The light-scattering properties of dental enamel and dentin were measured at 543, 632, and 1053 nm. Angularly resolved scattering distributions for these materials were measured from 0° to 180° using a rotating goniometer. Surface scattering was minimized by immersing the samples in an index-matching bath. The scattering and absorption coefficients and the scattering phase function were deduced by comparing the measured scattering data with angularly resolved Monte Carlo light-scattering simulations. Enamel and dentin were best represented by a linear combination of a highly forward-peaked Henyey-Greenstein (HG) phase function and an isotropic phase function. Enamel weakly scatters light between 543 nm and 1.06 µm, with the scattering coefficient (µ(s)) ranging from µ(s) = 15 to 105 cm(-1). The phase function is a combination of a HG function with g = 0.96 and a 30-60% isotropic phase function. For enamel, absorption is negligible. Dentin scatters strongly in the visible and near IR (µ(s)≅260 cm(-1)) and absorbs weakly (µ(a) ≅ 4 cm(-1)). The scattering phase function for dentin is described by a HG function with g = 0.93 and a very weak isotropic scattering component (˜ 2%).

The anti-caries activity of fluoride is contributed to in several ways. Two major aspects of fluoride action are (i) the inhibition of demineralization at the crystal surfaces within the tooth, and (ii) the enhancement of subsurface remineralization resulting in arrestment or reversal of caries lesions. Fluoride present in the aqueous phase at the apatite crystal surface may play a determining role in the inhibition of enamel or dentin demineralization. In one part of the present study, the initial dissolution rate of synthetic carbonated-apatite in acetate buffers was measured with fluoride present in the buffer in the 0-2.6 mmol/L (0-50 ppm) range. Inhibition of demineralization was shown to be a logarithmic function of the fluoride concentration in solution. In the second part of the present study, an in vitro pH-cycling model was used for determination of the effect on net de/remineralization of enamel by treatment solutions containing fluoride in the 0-26 mmol/L (0-500 ppm) range. The net mineral loss was shown to be negatively related to the logarithm of the fluoride concentration. These studies have demonstrated an exponential quantitative relationship between fluoride concentration and inhibition of apatite demineralization or enhancement of remineralization. The clinical implications are (i) that simply increasing fluoride concentration may not necessarily give increased cariostatic benefit, and (ii) that improving the means of delivery of relatively low fluoride concentrations for longer times should be more appropriate for enhancing clinical efficacy.

Studies of the effects of carbon dioxide (CO2) lasers on dental enamel have demonstrated that surface changes can be produced at low fluences (< 10 J/cm2) if wavelengths are used which are efficiently absorbed by the hard tissues. In this study, scanning electron microscopy (SEM) was used to characterize the wavelength dependence of surface changes in dental enamel after exposure to an extensive range of CO2 laser conditions. Bovine and human enamel were irradiated by a tunable, pulsed CO2 laser (9.3, 9.6, 10.3, 10.6 microns), with 5, 25, or 100 pulses, at absorbed fluences of 2, 5, 10, or 20 J/cm2, and pulse widths of 50, 100, 200, 500 microseconds. SEM micrographs revealed evidence of melting, crystal fusion, and exfoliation in a wavelength-dependent manner. Crystal fusion occurred at absorbed fluences as low as 5 J/cm2 per pulse at 9.3, 9.6, and 10.3 microns, in contrast to no crystal fusion at 10.6 microns (< or = 20 J/cm2). Longer pulses at constant fluence conditions decreased the extent of surface melting and crystal fusion. The total number of laser pulses delivered to the tissue did not significantly affect surface changes as long as a minimum of 5 to 10 pulses was used. Within the four easily accessible wavelengths of the CO2 laser, there are dramatic differences in the observed surface changes of dental hard tissue.

The mineral profiles of artificial caries-like lesions formed in intact human dental enamel and pretreated with pulsed, infrared laser radiation were determined using a longitudinal microhardness technique. Laser pretreatment with 100–200 ns pulses with 10–50 J cm^––2 energy densities resulted in peak power densities of 107–108 W cm^––2 and caused significant inhibition of lesion formation with the greatest inhibition occurring at the highest pulse energies. In some cases, laser-treated enamel produced lesions that were 50% less demineralized than the controls. The inhibitory effect was also wavelength dependent with two of the four wavelengths chosen (9.32 and 10.59 μm) being more active than the other two (9.57 and 10.27 μm), although the 9.32 μm wavelength was the most effective.

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