| Neodymium: yttrium-aluminum-garnet laser( Nd: YAG laser) has been widely used in dentistry, especially in incision and hemostasis of oral soft tissue. In recent years, many studies were carried out to investigate its possible applications in operative dentistry and endodontics. In caries prevention, however, most researches were focused on the effects of low-energy lasers irradiation and most experiments were in vitro. We still have a long way to go to have laser irradiation a routine therapy in caries prevention. In this study, human enamel blocks were exposed to different energy dose of laser irradiation (energy density varied from 1 OOJIcm2 to I 000JIcm2). After that the processes of demineralization and remineralization of enamel were investigated to determine the influence of laser irradiation on them. Morphological and mechanical changes of enamel were also studied. In animal experiment, we used rats to set up a caries model and observed the effects of low energy laser on caries process. From the results of these experiments we tried to find a suitable parameter of laser irradiation for clinical application. In the first experiment, human enamel blocks were exposed to 3 different energy dose of laser irradiation (energy density: IOOJ/cm , 500J/cm and 10001/cm2, respectively) and then were immersed in demineralizat ion solution for 14 days. By microhardness test we found that the volumes of mineral loss in irradiation groups were much lower than that of control group and the percent inhibition of demineralization were 37.25%.. 70.36% and 82.01% respectively. The results support the theory that laser irradiation can inhibit the process of demineralization and enhance acid resistance of enamel, and its effect become obvious when higher energy dose is used. The effect of inhibition appeared earliest in low energy group (in the 1st hour), but became insignificant very soon. It appeared later in middle and high energy groups (from 2nd to 4~?hour) and lasted for a longer time. In the remineralization test, 3 different energy dose of laser irradiation were applied on the enamel blocks that had been partially demineralized. After remineralized for 14 days, the blocks were taken out and tested for their volume of mineral. The results showed that the volumes of mineral loss in irradiation groups were much lower than that of control group and the percent inhibition of demineralization were 122.89%, 69.22% and 56.14% respectively. Remineralization rate seemed to be the highest when irradiated with low energy dose of laser. The results suggest that low energy laser irradiation be helpful in remineralization therapy. After laser irradiation, the surface of enamel became rough with the formation of glaze-like material and craters. Deep fissures appeared in high-energy group. With the increment of irradiation dose, the hardness of enamel decreased and its abrasion rate increased significantly. Enamel hardness dropped to as low as 85.1%.. 52.1% and 41.2% of normal level respectively after laser irradiation. In abrasion test, R value (abrasion rate) in low energy group(0.24978mm3/cm2) was slightly lower than that of control group, but the difference was statistically insignificant. However, the values in middle and high energy groups (0.9064mm3/cm2 and 1.7 1337 mm3/cm2) were much higher than that of control group, reaching 2.025 and 3.830 times of the latter. The increment of abrasion rate has adverse influence on the long-term c...
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