Ultrastructural change of enamel exposed to a normal pulsed Nd-YAG laser

The ultrastructural changes of the enamel surface occurring due to normal pulsed Nd-YAG laser irradiation were examined by scanning electron microscopy. The melted and recrystallized enamel in the lased enamel surface was observed. A coalescence of enamel globules was caused by 10 pulses of laser irradiation. In the case of excessively repeated irradiation (3 applications of 10 pulses), new particles of enamel crystal were produced which had a larger size than the original and measured about 0.2-1.0 microns in diameter. In some portions, large-sized hexagonal particles were also observed. In fractured surfaces of enamel, the shapes of the new particles of enamel crystal varied and included granules, needles and columns. The column structure had various appearances such as being well packed, along with piled granules and hollow columns, as well as having intercrystal or intracrystal voids. It was suggested that a distinctive polygonal column shape of more than 2 microns in length might be formed in the process of homogeneous melting at high temperatures followed by uniform cooling and contraction. The recrystallized layer demonstrated significant acid resistance but was a little fragile. These results greatly support the hypothesis that a larger crystal size may cause acid resistance in enamel exposed to pulsed laser.