RESUMO
PURPOSE: To determine the effects of bleach on dentin fatigue resistance and flexural strength. MATERIALS AND METHODS: Eighty bovine dentin specimens (2 × 2 × 17 mm) were treated with: placebo or 10% carbamide peroxide bleach. Treatment was applied for 6 hours/day for 2 or 8 weeks. After treatment, 10 specimens per group were subjected to fatigue testing (10(6) cycles) whereas the other 10 were stored in artificial saliva as fatigue controls. The specimens undergoing fatigue were checked daily for visible signs of fracture and excluded from subsequent flexural strength tests if fractured. Fatigue control and surviving fatigued specimens were subjected to flexural strength testing. Chi-square, Kruskal-Wallis, factorial analysis of variance (p < 0.05) and Mann-Whitney (p < 0.002) tests were performed. RESULTS: There were significant differences in fatigue resistance (p = 0.003) and flexural strength rank scores (p < 0.0001) among the groups. None of the specimens in the "8-week bleach" group survived the fatigue testing. Fatigue (p = 0.005) and interaction of time and treatment (p = 0.039) were significant factors in the flexural strength results. Fatigued specimens had lower flexural strength than nonfatigued and "8-week bleach" had lower flexural strength than placebo and "2-week bleach" groups. CONCLUSIONS: Prolonged direct bleaching of bovine dentin reduces its fatigue resistance and flexural strength in vitro. Further research is needed in this area. CLINICAL SIGNIFICANCE: It remains prudent to advise patients to limit their exposure to tooth bleaching materials by avoiding direct application of bleach to exposed dentin and by minimizing the duration of bleach treatment.