Morphological study of dentin irradiated by solid-state ultraviolet lasers: a comparison of wavelength and repetition rate.

Objective: In this study, we examined the surface morphology of dentin after being ablated by the third, fourth, and fifth harmonics of the Nd:YAG laser. The influences of the repetition rate with respect to the wavelength were also investigated.

Background data: Typically, excimer lasers have been used as the primary sources of ultraviolet (UV) laser wavelengths to investigate laser ablation of dentin. In the past decade, developments in nonlinear optical technology have given rise to higher conversion efficiencies of the fourth and fifth harmonics of the Nd:YAG laser. To this end, sufficient energy densities of the fourth and fifth harmonics of the Nd:YAG laser have been generated to ablate dentin.

Materials and methods: Thin dentin samples (typically 1 mm) were irradiated by the third, fourth, and fifth harmonics of the Nd:YAG laser. Ablation occurred at repetition rates of 1, 5, 10, and 20 Hz, using similar energy densities for each laser wavelength. An environmental scanning electron microscope was used to assess the resultant surface morphology.

Results: Dentine surfaces after 355-nm ablation exhibited plugging at each repetition rate. Similar surfaces were exhibited after 266-nm ablation. "Plugging" over dentine tubules was less obvious after 213-nm laser ablation. The results may highlight the impact of different absorption characteristics of each wavelength.

Conclusion: Solid-state UV laser ablation of dentin exhibits similar properties to excimer laser ablation. At similar energy densities, the deeper UV laser wavelengths exhibit less "plugging" of dentin tubules, suggesting a lower thermal impact.