Purpose: The aim of this in vitro study was to evaluate the microtensile bond strength (μTBS) and hybrid layer morphology of different adhesive systems, either followed by treatment with Nd:YAG laser irradiation or not. Previous studies have shown the effects of Nd:YAG laser irradiation on the dentin surface at restoration margins, but there are few reports about the significance of the irradiation on the hybrid layer. Materials and Methods: The flattened coronal and root dentin samples of 24 bovine teeth were randomly divided into 8 groups, according to the adhesive system used – Scotchbond Multi Purpose (SBMP) or Clearfil SE Bond (CSEB) – and were either irradiated with Nd:YAG or not, with different parameters: 0.8 W/10 Hz, 0.8 W/20 Hz, 1.2 W/10 Hz, 1.2 W/20 Hz. The left sides of specimens were the control groups, and right sides were irradiated. A composite crown was built over bonded surfaces and stored in water (24 h at 37°C). Specimens were sectioned vertically into slabs that were subjected to μTBS testing and observed by SEM. Results: Control groups (27.81 ± 1.38) showed statistically higher values than lased groups (21.37 ± 0.99), and CSEB control group values (31.26 ± 15.71) were statistically higher than those of SBMP (24.3 ± 10.66). There were no significant differences between CSEB (20.34 ± 10.01) and SBMP (22.43 ± 9.82) lased groups. Among parameters tested, 0.8 W/10 Hz showed the highest value (25.54 ± 11.74). Nd:YAG laser irradiation caused dentin to melt under the adhesive layer of both adhesive systems tested. Conclusion: With the parameters used in this study, Nd:YAG laser irradiation of the hybrid layer promoted morphological changes in dentin and negatively influenced the bond strength of both adhesive systems.

Keywords: adhesion, dentin, microtensile bond strength, Nd:YAG laser, scanning electron microscopy