Subsurface microhardness of enamel and dentin after different external bleaching procedures

PURPOSE: To evaluate the influence of various bleaching systems on subsurface microhardness of enamel and dentin. METHODS: 60 bovine crowns were distributed among seven groups (A: Opalescence Xtra Boost, B: Opalescence Quick, C: Rapid White, D: Whitestrips, E: Opalescence 10%, F: Opalescence PF 15%). The crowns were sectioned and baseline hardness (Knoop) of enamel and dentin was assessed on the sectioned surface at various distances from the enamel surface. The sectioned surface was covered with wax and the enamel was treated for 10 days with the bleaching agents A-F according to manufacturers' instructions. Bleaching with C-F was conducted each day (C: twice per day for 10 minutes, D: twice per day for 30 minutes, E: 8 hours, F: 4 hours), systems A-B were applied on first and fifth days (A: twice for 10 minutes, B: 1 hour). Finally, hardness was re-assessed. Data were statistically analyzed to compare baseline hardness and final hardness in the respective groups (P< 0.05). RESULTS: Analysis showed that in Group C, hardness was significantly reduced in both enamel and dentin. In the remaining groups, significant reduction of hardness was observed up to the following depths [microm] in enamel; A: 250, B: 700, D: 300, E: 150, F: 150. In these groups no significant hardness changes were recorded in subsurface dentin.