Specimens of human dental enamel were topically applied with solutions of sodium fluoride (NaF) or acidulated phosphate fluoride (APF) before and after laser irradiation. APF application after laser irradiation caused a remarkable increase in acid resistance of the enamel, while APF application before laser irradiation showed a lesser effect, similar to either APF treatment alone or laser irradiation alone. These results were consistent with observations made using a scanning electron microscope. APF application after laser irradiation produced a greater fluoride uptake in the enamel than APF application before laser irradiation, as shown by electron probe microanalyzer. NaF application caused lesser acid resistance and lesser fluoride uptake than APF application, even when the enamel was treated with laser irradiation.

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 μm 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 μm 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.

The effects of normal pulsed Nd-YAG laser irradiation on the acid resistance of human dental enamel of pits and fissures, the cleaning of the pit and fissure contents and fluoride uptake into deep pits and fissures were examined. The acid resistance of the pit and fissure enamel was evaluated by the amount of dissolved calcium per square millimeter of the surface area. The pit and fissure enamel treated with laser irradiation obtained an acid resistance 30% higher than that of the unlased controls. The cleaning effect of laser irradiation on the pit and fissure contents was compared with chemicomechanical and mechanical methods. The laser irradiation was found to clean the pits and fissures to a greater depth without alterating the shape of pits and fissures, compared with the other two methods. The distribution of calcium, phosphorus and fluoride in the enamel of the pits and fissures was then measured by electron probe microanalyzer. At the entrance and in the deep part of the pits and fissures, the fluoride content of the enamel treated with acidulated phosphate fluoride after laser irradiation was higher than that of the enamel treated with acidulated phosphate fluoride alone. These results thus suggest that Nd-YAG laser irradiation might be effective in increasing the acid resistance of the pit and fissure enamel, while removing the pit and fissure debris contents and increasing the fluoride uptake into the pit and fissure enamel.

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