Effect of Pulsed Nd:YAG Laser Irradiation on Acid Demineralization of Enamel and Dentin

Abstract

Objective: The purpose of this study was to investigate the effect of Nd:YAG laser irradiation on the acid demineralization of enamel and dentin by spectrophotometry. A mechanism of acquired acid resistance is also proposed. Summary Background Data: The ability of Nd:YAG laser irradiation to the enhanced resistance to artificial caries formation is still controversial. Methods: A pulsed Nd:YAG laser at 1.064-µm wavelength was used to irradiate the human enamel and dentin samples from 20 extracted human molars at the parameters of 1, 2, and 3 W and 20 pps for a total of 9 sec after painting with black ink. Samples were then subjected to 2 µl of 0.1 M lactic acid solution (pH 4.8) for 24 h at 36°C. The parts per million (ppm) of calcium ion (Ca²⁺) dissolved in each solution was determined by atomic absorption spectrophotometry and the morphological changes were also investigated by scanning electron microscopy (SEM). Results: The lowest mean Ca²⁺ ppm was recorded in the samples irradiated at 3 W, in those by irradiated at 2 W and 1 W. The unlased samples showed the highest Ca²⁺ ppm. SEM observation showed that in the lased areas, the smear layer was partially melted and the underlying primary enamel or dentin seemed to be thermally degenerated. Conclusions: The results of this study suggested that melted smear layer and underlying enamel or dentin degenerated thermally by the heat treatment of Nd:YAG laser might play a major role to enhance resistance to artificial caries-like formation.