Susceptibility of Bleached Dental Enamel to Acid Demineralization

Method: One hundred bovine enamel blocks (6x6x3mm) were divided in 10 groups (n=10). Groups 1 and 2 received no bleaching. Groups 3 to 6 were submitted to the at-home bleaching technique using 6% hydrogen peroxide (HP; G3 and G4) or 10% carbamide peroxide (CP; G5 and G6). Groups 7 to 10 were submitted to the in-office bleaching technique using 35% HP (G7 and G8) or 35% CP (G9 and G10). During bleaching, a daily fluoridation regime with 0.05% of sodium fluoride (NaF) solution was done in groups 3, 5, 7 and 9, while a weekly fluoridation with  a 2%NaF gel was applied in groups 4, 6, 8 and 10. The samples of the groups 2 to 10 were pH cycled during 14 consecutive days. Afterwards, the samples of all groups were assessed by cross-sectional Knoop microhardness at different depths from the outer enamel surface.  The average Knoop hardness numbers (KHN) were compared by Two-Way Anova and Tukey´s tests (α=0.05). 

Result: The comparison between groups 1 and 2 showed that the demineralization method was effective. The samples of the groups 2 to 6 showed the same susceptibility to acid demineralization. The treatments done in groups 8 and 10 let the enamel more susceptible to acid demineratization in up to 12% (p<0.05). Groups 7 and 9 showed KHN similar then group 2, but higher then groups 8 and 10.

Conclusion: The use of HP 6% and CP 10% associated with daily or weekly fluoridation regime do not increase the susceptibility of enamel to acid demineralization. However, the use of HP 35% and CP 35% must be associated with a daily fluoridation regime, otherwise the in-office bleaching can let the bleached enamel more susceptible to acid demineralization.