Effect of carbamide peroxide bleaching agents on enamel microhardness in vitro

AIM:To evaluate the effect of carbamide peroxide(CP) bleaching agents of different concentration and carrier on enamel microhardness.METHODS:Eighteen premolars with 2 mm×2 mm flat on buccal and lingual surfaces were randomly assigned to six groups.Carbopol,PVP and Poloxamer were used as thickening carrier respectively.CP bleaching agents with density of 11% and 21% were applied on the enamel specimens for 2 weeks(6～8 h/d,37℃,100% relative humidity).Surface microhardness was measured before and after treatment.After the bleaching treatment,groups treated with 21%CP were immersed in artificial saliva for 14 days,then measured again.RESULTS:The microhardness of enamel treated with 11% of CP has no significant difference before and after treatment(P0.05).The microhardness of enamel treated with 21% of CP was significantly reduced after bleaching((P0.01)),but more microhardness was increased after immersed in artificial saliva for 2 weeks more again((P0.05)),but still lower incomparion with that of no bleaching(P0.05).There were no significant difference in microhardness among groups with different carriers at the same concentration(P0.05).CONCLUSION:Bleaching agent with 11% of CP does not have any significant effect on the microhardness of enamel,while 21% of CP can significantly reduce the microhardness of enamel.However,artificial saliva presented a remineralizing effect on the bleached enamel.Different thickening carriers does not have any significant effect on the microhardness of enamel.