Effect of in-office tooth bleaching on the microhardness of six dental esthetic restorative materials

Objectives The aim of this in vitro study was to evaluate the effect of the in-office bleaching technique on the microhardness of six dental esthetic restorative materials. Methods Four composite resins (a hybrid, a flowable, a micro-hybrid and a nano-hybrid), an ormocer and a ceramic were tested, after the use of an in-office bleaching product. Fourteen specimens of each composite and the ormocer were fabricated and randomly divided into two groups of seven samples each. One group was polished and the other group remained unpolished. For the ceramic, seven polished samples were fabricated. Two samples of each group were used as negative controls. The specimens were bleached for 15, 30 and 45 min. Five Knoop microhardness measurements were made on each sample, for each of the following periods tested: before bleaching, after 15, 30 and 45 min of bleaching, 24 h and 1 month after the bleaching procedure. Data were analyzed by the repeated measures analysis of variance with three between factors and one within. Results The differences in the microhardness values between the bleached and the control samples for the composites and the ceramic, were not statistically significant (hybrid: p = 0.264; flow: p = 0.584; micro-hybrid: p = 0.278; nano-hybrid: p = 0.405; ceramic: p = 0.819). For the ormocer, although bleaching did not have any significant effect on the unpolished samples (p = 0.115), it caused an increase on microhardness of the polished samples. Significance Bleaching with 38% hydrogen peroxide does not reduce the microhardness of the restorative materials tested. Therefore, no replacement of restorations is required after bleaching.