Permanent and transient changes in the reflectance of CO2 laser-irradiated dental hard tissues at lambda = 9.3, 9.6, 10.3, and 10.6 microns and at fluences of 1-20 J/cm2.

BACKGROUND AND OBJECTIVE: Effective use of lasers for preventive dental treatments requires accurate knowledge of the amount and distribution of laser energy deposited during irradiation. At CO2 wavelengths, the reflection losses are considerable and reduce the laser energy absorbed by the tissue surface. STUDY DESIGN/MATERIALS AND METHODS: The specular and diffuse reflectance of enamel and dentin were measured at the 10.6-, 10.3-, 9.6-, and 9.3-microns wavelengths of the CO2 laser. Changes in reflectance during and after laser irradiation were investigated. RESULTS: The low-fluence reflectance (< 1 J/cm2) of calcified dental tissues at CO2 wavelengths varies between 9% and 50%. Permanent and transient changes in the reflectance are induced at higher irradiation intensities. CONCLUSION: These changes resulted in increased energy coupling during irradiation.

Scanning electron microscope observations of CO2 laser effects on dental enamel.

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.

Nature of light scattering in dental enamel and dentin at visible and near-infrared wavelengths.

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%).

Proton-induced physicochemical calcium release from ceramic apatite disks.

When bone is cultured in acid medium there is net calcium efflux (JCa) and proton influx (JH) relative to the mineral. The acid medium appears to induce physicochemical mineral dissolution as well as cell-mediated bone resorption. To determine the independent effect of acid medium on physicochemical dissolution, we utilized cell-free synthetic ceramic apatite (CAP) disks, which contain carbonate (5.5%) in an apatite structure chemically similar to mammalian bone. CAP disks were cultured in control (Ctl, pH approximately equal to 7.44) or acid (Met, pH approximately equal to 7.11) medium for 48 h and compared to similarly treated neonatal (4-6 days old) mouse calvariae. Medium was changed and analyzed at 3, 24, and 48 h. At 3, 24, and 48 h there was significantly greater JCa from the CAP disks and calvariae incubated in Met compared to Ctl; over the entire 48 h time period there was a greater progressive increase in JCa from the CAP disks than the calvariae incubated in Met. There was no significant JCa at 3, 24, or 48 h from CAP disks or calvariae incubated in Ctl. At 3 h there was significantly greater JH into the CAP disks and calvariae incubated in Met compared to Ctl; JH was greater into the CAP disks than the calvariae. Utilizing a synthetic model of bone mineral we demonstrated that acid medium induces physicochemical calcium efflux and proton influx relative to the mineral.