title: The influence of the Nd:YAG laser bleaching on physical and mechanical properties of the dental enamel.
abstract: OBJECTIVE
  The Nd:YAG laser can be used in Dentistry to remove soft tissue, disinfect canals in endodontic procedures and prevent caries. However, there is no protocol for Nd:YAG laser application in dental bleaching. The aims of this in vitro study were: (a) to observe the tooth shade alteration when hydrogen peroxide whitening procedures are associated with dyes with different wavelengths and irradiated with Nd:YAG laser or halogen light; (b) to measure the Vickers (VHN) enamel microhardness before and after the whitening procedure; (c) to evaluate the tensile bond strength of two types of adhesive systems applied on bleached enamel; (d) to observe the failure pattern after bond strength testing; (e) to evaluate the pulpal temperature during the bleaching procedures with halogen light or laser; (f) to measure the kinetic reaction of hydrogen peroxide.
  
  
  METHODS
  Extracted sound human molar crowns were sectioned in the mesiodistal direction to obtain 150 fragments that were divided into five groups for each adhesive system: WL (H(2)O(2) + thickener and Nd:YAG), WH (H(2)O(2) + thickener and halogen light), QL (H(2)O(2) + carbopol + Q-switch and Nd:YAG), QH (H(2)O(2) + carbopol + Q-switch and halogen light), and C (Control, without whitening agent). Shade assessment was made with a shade guide and the microhardness tests were performed before and after the bleaching procedures. Immediately afterwards, the groups were restored with the adhesive systems Adper Single Bond 2 or Solobond M plus composite resin, and the tensile bond strength test was performed. The temperature was measured by thermocouples placed on the enamel surface and intrapulpal chamber. The kinetics of hydrogen peroxide was observed by ultraviolet analysis.
  
  
  RESULTS
  The shade changed seven levels for Nd:YAG laser groups and eight levels for halogen light. According to the student's t-test, there was no statistical difference between the VHN before and after the whitening protocols (p > 0.05). The tensile bond strength showed no statistical significance between the test groups and the controls, considering both adhesive systems tested by ANOVA and Tukey tests (p > 0.05). The predominant failure pattern after bond strength testing was mixed. The temperature was safe for laser and halogen light. The kinetic reaction showed that after 5 min all the hydrogen peroxide had been consumed.
  
  
  CONCLUSIONS
  Nd:YAG laser associated with hydrogen peroxide bleached the enamel, the shade being similar to that obtained with the traditional method performed with halogen light. Moreover, the Vickers' microhardness and bond strength values were not altered in comparison with those for nonbleached enamel.
authors.affiliation: Department of Restorative Dentistry, School of Dentistry, Pontifical Catholic University of Rio Grande do Sul (PUCRS), Porto Alegre, Rio Grande do Sul, Brazil.
authors.forename: Maurem
authors.initials: M
authors.lastname: Marcondes
authors.forename: Maria Paula Gandolfi
authors.initials: MP
authors.lastname: Paranhos
authors.forename: Ana Maria
authors.initials: AM
authors.lastname: Spohr
authors.forename: Eduardo Gonçalves
authors.initials: EG
authors.lastname: Mota
authors.forename: Isaac Newton Lima
authors.initials: IN
authors.lastname: da Silva
authors.forename: André Arigony
authors.initials: AA
authors.lastname: Souto
authors.forename: Luiz Henrique
authors.initials: LH
authors.lastname: Burnett
mesh_terms: D003116:Color; D004396:Coloring Agents; D003743:Dental Enamel; D006861:Hydrogen Peroxide / Q000737:chemistry; D007700:Kinetics; D053844:Lasers, Solid-State; D008855:Microscopy, Electron, Scanning; D009354:Neodymium; D013718:Tensile Strength; D014073:Tooth Bleaching
chemical_list: D004396:Coloring Agents; D009354:Neodymium; D006861:Hydrogen Peroxide