Objective: It has been shown that bleaching agents can adversely modify the characteristics of enamel, including surface hardness. The purpose of this study was to evaluate the effect of remineralizing agents such as fluoride or amorphous calcium phosphate (ACP) during bleaching on surface enamel microhardness. Methods: Nine human central incisors were utilized. The incisors were sectioned longitudinally into halves for a total of 18 surfaces. The sections were then embedded in acrylic with the facial surfaces exposed. These specimens were then randomly divided into 3 groups (6 per group): Group 1, 16% carbamide peroxide with ACP (NiteWhite ACP); Group 2, 15% carbamide peroxide with fluoride (Opalescence PF); Group 3, 9% hydrogen peroxide with fluoride (Tres White). Prior to treatment, the Vicker's Hardness (VHN) of each specimen were measured as the baseline data. Six readings from each specimen were obtained on a Micromet 2100 (Buehler, Lake Bluff, IL) with 500 gm load for 15 seconds. A total of 36 reading per group were performed. Six consecutive one-hour cycles of bleaching were performed with each bleaching system. Upon completion of bleaching, the specimens were tested for hardness with 6 readings per specimen. All data gathered were analyzed using ANOVA with p<0.05 for significant differences. Results: After 6 cycles of bleaching, all bleaching systems produced a decrease in enamel hardness. The VHN of Group 1 decreased by 5.75%, Group 2 decreased by 10.49%, and Group 3 decreased by 11.91%. Group 1 hardness decreased significantly less than the other groups. Conclusion: All 3 bleaching systems decreased enamel hardness. Under the conditions of this study, the system with ACP produced the least decrease in surface enamel hardness.

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