Abstract: Objective: To determine the enamel surface structure and hardness of the extracted teeth after bleached with three kinds of peroxide bleaching agent: 30% hydrogen peroxide (HP), 22% carbamide peroxide (result peroxide, CP) and 3% boric acid (sodium perborate, SP) commonly used in clinics. Methods: Specimens of the extracted teeth were randomly divided into four groups (ABCD) which were treated with normal saline, 30% hydrogen peroxide, 22% had carbamide peroxide and 3% boric acid sodium respectively. Then the teeth were observed by scanning electron microscope (2000) x and evaluated with Vickers hardness measurement, followed by statistical analysis. Results: Scanning electron microscopy showed that the enamel surface of teeth treated with 30% hydrogen peroxide and 3% boric acid sodium presented scattered honeycomb erosion, suggesting different degrees of tooth demineralization and dissolution. While the enamel surface of teeth treated with 22% over oxidation urea was intact, showing no obvious tooth demineralization and dissolution. Hardness determination results showed that statistical differences existed between groups with pairwise comparison (P<0.05). The hardness comparison of teeth treated with different groups were control group > 22% carbamide peroxide group > 3% over sodium borate > 30% hydrogen peroxide. Conclusion: Among the three tooth whitening agents, 22% cardamide peroxide showed the minimal impact on tooth enamel structure and hardness.

Key words: Bleaching agents, Hardness, Scanning electron microscope