| Research Abstract |
Laser ablation [Er:YAG (Erwin^R, 60mJ, 10pps), CO_2 (Topal^R, 1W, 0.3sec)] of dentin reduced adhesive properties of resin bonding systems. The so-called "laser-etching" might not be effective in promoting the adhesion. By the TEM observations of non-decalcified specimens, such morphological changes as observed in the acid-etched dentin surface could not be detected in the subsurface layer of the lased dentin. It was mechanically weakened due to the formation of structural defects and heat-denatured layer, leading to considerably lower adhesive properties. Significantly lower bond strength was obtained in the wedge-shaped defects, as compared with the flat surfaces. Configuration factor (C-factor) might be an important factor especially for the adhesion to lased dentin.
However, high-energy laser ablation of dentin (Er:YAG : 250mJ, 10pps, CO_2 : 3W, 0.3sec) hardly reduced the bond strength. In the subsurface layer of low-power lased dentin (Er:YAG : 60mJ, 1pps, CO_2 : 1W, 0.3sec), TEM observations of decalcified specimens revealed that the amorphous matrix substances containing denatured collagen fibers were generated (Er:YAG : 3〜4μm, CO_2 : 1〜2μm). On the other hand, in the high power lased dentin (Er:YAG : 250mJ, 1pps, CO_2 : 3W, 0.3sec), such morphological structure as hardly distinguished from sound dentin (Er:YAG) or the subsurface dentin layer having both little amorphous matrix and torn collagen fibers with cross-striation (CO_2) was observed. It was considered that the effects of high-energy laser ablation might be limited to superficial layer. CO_2 laser irradiation (1w, 0.3 sec) of carious dentin produced almost the same morphology with little heat-denatured layer as that by low-speed steel burs.
It was concluded that the amount of irradiation energy and/or the value of c- factor had to be considered when evaluating the adhesive properties. Further investigation was necessary when caries-affected dentin was used as an adherent of resin bonding systems.
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