Methods: Eighty-eight bovine enamel blocks were obtained: 08 enamel blocks were prepared for surface analysis (n=2), 40 enamel blocks for microhardness (n=10) and 40 enamel blocks for analysis of color change (n=10). After planning/polishing, the enamel blocks were subjected to surface treatment with fluoride, CPP-ACP or Nd:YAG laser associated with fluoride. After 24 h they were bleached with 35% hydrogen peroxide. Microhardness and color changes were accessed under microhardness and spectrophotometry, respectively, before and after bleaching (immediately and 7 days). An intermediate reading was made immediately after the pretreatment of the enamel surface.
Results: Microhardness were: G1 – 287.7± 25.78; 279.70 ± 21.04; 206.20 ± 13.14; 242.30 ± 19.28; G2 – 260.90 ± 17.57; 269.40 ± 29.74; 248.80 ± 30.33; 260.40 ± 24.41; G3 – 274.40 ± 38.29; 275.90 ± 33.94; 245.90 ± 25.66; 265.40 ± 23.75; G4 – 271.00 ± 25.78; 277.00 ± 19.33; 241.80 ± 22.68; 263.00 ± 18.62. Regarding color changes coordinates L * a * b * were used to calculate deltas and the final color change. There was an increase of lightness and decrease in yellow color, regardless of pre-treatment performed on the enamel surface. Surface morphology analysis showed a more porous enamel surface after bleaching in G1. The G2, G3 and G4 showed a surface modified with deposition of some amorphous precipitates.
Conclusion: Bleaching with 35% hydrogen peroxide caused decrease in microhardness of bovine enamel and the treatments performed in enamel before bleaching were able to prevent significant reduction in microhardness. In addition, pretreatments performed on enamel surface did not influence bleaching treatment.