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.