Abstract
By using the normal pulse and the AO Q-switched pulse of Nd-YAG laser, an extent of resistance of the dental enamel to acid demineralization with a intensity of laser irradiation was measured. The depth of enamel which obtain the acid resistance by the AO Q-switched pulse laser is thicker than by the normal pulse laser. Consequently, some cracks as to reach the dentin by the AO Q-switched pulse and fine surface mosaic-like pattern by the normal pulse was occured. On the other hand an increment of the temperature of the lased enamel was calculated by a formula. It makes clear that the enamel crack by the AO Q-switched pulse and the surface change of enamel by the normal pulse was caused by an increasing temperature of deeper and shallower portion of enamel, respectively. Therefore, the normal pulse is more suitable than the AO Q-switched pulse from the view point of both the acid resistance and the crack formation of the enamel. From the calculation analysis it found that the optimum irradiation intensity by the normal pulse to obtain the acid resistance without the damage of enamel structure is 50 J/cm2. In this study it suggested that three conditions to attain the acid resistance of the enamel surface by the laser irradiation are as follows: 1) an increment of surface temperature must be above 300°C, 2) the surface temperature must not exceed 1,000°C, 3) zone of increased temperature above 300°C must be limited under 30μm thick to avoid the crack.
