Efficacy of In-office Bleaching on Microhardness of Permanent Teeth with Antioxidant Re-hardening

Bleaching procedures affect surface enamel structure and decrease its bonding ability to resin composite restorative materials. The application of re-hardening materials to bleached enamel surfaces may prevent this decrease in micro-hardness.

This in-vitro study aims to evaluate the surface micro hardness of human teeth enamel subjected to bleaching with Zoom Advanced Power 2 AP (Phillips, USA), and Opalescence Boost (Ultradent, USA) and compare the re-hardening effects of 10% Sodium Ascorbate, 2% acidulated phosphate fluoride gel, and a 5% Potassium nitrate 0.22% Sodium Fluoride + Calcium Nitrate gel.

Ninety human third molar teeth were used. The specimens were randomly assigned to 5 groups. After the bleaching procedure, the specimens were treated with APF, Sodium Ascorbate or Relief gel as re-hardening agents with 30 teeth in each group. Enamel micro-hardness was measured with Vickers Micro-hardness Tester. The data were evaluated with Kolmogorov-Simirnov, one-way ANOVA, Dunnett’s test, post-hoc Tukey and T-tests.

Statistical analysis revealed no significant differences among initial enamel groups’ micro-hardness values (P>.05); however, significant differences occurred between initial and after bleaching treatment group value for G3 (P< .05). After re-hardening, only the Sodium Ascorbate group showed a statistically significant increase with hardness values (P< .05) for G4 and G5.

Bleaching treatment conducted with light had no adverse effect on enamel micro-hardness. Sodium Ascorbate can be useful after bleaching to change the adverse effects of bonding on the enamel.