Dental bleaching with violet LED: Effects on dentin color change, resin-dentin bond strength, hybrid layer nanohardness and dentinal collagen biostability

Background: The purpose of this study was to investigate the effect of a novel dental bleaching technique with Violet LED on enamel color change, bond strength and hybrid layer nanomechanical properties in resin-dentin restoration, and dentin biostability. Methods: A total of 125 bovine incisors were distributed into a control group, violet LED group (LED), and 35 % peroxide hydrogen bleaching gel (BLG) groups (n = 15). Three 45-minute sessions were performed for both bleaching procedures every week. Enamel color change (AE, AL, and Ab) was determined after every bleaching session. After color analysis, dentin was exposed for the resin-dentin bond strength analysis using microtensile test and evaluation of the nanomechanical properties at the hybrid layer (nanohardness). While half of the specimens were tested immediately, the remaining were evaluated after 10,000 thermal cycles (TC). Thirty additional teeth were used to investigate dentin ultimate tensile strength (UTS) after the bleaching treatments. UTS was evaluated before and after an enzymatic challenge. Two-way repeated measures ANOVA and Tukey's post-test were used for the statistical analysis (alpha = 0.05). Results: Enamel bleaching effect was observed in the LED and BLG groups with significant alterations in the AE, AL, and Ab in the BLG group. No difference was observed in the resin-dentin bond strength among the groups (p > 0.05), however, TC negatively affected the bond strength values for all the groups. Nanomechanical properties remained unchanged when comparing immediate and after TC results (p > 0.05). Bleaching with BLG reduced significantly the dentin UTS, while all groups showed major decrease in UTS after the enzymatic challenge. Conclusions: Although violet LED was able to promote a bleaching effect, less color changes was observed when compared to BLG. None of the bleaching techniques effected the resin-bond strength or the nanomechanics of the hybrid layer. Violet LED did negatively effect dentin biostability as observed for BLG and it may promote less changes to the organic content of dentin.