Non-linear in vitro wear of posterior composites with time

Several clinical studies have found the wear rate of posterior composites to be nonlinear, with greater wear occurring initially. This relationship and its causes were sought in an in vitro test that used the Taber Abraser. Three commercial composites--Marathon, Occlusin, and P-30--were tested. Sixteen cured samples of each were initially finished with 600-grit paper and then abraded by medium-grit wheels for 30,000 cycles. Periodically, the weight losses were recorded and then converted to volume losses. Analysis included comparison of volume losses per 5000-cycle interval and comparison of first derivatives (wear rate) and second derivatives of the best-fit second-order regression line. Significant differences (p less than 0.05), as analyzed by ANOVA, were consistently realized between the volume losses for the first interval and those for subsequent intervals. Rank, with regard to wear resistance, depended on whether volume loss or initial wear rate was compared. The Taber Abraser did show non-linear trends similar to those observed clinically. Initial surface preparation, which apparently contributes to the greater early wear rates recorded in this in vitro study, may also be a factor clinically.