Wear and marginal breakdown of composites with various degrees of cure

Loss of anatomical form due to wear has been cited as one factor limiting the clinical use of posterior composites. The physical properties and possibly the wear resistance of composite are influenced by the extent to which it is cured. The aim of this study was to vary degree of conversion (DC) in composites to test the hypothesis that resistance to wear and marginal breakdown could be improved by enhanced curing. A light-cured hybrid composite containing a 50% Bis-GMA/50% TEGDMA resin and 62 vol% of strontium glass (1 to 2 microm) with microfill silica was formulated (Bisco). Composite was placed into two 2.5-mm-diameter cylindrical holes in Co-Cr teeth replacing first and second molars in the mandibular dentures of 50 edentulous patients. The composites were light-cured for different time periods (9 s, 12 s, 25 s, 40 s, and 40 s + 10 min at 120 degrees C) and then polished. The microfill Heliomolar was also tested. DC (%) was measured by FTIR and ranged between 55% for 9 s of light-curing and 67% for 40 s of light-curing followed by heat application. Impressions were evaluated at baseline, 6 mo, 1 yr, and 2 yrs. Stone casts were evaluated independently by three observers to determine the % of the total margin exhibiting breakdown. Epoxy replicas were measured with a profilometer for wear. Wear of the hybrid composite at 2 yrs ranged from a high of 144 microm with 9 s of light-curing to a low of 36 microm with 40 s of light-curing followed by heat. Heliomolar exhibited from 11 to 16 microm of wear at 2 yrs. There was a strong negative correlation (r2 = 0.91) between the degree of cure and the abrasive wear of the hybrid composites. Marginal breakdown was negligible for the hybrids, and was reduced for the microfill from 40% to 15% of the margin by heat treatment. This study showed that the resistance to abrasive wear of a dental composite could be improved by enhancement of its degree of conversion.