Time-dependent corrosion potential of newly-placed admixed dental amalgam restorations

It was the objective of this study to measure the corrosion potential over time of newly-placed admixed dental amalgam restorations as a possible indicator of corrosion activity.

The corrosion potentials of 271 amalgam restorations, 4-min to 24-month old were measured in a convenience sample of 81 subjects. The selected restorations had no occlusal or interproximal contact with other metallic restorations. Eighty-one percent of the restorations aged 4 min to 7 months were made from zinc-containing admixed high-copper amalgam. The remaining amalgams were an unknown mixture of admixed high-copper and single-composition-alloy amalgams.

The data were fit to a mixed-effects model with random patient effects to allow for the correlation, using the maximum likelihood method. The early data best fit a quadratic model with an initially rapidly rising corrosion potential that leveled off to a constant mean value of -146 (60)mV (versus Ag/AgCl, 3M KCl at 35 degrees C) by approximately 7 months. Most of the ennoblement occurred within the first 4 months after restoration placement.

This in vivo study observed a slower rate of corrosion potential ennoblement for admixed amalgam restorations than would be predicted from in vitro studies. The slower rate is believed due to the combination of mechanical, chemical and biological forces in the mouth that are generally absent using simulated conditions. The slower rate of ennoblement suggests a slower rate of achieving maximum corrosion resistance than would be predicted using in vitro studies.