To elucidate the structural changes caused in human dental enamel by laser irradiation, the optical properties and permeability of heated human enamel and lased enamel after treatment with various solutions were investigated. Heated enamel at 400°C for 24 hours acquired the highest resistance to acid decalcification and showed a high positive birefringence which was similar to that of lased enamel. Carbolic fuchsin, acidulated phosphate fluoride (APF), and calcifying fluid did not penetrate the heated enamel, though those solutions penetrated the lased enamel well. Changes in birefringence after treatment with lactate buffer was not observed in the heated enamel, but was observed in the lased enamel.
APF penetrated the lased enamel well and the affected area had an optically isotropic appearance (R region). Carbolic fuchsin could not penetrate the APF-treated area of lased enamel. However, the high positive birefringence in the layer below the R region changed to a low negative one after treatment with lactate buffer for 3 hours. Slight surface decalcification was also observed on the surface of the R region at this time.
These results suggested that the structural changes in lased enamel are different from those of heated enamel. It appears that lased enamel has micropathways through which solutions can penetrate into the enamel, and also microspaces which could act as sites of deposition for ions released by acid decalcification.
