed or received a cleaning for ≥20min prior to the measurements. Potentials are reported vs. the Ag/AgCl reference electrode (@35°C). Results: Statistical analysis used mixed-effects models to examine linear, quadratic and log transform models for observations with times <12mo. All models were significant (p≤0.0001) and showed a positive slope for potential vs. time. Diagnostics based on residuals showed the quadratic model of initially rapidly increasing potential gave the best fit for <12mo, with the residual plots conforming most closely to the model assumptions. For 12 to 24mo, potentials were relatively constant, mean: –146±60mV, which contrasts sharply with the three newest amalgams (≤30 min old) that had much more negative potentials of –802, -709 and -648mV. Conclusion: Based on corrosion potential measurements, corrosion resistance of dental amalgam restorations rapidly improves during the first 12mo after placement and then changes little during the next 12mo.