The effects of fractional CO2 laser, Nano-hydroxyapatite and MI paste on mechanical properties of bovine enamel after bleaching.

Background: This study investigated the effect of post bleaching treatments to the change of enamel elastic modulus and microhardness after dental bleaching in- vitro .

Material and Methods: Fifty bovine incisor slab were randomly assigned into five groups (n=10). The samples were bleached for three times; 20 minutes each time, by 40% hydrogen peroxide. Next it was applied fractional CO2 laser for two minutes, Nano- hydroxy apatite (N-HA) and MI-paste for 7 days and 2 minutes per day. The sound enamel and bleached teeth without post treatment remained as control groups. The elastic modulus and microhardness were measured at three times; 24 hours, 1 and 2 months. Data were statistically analyzed by two-way analysis of variance with 95% confidence level.

Results: Different methods of enamel treatment caused a significant increase in elastic modulus compared to bleached group ( P <0.05). Modulus was significantly increased in 1 and 2 months ( P <0/001: bleach, P = 0/015: laser, P = 0/008: NHA, P =0/010: MI paste) but there were no significantly difference between 1 and 2 months ( P >0.05). There was any significance difference for hardness among treated and control groups, but hardness increased significantly by increasing storage time ( P <0.05).

Conclusions: The use of the protective tested agents can be useful in clinical practice to reduce negative changes of enamel surface after whitening procedures. Key words: Bleaching enamel, CO2 laser, MI pastes, Nano-hydroxy apatite, Microhardness, Elastic modulus.