Thu! purpose of this study was to examine the combined effects of a pulse d NdYAG laser and acidulated phosphate fluoride(APF) on acid resistance of bovine tooth enamel. The materials were 54 freshly extracted bovine permanent teeth with intact smooth enamel surfaces and sectioned sagitally and horizontally for 214 experimental specimens.
To determine the most effective energy density of laser in increasing acid resistance of tooth enamel, the authors divided 70 specimens into 7 groups including unlaced control and 6 experimental groups. Among them, 60 specimens for 6 experimental groups were exposed to a pulsed NdYAG laser under conditions ranged from 10 to 60 J/cm2 with an increment of 10J/cm2. Total 70 specimens for unlaced control and 6 experimental groups were etched with lml of 05N HC1O4 for 30sec at room temperature and then the amount of dissolved calcium in the acidic solution was determined with an atomic absorption spectrophotometer. Scanning electron microscopic(SEM) observations were also made to examine the enamel surfaces of the lased teeth with 14 specimens for unlased control and 6 experimental groups.
To survey the combined effects of a pulsed NdYAG laser and APF on acid resistance of enamel, the authors divided 90 specimens into 9 groups according to treatment method(APF only, APF after lasing, lasing after APF) and fluoride application time(4min, 60min, 24hr). After determination of the most effective laserAPF combination method on acid resistance by the amount of dissolved calcium in the acidic solution, the rest 40 specimens for SEM were divided into 20 groups according to treatment method(untreated control, APF only, the most effective laser+ APF imethod) and etching time(nonetching, 15, 30, 60, 90 sec). And then, the surfaces of each treated specimens were also observed with SEM.
The results were as follows :
1. The acid resistance of enamel surface was increased significantly compared to untreated control at the laser energy density of 40J/cm2 and 50J/cm2, but more increased at 50J/cm2 without statistical difference between both groups. Laser energy density of 40J/an´ produced the almost uniform fusion of the lased enamel surface, while that of 5OJ/cm2 produced the fusion of enamel surface and fine cracks of a mosaic pattern.
2. In the groups etched for 90 sec, the enamel surface of lasing(50J/cm2) only group showed its prism structure as clearly as in the control, APF only group. But a part of enamel structure was still resistant to acid in the group treated with APF after lasing.
3. Iaxnger incubation with APF did bring about an increase in acid resistance but there was no significant increase in acid resistance from 4min treated group with the same treatment method except the group treated ApF for 24hr after lasing(50J/cm2).
4. The group of APF application after lasing(50J/cmz) caused a remarkable increase in acid resistance of the ena. . meL while the group of lasing after APF application showed a lesser effect similar to either APF only or lasing only group.
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