Fast Curing with High-power Curing Lights Affects Depth of Cure and Post-gel Shrinkage and Increases Temperature in Bulk-fill Composites.

Abstract

High-power LED curing lights and bulk-fill resin composites are intended to reduce chair time. This study investigated depth of cure, post-gel shrinkage (responsible for shrinkage stress), and heat generation in bulk-fill composites when cured according to minimum curing times recommended by manufacturers of curing lights and composites. A regular LED curing light (Demi Ultra, 1350 mW/cm2, Kerr Dental) and two LED curing lights with high-power modes (VALO Grand, 3117 mW/cm2 Xtra Power, Ultradent; and Bluephase PowerCure, 2435 mW/cm2 Turbo and 3344 mW/cm2 3sCure, Ivoclar Vivadent) were tested on three bulk-fill composites (Filtek One Bulk Fill, 3M Oral Care Solutions; Tetric EvoCeram Bulk Fill, Ivoclar Vivadent; Tetric Powerfill, Ivoclar Vivadent). Using minimum times recommended by manufacturers (3, 5, 6, 10, or 20 seconds), depth of cure was determined by Vickers hardness of specimens cured in a slot (n=10). Post-gel polymerization shrinkage was measured using a strain gauge (n=10) and temperature with a thermocouple (n=5). Results were analyzed using two- and one-way analysis of variance (ANOVA) followed by pairwise comparisons or Student-Newman-Keuls post hoc tests (α=0.05). Curing lights and curing protocols significantly affected depth of cure, post-gel shrinkage, and temperature rise (p<0.001). Cure decreased with depth whereby best overall curing performance was achieved by the 20 second exposure at lowest irradiance (Demi Ultra). Fast curing (3-5 seconds) at high irradiance resulted in lesser depth-of-cure performance, except for the BluePhase-Tetric PowerFill combination. Post-gel shrinkage was higher in all composites when cured at high irradiance (p<0.001), while heat generated also tended to be higher. Although the high-power LED curing lights advertise time savings, not all manufacturer recommended minimum curing times cured bulk-fill materials to the same extent. Moreover, these time savings came at a cost of higher post-gel shrinkage and generated more heat in the bulk-fill composites than the lower irradiance curing protocol.