Fast Curing with High-power Curing Lights Affects Depth of Cure and Post-gel Shrinkage and Increases Temperature in Bulk-fill Composites
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.SUMMARY
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Depth-of-cure test procedure in which composite was extruded into a plaster mold (A,B) and cured from one direction while the slot was covered with an orange glass plate (C). Microhardness was determined at the irradiated surface (0 mm) and in 1 mm increments along the composite surface shielded from direct curing light exposure during the light-curing (D).
Irradiance during the light-curing and wavelength spectrum of the curing lights tested.
(A) Radiant exposure (irradiance × curing time), (B) Relative hardness at 2-mm depth (%), (C) Post-gel shrinkage at 10 minutes (Vol%), and (D) Temperature increase at the bottom of the resin composite (°C). Columns show mean values, error bars represent standard deviation, and different letters indicate significant differences.
Mean and standard deviation of hardness profiles (depth-of-cure) of bulk-fill composite-curing protocol combinations (9 protocols in Table 2). Different letters indicate significant differences within each material.
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