Objective:The purpose of this study was to investigate morphological changes of dentine after exposure to Nd: YAC laser. Methods: The second maxilla and mandible molar was selected, and slices with 1.5 mm thickness were prepared from each tooth by suing a high-speed bur. Two horizontal sections of shces were obtained, one at the occlusion enamel-dentinal junction and the other from the position of 1.5 mm lower than the enamel-dentinal junction. A square of 3 mm x 4 mm area on each of the four dentinal slices was irradiated under the laser with four different parameters (1 W, 10 Hz, 100 mj and with different time of 5 s, 10 s, 15s, 20s). After the specimens became dry and covered with gold, the morphological changes of each slice were observed with a scanning electronic microscope (SEM). Results: Laser with the above four parameters blocked dentinal tubule orifices. With the time of irradiation increased, the melting level and sealing depth of dentinal slices increased, but crack and pit were found in the region of irradiation. Conclusion: The density of 41.6 J/cm (10 Hz, 100 mj) can blocked dentinal tubule orifice, and will not cause large crack and pit structure.
