Acquired Acid Resistance of Human Enamel by Nd:YAG Laser Irradiation

Objective: This is a subsequent study on acquired acid resistance by a CO2 laser irradiation presented at the previous IADR(2006), on the basis of our previous studies (1997~2002) dealt with the alterations of the HAP (Hydroxyapatite) caused by the laser irradiation. The aim was to clarify how the lased teeth acquire acid resistance by Nd:YAG laser irradiation, analyzing the lased teeth before and after soaking in acid solutions. Methods: 16 extracted human molars were used, of which irradiated enamel surfaces were applied with Chinese ink. Two types of Nd:YAG laser (λ=1.06 µm) were used :(1)A cw Nd:YAG laser (OSADA,N-40), spot size 1mm, 5W and 8W for 2 seconds, (2)A pulsed Nd:YAG laser (SOKKIA, SPL-7200), spot size 0.32mmm, 50~200mJ, pulse-width 90µsec, 10~30pps, 4 shots. The irradiated teeth were cut longitudinally and reduced to about 150µm by grinding. The specimen were observed by a polarizing microscope, then analyzed by a MDG (RIGAKU/PSPC-MDG) equipped with a collimator 100µm in diameter at 50kV, 200mA. After the analysis, the specimens were soaked in 2% lactic acid solution(pH=2.09~2.19) for 8 hrs and the substances remained unsolved were identified by the MDG. Results: Before soaking in the acid solution; under the polarizing microscopic observation, there were diffusely altered regions around the irradiated central region. From the analysis by the MDG, a mixture of high-temperature phases of HAP such as α- TCP[Ca3(PO4)2] and semi-high temperature phases such as β-TCP, PYR[Ca2P2O7], MET[Ca(PO3)2] were identified in the altered central regions. As for these thermal effects,the pulsed laser caused less effects than those of the cw laser. Conclusion: After soaking in the acid solution; almost all the enamel solved except the altered regions remained unsolved after soaking in the acid solutions. In the regions remained unsolved, the semi-high temperature phases of PYR and MET were identified predominantly instead of the high-temperature phases.