Effect of Home-Bleaching Systems on Enamel Hardness and Elastic Modulus

Objective: The purpose of this study was to evaluate the nanohardness and elastic modulus of human enamel after treatment with tray and strip bleaching systems. Methods: Fifty-five human enamel samples were exposed to five different bleaching substances. Nanoindentations were made before and after bleaching treatments following manufacturer's directions using a Nanoindenter XP® (MTS Systems Corporation Oak Ridge, TN, USA). Nanohardness and elastic modulus measurements were obtained and the results were statistically analyzed using a repeated measures analysis of variance with a post-hoc Tukey-Kramer multiple comparison test. Results: Hardness and elastic modulus measurements showed decrease of mean values for each group except the control which remained the same (Table 1).

Group (n)	Hardness Change	Elastic Modulus Change
	Mean (GPa)/SD	Mean(GPa)/SD
Control (5)	-0.04/0.38	-1.34/4.53
Crest Premium [CP] (10)	1.14/0.33	19.12/11.79
Crest Supreme [CS] (10)	1.18/0.91	10.47/12.90
NiteWhite ACP [NW] (10)	1.15/1.21	7.09/9.23
Oral B Rembrant® [OB] (10)	1.72/0.65	11.2312.51
Treswhite-Opalescence [TO] (10)	1.92/0.97	19.57/15.70

There were significant differences due to time (before and after treatment) and material effects in nanohardness (p<.0001) and elastic modulus (P<0.0094). Measurements after treatment showed significant differences in hardness between each group and the control group. There was a significant difference in hardness between CP and TO after treatment. Additionally, there were significant differences in elastic modulus between the control group when compared to OB and TO after treatment. While there were significant changes in the elastic modulus due to treatment for both CP and TO, there were no significant differences between any of the groups after treatment. Conclusion: Nanohardness of human enamel was significantly decreased after usage of home-bleaching systems. The elastic modulus of enamel was also affected by some bleaching systems.