Abstract
Since Sognnaes and Stern [J. S. Calif. Dent. Assoc. 33 (1965) 328] found that lased teeth had acquired acid resistance, different and competing explanations for its mechanism have been proposed. Human teeth were irradiated under the following conditions: a CO 2 laser, 2, 5 W for 0.5 s; a Nd:YAG laser, 5, 10 W for 2 s. An Er:YAG laser, 150, 250 mJ, 2 Hz, 200 pulses. The irradiated teeth were reduced to about 150 μm. The specimens were observed by a polarizing microscope, then analyzed by a Micro X-ray diffractometer at 50 kV, 200 mA. Lased enamel surfaces showed different morphological appearance by the laser. From the analysis by the Micro X-ray diffraction, the hydroxyapatite (HAP) crystals were altered to high temperature phases and semi-high temperature phases. It was noteworthy that calcium pyrophosphate (PYR) and calcium metaphosphate (MET) were detected from the irradiated central regions to its surrounding altered regions. PYR is known to be soluble in acids, however, strongly inhibits the dissolution of HAP crystals. MET is known to be very insoluble in acids due to the stout long-chain chemical structure. Considering together with the previous experimental evidence, it is probable that the formation of PYR and MET in lased teeth would have an effect on reducing solubility rate.
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