WEAR AND MARGINAL BREAKDOWN OF COMPOSITES WITH VARIOUS DEGREES OF CURE

Loss of anatomical form due to wear has been cited as one factor limit ing the clinical use of posterior composites. The physical properties and possibly the wear resistance of composite are influenced by the ex tent 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 resi n and 62 vol% of strontium glass (1 to 2 mu m) with microfill silica w as formulated (Bisco). Composite was placed into two 2.5-mm-diameter c ylindrical holes in Co-Cr teeth replacing first and second molars in t he 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 Heliomol ar 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 re plicas were measured with a profilometer for wear. Wear of the hybrid composite at 2 yrs ranged from a high of 144 Fun with 9 s of light-cur ing to a low of 36 Fun with 40 s of light-curing followed by heat. Hel iomolar exhibited from 11 to 16 Fun of wear at 2 yrs. There was a stro ng negative correlation (r(2) = 0.91) between the degree of cure and t he abrasive wear of the hybrid composites. Marginal breakdown was negl igible for the hybrids, and was reduced for the microfill from 40% to 15% of the margin by heat treatment. This study showed that the resist ance to abrasive wear of a dental composite could be improved by enhan cement of its degree of conversion.