Water spraying is usually used in the removal of hard dental tissue with laser for the clinical cavity preparation which is previous to the prosthesis. However, the effect of water on the dental tissue irradiated with laser has not been evaluated quantitatively. In this paper, the cavity preparation is performed for human dentin tissue with Nd:YAG laser and Er:YAG laser to investigate the effects of water. The thermal damages caused on dentin tissue are microscopically observed and the perforatability is evaluated from the dimensions of cavity perforated. The thermal damages caused on dentin tissue is reduced by water, especially very fine cavity can be achieved in the cavity preparation with Er:YAG laser, while the perforatability with laser beam is decreased by water. The audible sound pressure level is also measured to investigate the feasibility of sound monitoring as the removal-monitoring method. It is found that the sound pressure level increases linearly with the extent of removed dental tissue.
