Dentin Ablation with Free Electron Laser

Abstract

Purpose: Since a free electron laser (FEL) not only has wavelength variability but also can irradiate a micropulse having an ultrashort pulse width, FEL can provide useful information on the effect of differences in pulse structure on irradiated materials, by irradiating laser beams tuned to the same wavelength as another laser for testing but with a different pulse structure. Methods: Consequently, the present study focused on the effect of ultrashort pulses emitted by an FEL, and a high-speed camera was used to observe the aspects of bovine dentin transpiration using the FEL tuned to the same wavelength as erbium-doped yttrium aluminum garnet (Er: YAG) lasers (2.94μm). The ejected transpiration particles were also observed using a scanning electron microscope (SEM), and the depth of transpiration was compared with that obtained using an Er: YAG laser. Additionally, irradiation was performed on bovine dentin with varying FEL irradiation wavelengths of 2.50μm, 2.94μm, 3.50μm, and 4.00μm, the depth of transpiration was measured, and increases in temperature at the irradiated site (irradiated side) and the opposite side of the sample directly under the irradiated site (dental pulp side) were also measured using a radiation thermometer. Result: 1. From 8,000 fps high-speed camera images of the bovine dentin at the time of ablation, a scattering of dentin particles was confirmed in the case with the irradiation by Er: YAG laser, and the SEM images of the collected ejected particles produced by Er: YAG laser irradiation showed dentin particles with sizes in the range of 10 to 100μm. On the other hand, in FEL, no scattering of ejected particles was observed with high-speed camera images taken with a 50-power lens. However, many microscopic particles with sizes in the range of 10 to 20μm were found with SEM. 2. Under the conditions of the experiment, the depths of transpiration exhibited by the Er: YAG laser and the FEL were 208μm and 670μm, respectively, with the latter being three times deeper. 3. The depths of transpiration under varying FEL wavelengths of 2.50μm, 2.94μm, 3.50μm, and 4.00μm were 35μm, 671μm, 414μm, and 220μm, respectively, demonstrating that the depth of transpiration was significantly larger at the wavelength of 2.94μm. 4. The peak temperature increases under varying FEL wavelengths of 2.50μm, 2.94μm, 3.50μm, and 4.00μm were 2.4℃, 2.2℃, 2.0℃, and 2.2℃ on the irradiation side, and 1.1℃, 1.1℃, 1.3℃, and 1.3℃ on the dental pulp side, respectively. Conclusion: These results showed that dentin could be ablated effectively by using a laser, having a pulse width shorter than the thermal relaxation time and an approximate wavelength of 2.94μm.