Effects of Extended Bleaching on Human Root Dentin

Vital tooth bleaching is administered in a variety of forms including trays, strips and paint on high concentration treatments.  The concentrations and conditions of bleaching vary considerably between these treatment forms.    Objectives: This study compared physical and micromorphological surface changes associated with exposure of human dentin specimens to a cycle of chronic bleaching with hydrogen peroxide bleaching strips designed for daily application in vitro.  Methods: Extracted human canines were used to prepare root dentin blocks in Durabase acrylic with final polishing 12 µm grit aluminum oxide for a smear layer.  Blocks were pre-measured for surface microhardness (Vickers 200 g).  Bleach-cycled teeth were bleached with 6.0 % hydrogen peroxide bleaching strips (Crest Whitestrips^® Daily Multicare^TM) 4x/day for a total of 720 treatment cycles.  The bleached group also received bid daily treatment with Crest Regular dentifrice as a 25 % slurry for 5 minutes per day which was followed by a brief surface brushing under tap water.  A cycled-control group received dentifrice treatments only while a third group was saved as a non treated control.  Following 720 exposures teeth were compared for final surface microhardness and also surface roughness measured with Taylor Hobson stylus profilometer.  Results:   VHN measurements for dentin at T=720 treatments:  Bleach = 57±4ab; Cycle/Control = 56±2b; Untreated = 60±4a (a ¹ b p<0.05 Students t).  Surface roughness T=720:  Bleach = 0.041±0.004a; Cycle/Control = 0.048±0.006b; Untreated = 0.055±0.023 (a ¹ b p<0.05 Students t).   Profilometry did not reveal any smear layer etching.     Conclusions:  Dentin surface microhardness and roughness were not significantly changed following simulation of 2 years daily bleaching as compared to non-bleach cycled and untreated specimens.  The cycling protocol resulted in slight smoothing of dentin surfaces as compared to non-cycled and non-treated controls – likely due to smear layer polishing from paste abrasives.