Objective: The purpose of this study was to evaluate the hardness of the adhesive interface between resin and Er:YAG laser-irradiated bovine dentin by nano-indentation.
Background data: It has been reported that laser output energy and pulse repetition rate affect the tensile bond strength in Er:YAG laser-irradiated dentin.
Materials and methods: Three laser settings were evaluated at the same total energy level (approximately 1.0 W): 100 mJ/pulse-10 pps (100-10), 50 mJ/pulse-20 pps (50-20), and 33 mJ/pulse-30 pps (33-30). Laser-irradiated dentin in each group was conditioned with 10% citric acid solution containing 3% ferric chloride for 15 sec, rinsed with distilled water for 30 sec, and bonded to PMMA rods with 4-META/MMA-TBB resin. The bonded specimens were sectioned vertically, embedded in epoxy resin, and their nano-hardness measured. A non-irradiated control group was also investigated.
Results: The adhesive resin interface in the controls showed the lowest level of hardness, which gradually increased from the top of the hybrid layer (0 microm) through the bottom of the hybrid layer (5 microm) and into the underlying dentin ( approximately 10 microm). Significant differences in hardness were observed between the 5 microm point in the controls, the 10 microm and 15 microm points in the 100-10 group, and the 10 microm point in the 50-20 and 33-30 groups.
Conclusion: The results suggest that laser settings affect hybrid layer thickness, even when the total energy level is constant.