title: The effect of bleaching time on dentin fracture toughness in vitro.
abstract: BACKGROUND
  A recent study reported a decrease in dentin fracture toughness after the application of peroxide bleaching products to dentin in vitro.
  
  
  OBJECTIVE
  The objective of the present study was to investigate this in vitro decrease in fracture toughness further by evaluating the effect of different peroxide application times on dentin fracture toughness.
  
  
  METHODS
  Compact test fracture toughness specimens were prepared from coronal human. These were divided into five groups (N = 12) and subjected to either bleach (10% carbamide peroxide) and/or placebo gel for a total of 336 consecutive hours (0 and 336, 84 and 252, 168 and 168, 252 and 84, 336 and 0 hours of bleach and placebo application time, respectively). The gel materials were changed every 6 hours. Fracture toughness testing was done 24 hours after the end of bleaching using tensile loading at 10 mm/min. Results were analyzed by analysis of variance and linear regression (p < 0.05).
  
  
  RESULTS
  Dentin fracture toughness after 252 and 336 hours was significantly reduced compared to the 0- and 84-hour bleach times. An association between fracture toughness and bleach time (r(2) = 0.82) with an inverse linear regression line (K(1C) = -0.0032 [hour] + 3.386) was found.
  
  
  CONCLUSIONS
  A significant correlation was found between bleach time and dentin fracture toughness. Dentin fracture toughness was reduced over time during the 336-hour course of in vitro bleaching.
  
  
  CONCLUSIONS
  The results suggest that it would be prudent to minimize the length of time for clinical bleaching procedures when dentin is directly exposed to bleach.
authors.affiliation: Restorative Dentistry, Faculty of Dentistry, University of Toronto, 124 Edward Street, Toronto, ON, Canada M5G 1G6.
authors.forename: Johnathan M
authors.initials: JM
authors.lastname: Woo
authors.forename: Stephanie
authors.initials: S
authors.lastname: Ho
authors.forename: Laura E
authors.initials: LE
authors.lastname: Tam
mesh_terms: D000704:Analysis of Variance; D000077463:Carbamide Peroxide; D003799:Dental Stress Analysis; D003804:Dentin / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D016014:Linear Models; D008963:Molar; D016877:Oxidants / Q000009:adverse effects*; D010545:Peroxides / Q000009:adverse effects*; D013718:Tensile Strength; D013997:Time Factors; D014073:Tooth Bleaching / Q000009:adverse effects*; D014082:Tooth Fractures / Q000209:etiology; D014508:Urea / Q000009:adverse effects / Q000031:analogs & derivatives*
chemical_list: D016877:Oxidants; D010545:Peroxides; D000077463:Carbamide Peroxide; D014508:Urea