title: Effect of two different tooth bleaching techniques on microhardness of giomer.
abstract: BACKGROUND
  Tooth bleaching is a safe and conservative treatment modality to improve the esthetic appearance of discolored teeth. One of the problems with the use of bleaching agents is their possible effect on surface microhardness of resin-based materials. The present study was carried out to evaluate the effect of in-office and at-home bleaching on surface microhardness of giomer.
  
  
  METHODS
  Seventy-five disk-shaped giomer samples (Beautifil II) were prepared and cured with a light-curing unit. The samples were randomly assigned to three groups (n=25). In group 1 (control), the samples were stored in distilled water for 14 days. The samples in groups 2 and 3 underwent a bleaching procedure with 15% carbamide peroxide (CP) (8 hours daily) and 45% CP (30 minutes daily), respectively, for 14 days. Finally, the microhardness of samples was measured with Vickers hardness tester using a 100-g force for 20 seconds. One-way ANOVA was used to compare the mean microhardness values among the study groups, followed by post hoc Tukey test for two-by-two comparison of the groups. Statistical significance was set atP<0.05.
  
  
  RESULTS
  One-way ANOVA showed significant differences in the mean microhardness values among the study groups (P<0.001). Based on the results of Tukey test, microhardness in the bleached groups was significantly less than that in the control group (P<0.0005). In addition, microhardness in the 45% CP group was significantly less than that in the 15% CP group (P<0.0005).
  
  
  CONCLUSIONS
  Use of both bleaching agents during in-office and at-home bleaching techniques resulted in a decrease in surface microhardness of giomer. The unfavorable effect of in-office bleaching (45% CP) was greater than that of at-home bleaching (15% CP).Key words:Dental restorations, hardness, tooth bleaching.
authors.affiliation: Dental and Periodontal Research Center, Faculty of Dentistry, Tabriz University of Medical Sciences, Tabriz, Iran; Professor, Department of Operative Dentistry, Faculty of Dentistry, Tabriz University of Medical Sciences, Tabriz, Iran.
authors.forename: Soodabeh
authors.initials: S
authors.lastname: Kimyai
authors.affiliation: Assistant Professor, Department of Operative Dentistry, Faculty of Dentistry, Tabriz University of Medical Sciences, Tabriz, Iran.
authors.forename: Mahmoud
authors.initials: M
authors.lastname: Bahari
authors.affiliation: Post graduate student, Department of Operative Dentistry, Faculty of Dentistry, Tabriz University of Medical Sciences, Tabriz, Iran.
authors.forename: Fereshteh
authors.initials: F
authors.lastname: Naser-Alavi
authors.affiliation: Under graduate student, Department of Operative Dentistry, Faculty of Dentistry, Tabriz University of Medical Sciences, Tabriz, Iran.
authors.forename: Soodabeh
authors.initials: S
authors.lastname: Behboodi