Effects of Bleaching and Remineralising Agents on the Surface Hardness of Enamel

Statement of Problem: Home bleaching is a common method for whitening the teeth. However, bleaching may lead to a decrease in the hardness of the enamel. Purpose: The purpose of this study was to investigate the effects of two different concentrations of carbomide peroxide (CP) on the hardness of the enamel and also to evaluate the effects of the remineralising agents on the hardness of bleached enamel. Materials and Method: Crowns of 100 intact extracted human anterior teeth were resected from their roots and mounted in acrylic resin in a way that the buccal surface was parallel to the floor (horizontal). The samples were then divided into 10 groups. The baseline hardness in the middle of the buccal surface was measured through Vickers Micro-hardness test and at a load of 500 gram per second. Then five groups were bleached with 10% carbomide peroxide and other five groups with 22% carbomide peroxide. The bleaching was performed for 21 days and was applied 4 hours each day. The samples were kept in distilled water in the interval time between the applications of bleaching agents. In the next phase the enamel micro-hardness was measured and different demineralising agents were used for each group. The Tooth Mousse (TM) paste; MI paste plus (MI); and Crest fluoridated toothpaste was applied for 4 hours to the surface of the enamels in three groups. In the forth group, samples were embedded in fresh cow milk for the same period and the fifth group was kept in distilled water as a control group. Then, the final hardness was measured and the collected data were analyzed by t-test, paired sample t-test and One-way ANOVA test. Results: Bleaching with the aforementioned concentration of CP had no effects on enamel microhardness. In the groups with a 10% CP, none of the demineralising agents had any effect on the hardness value. However, the application of milk in-creased the hardness. In the groups with a 22% CP, TM paste reduced the enamel microhardness value while Crest, increased it. MI paste and milk didn’t have any effect on it. Conclusion: The use of TM paste results in lower hardness of the bleached enamel. It seems that the high concentration of fluoride in MI paste may be responsible for increased microhardness of enamel. Milk and fluoridated toothpaste have propensity to increase the enamel hardness.