Effect of different photo-activation methods on push out force, hardness and cross-link density of resin composite restorations

Objectives. To evaluate push out force, hardness and cross-link density (CLD) of composite restorations photo-activated by different methods. Methods. 2250 (3M ESPE) and XL2500 halogen unit (3M ESPE) were used. For push out force and hardness tests, conical restorations were made in bovine incisors. For CLD evaluation, cylindrical specimens were prepared. Different activation methods were tested: high-intensity continuous (HIC), low-intensity continuous (LIC), soft-start (SS) or pulse-delay (PD), with constant radiant exposure. Knoop readings were performed on bottom and top surfaces. Data were submitted to two-way ANOVA and Tukey's test (alpha = 0.05). Push out force data were submitted to ANOVA and Tukey's test (alpha = 0.05). Failure modes were classified under magnification (40x). CLD was estimated by hardness readings before and after storage in ethanol. Data were submitted to RM-ANOVA and Tukey's test (alpha = 0.05). Results. No significant differences in top hardness (KHN, N/mm(2)) were observed for HIC (598), LIC (564), SS (585) and PD (573). LIC presented significantly lower bottom hardness (520) than HIC (574), SS (562) and PD (572). Push out force (N) for SS (246) and PD (238) were similar, but significantly higher compared to LIC (198) and HIC (193). For HIC and LIC, only adhesive and mixed failures were observed. For SS and PD, cohesive failures also occurred. After storage, HIC and LIC presented significantly lower softening than PD. HIC also presented lower softening than SS, and similar results were observed for SS and PD. Significance. Different activation methods can interfere with push out force, hardness and CLD of composite restorations. (C) 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.