Bleach Effects on Dental Enamel Surface Hardness and Roughness

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 applied an accelerated exposure model to evaluate tooth surface reactivity of a novel bleaching gel containing hydrogen peroxide and sodium pyrophosphate.   Methods: Bovine molars were cross sectioned and blocks were mounted in Durabase acrylic with final polishing on gamma alumina.  Blocks were pre-measured for surface microhardness (Buehler - Vickers diamond 200 g load) and surface roughness (Taylor Hobson stylus profilometer).  Specimens were allocated to one of 4 treatment groups (N=4):  non treated control (stored humid at 37°C);  and three groups which were exposed to treatments in 4 – 2 hour cycling blocks with pooled saliva immersion in between:  toothpaste treated control (25 % Crest® Regular flavor paste/water slurry placed on surface fresh for 4 consecutive 30 min. periods);   Crest Whitestrips^® Daily Multicare^TM  strips – applied directly on the enamel surface for 4 consecutive 30 min. periods;  experimental H₂O₂ + pyrophosphate bleaching strip applied directly on the tooth surface for 4 consecutive 30 min. periods.  Post treatment (8 hour exposure) teeth were compared for final surface microhardness and also surface roughness.  Results:   Treatment groups showed nsd in hardness or roughness at baseline.  Post treatment VHN:  NonTreat=320±17a; PasteOnly=320±1a; DayStrip=287±2b; H₂O₂/PyroStrip=309±5ab. Post treatment surface roughness:  NonTreat=0.076±0.031a; PasteOnly=0.074±0.040a; DayStrip=0.087±0.013a; H₂O₂/PyroStrip=0.098±0.009a.  a ¹ b p<0.05 Students t.   Conclusions:  An accelerated exposure model permitted the rapid assessment of bleach strip reactivity with enamel surfaces.  Bleaches proven passive to dental enamel in chronic (realistic) cycling (Daystrip at 2 years use) produced a good dynamic range for test comparisons (small ns hardness decrease and roughness increase).  A H₂O₂/PyroStrip was found to be equivalent to standard day-strip in effects on surface hardness and roughness in vitro.