Influence of Irradiation Conditions of Er:YAG Laser on Bond Strength of Dentin to Resin

Abstract

 Purpose: Er:YAG lasers are clinically applied to specific cases in which laser use is preferable. However, the energy irradiated to dentin is converted to heat in the irradiation area, causing a degradation layer to develop on the dentin surface, and this is considered to decrease the bond strength of adhesive restorations. Details of the degradation layer have not been sufficiently clarified, and so the irradiation conditions and a protocol for manual techniques need to be clarified for the clinical application to adhesive restorative treatment. Therefore, in this study, we evaluated the influence of the irradiation conditions of Er:YAG laser on the bond strength of the dentin surface to resin.
 Materials and Methods: Fresh extracted human molars were cut vertically or horizontally to the tooth axis, and adjusted dentin specimens were used. The influence of the irradiation output of the Er:YAG laser and energy density varying with the tip diameter on the evaporation depth, thickness, and hardness of the degradation layer, and temperature at the dentin irradiation surface was evaluated. Furthermore, we applied the composite resin bonding system to the adhesion of resin to the degradation layer formed under each laser irradiation condition, and evaluated the relationship between the thickness of the degradation layer and bond strength. We also performed prior treatment (chemical treatment) of the degradation surface, and evaluated the conditions that resulted in improved adhesion properties.
 Results: The evaporation depth and thickness of the degradation layer significantly increased when the energy density was high and irradiation was performed in the direction of the orifice of the dentinal tubules. When the energy density was high, the temperature of the dentin surface significantly increased, and the indentation hardness of the dentin surface significantly decreased. Furthermore, the bond strength increased when the energy density was low, and the degradation layer was treated with sodium hypochlorite, a reducing agent, and a mild acid.
 Discussion and Conclusion: The irradiation energy density of the Er:YAG laser and course of the dentinal tubules influenced the evaporation depth, thickness, and hardness of the degradation layer. Furthermore, it was suggested that, in cases of adhesive composite resin restoration, the selection of low energy density and chemical treatment of the degradation layer with sodium hypochlorite and a reducing agent can achieve the same bond strength as dentin cut with rotary cutting instruments.