title: Do different bleaching protocols affect the enamel microhardness?
abstract: OBJECTIVE
  Tooth bleaching tends to increase enamel roughness and porosity, in addition to reducing surface microhardness. The aim of this in vitro study was to evaluate the effects of bleaching treatments using different hydrogen peroxide (HP) concentrations, with and without light activation on bovine enamel microhardness.
  
  
  METHODS
  The buccal surfaces of sixty bovine incisors were flattened and polished and the enamel specimens were divided into six groups: G1 : c0 ontrol, exposed to artificial saliva; G2: 35% HP applied in two sessions (45' each); G3: 35% HP applied in two sessions (3 × 15' each); G4: 35% HP applied in one session (3 × 7'30") plus hybrid light (HL); G5: 25% HP applied in one session (3 × 7'30") plus HL; and G6: 15% HP applied in one session (3 × 7'30") plus HL. After the treatment, the enamel specimens were stored in artificial saliva. The surface microhardness (Knoop) was measured at the baseline, 24 h and 7 days after bleaching. The data was analyzed using the ANOVA test, followed by the Tukey-Krummer test (P < 0.05).
  
  
  RESULTS
  All bleaching procedures lead to a decrease in surface microhardness when compared with the control group after 24 h. The lowest change in surface microhardness was found in the specimens treated with 15% HP plus HL. However, 35% HP plus HL induced the highest decrease in surface microhardness. After 7 days of remineralization, the surface microhardness returned to normal levels for all bleached specimens.
  
  
  CONCLUSIONS
  Therefore, it can be concluded that the bleaching protocols caused a slight enamel surface alteration. However, the remineralization process minimized these effects.
authors.affiliation: Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil.
authors.forename: Rafael Francisco Lia
authors.initials: RFL
authors.lastname: Mondelli
authors.affiliation: Postgraduate Student in Orthodontics, PROFIS, Bauru, São Paulo, Brazil.
authors.forename: Taisa R Conti Garrido
authors.initials: TRCG
authors.lastname: Gabriel
authors.affiliation: Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil.
authors.forename: Fabio Antonio Piola
authors.initials: FAP
authors.lastname: Rizzante
authors.affiliation: Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil.
authors.forename: Ana Carolina
authors.initials: AC
authors.lastname: Magalhães
authors.affiliation: Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil.
authors.forename: Juliana Fraga Soares
authors.initials: JFS
authors.lastname: Bombonatti
authors.affiliation: Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil.
authors.forename: Sérgio Kiyoshi
authors.initials: SK
authors.lastname: Ishikiriama
keywords: Hardness; tooth bleaching agents; tooth remineralization