Objective: Aim of the in vitro study was to evaluate the time period needed to re-establish enamel surface microhardness after bleaching with fluoridated (0.5% F(-) as NaF) and unfluoridated carbamide peroxide (CP) bleaching gels under a daily de- and remineralization protocol.
Materials and methods: Ninety enamel specimens were prepared from bovine incisors and prepared for Knoop microhardness determination. The samples were divided into five groups and bleached for 7 days (8h per day) with experimentally designed neutral and acidic 10% CP gels: A (0.5% F(-), pH 7.0), B (0.5% F(-), pH 5.5), C (no F(-), pH 7.0) and D (no F(-), pH 5.5). Group E served as control and was not bleached. After each 8-h bleaching period the samples were subjected to a de- and remineralization protocol. In the following 5 days, the specimens were not bleached, but stored in artificial saliva for 8h instead; the daily de- and remineralization cycles were maintained during this period. Knoop microhardness determinations were conducted at baseline, after each bleaching interval and after completion of each daily de- and remineralization cycle. A repeated measures two-way ANOVA statistical analysis and a comparison of confidence intervals was conducted to check for hardness recovery during the post-bleaching period.
Results: After completion of bleaching therapy on day 7 a statistically significant hardness loss was observed for all bleaching groups in the range of about 7-15% hardness reduction as compared to baseline. The fluoridated gels proved significantly less hardness loss than the unfluoridated gels. In the following period, from days 8 to 12 recovery of hardness to baseline data was observed in the fluoridated groups (A and B) after the first de-/remineralization cycle on day 8 (group A) and after the second 8-h remineralization period on day 9 (group B), respectively. Hardness of the specimens treated with the unfluoridated bleaching gel C returned to baseline values after day 10. The samples of group D did not show recover of hardness data during the 5-day post-bleaching period.
Conclusion: Fluoridated 10% carbamide peroxide bleaching gels support re-hardening of bleached enamel with a shorter period needed for hardness recover as compared to unfluoridated gels.