THE THRESHOLD EFFECTS OF ND AND HO-YAG LASER-INDUCED SURFACE MODIFICATION ON DEMINERALIZATION OF DENTIN SURFACES

Laser irradiation alters the structure of dentin and produces surface layers that give the appearance of being more enamel-like. The laser-m odified surface may be more resistant to demineralization; hence, many investigators are proposing continued development of the laser as a p ossible preventive treatment for caries. The purpose of this study was to explore the morphological changes that occur in dentin when treate d at threshold illuminance with two clinically interesting laser wavel engths, and to evaluate the effectiveness of the laser-treated surface at resisting demineralization in an acid-gel solution. The Nd:YAG las er (wavelength 1060 nm) produced significant recrystallization and gra in growth of the apatite, without the formation of second phases such as beta-tricalcium phosphate. This recrystallized surface layer showed resistance to demineralization; however, the layer did not provide pr otection of the underlying dentin from demineralization because of cra cks and macroscopic voids that allowed for penetration of the deminera lizing gel. The Ho:YAG laser-treated surface (wavelength 2100 nm) did not show significant evidence of recrystallization and grain growth, a nd only a trace amount of an acid-resistant layer was observed with de mineralization. It is speculated that the Ho:YAG laser is coupling wit h absorbed water, and that the heat transfer from the water to the min eral phase is inefficient. For the purposes of creating a demineraliza tion-resistant layer, threshold illuminance with both Nd:YAG and Ho:YA G was ineffective.