Ultrastructural properties of laser-irradiated and heat-treated dentin

Previous studies using scanning electron microscopy and infrared absorption spectroscopy reported that laser irradiation causes compositional changes in enamel. The purpose of this study was to evaluate the ultrastructural an d compositional changes in dentin caused by irradiation with a short-pulse laser (Q-switched Nd:YAG). The irradiated and non-irradiated areas of the l ased dentin samples were investigated by scanning (SEM) and transmission el ectron microscopy (TEM), micro-micro electron diffraction, and electron mic roprobe analysis of dispersive energy (EDX). Heat-treated dentin was simila rly investigated. This study demonstrated that laser irradiation resulted i n the recrystallization of dentin apatite and in the formation of additiona l calcium phosphate phases consisting of magnesium-substituted p-tricalcium phosphate, beta-TCMP, beta-(Ca,Mg)(3)(PO4)(2), and tetracalcium phosphate, TetCP, Ca-4(PO4)O. TEM analyses of the modified and unmodified zones of th e irradiated areas showed two types of crystal populations: much larger cry stals from the modified zone and crystals with size and morphology similar to those of dentin apatite in the unmodified zone. The morphology of crysta ls in the modified zones in the irradiated dentin resembled those of dentin sintered at 800 or 950 degrees C. In the irradiated areas (modified and un modified zones), the Ca/P ratio was lower compared with that in the non-irr adiated dentin. The Mg/Ca ratio in the modified zones was higher than that in the unmodified zones and in the nonirradiated dentin. In sintered dentin , the Mg/Ca ratio increased as a function of sintering temperature. The ult rastructural and compositional changes observed in laser-irradiated dentin may be attributed to high temperature and high pressure induced by micropla sma during laser irradiation. These changes may alter the solubility of the irradiated dentin, making it less susceptible to acid dissolution or to th e caries process.