Richard E. Glena

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%).

Dependence of in vitro Demineralization of Apatite and Remineralization of Dental Enamel on Fluoride Concentration

The Anticaries 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 del 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.

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 μm), 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 us. 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 μm, in contrast to no crystal fusion at 10.6 pm (≤ 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.

Effect of pulsed low energy infrared laser irradiation on artificial caries-like lesion formation

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 tech

Crystal morphology, composition, and dissolution behavior of carbonated apatites prepared at controlled pH and temperature

A range of carbonated apatites was prepared by aqueous precipitation at 37, 60, and 85°C and at controlled pH values varying from 6.00–9.50 in 0.25 increments. The products were analyzed for Ca, P, Sr, Mg, Na, F, and carbonate. Their initial dissolution rates were measured in a pH 4.5, 0.01 mol · liter−1 acetate buffer. Information about crystal morphologies and crystal defects was obtained by x-ray diffraction and high-resolution transmission electron microscopy (TEM). The CaP molar ratios of the products, together with their Sr and Mg contents, increased with increasing pH. Initial dissolution rates of the products, when adjusted for carbonate content, were in the order 37 > 60 > 85°C whereas apparent particle sizes determined by TEM and x-ray diffraction were ordered 37 < 60 < 85°C. Carbonated apatites precipitated at pHs of 7.0 or less were observed to have planar defects parallel to (100) that were identified as unit-cell-thick intergrowths of octacalcium phosphate. Carbonated apatites precipitated at higher pHs and noncarbonated apatites did not have these defects. A crystal growth mechanism is proposed to account for the presence of the (100) defects.

Infrared radiometry of dental enamel during Er: YAG and Er:YSGG laser irradiation

Time-resolved infrared (IR) radiometry was used to measure surface temperatures during pulsed Er:YSGG (l=2.79 mm) and Er:YAG (l=2.94 mm) laser irradiation of dental enamel. Scanning electron microscopy (SEM) was used to determine the melting and vaporization thresholds and to characterize other changes in the surface morphology. The magnitude and temporal evolution of the surface temperature during multiplepulse irradiation of the tissue was dependent on the wavelength, fluence, and pre-exposure to laser pulses. Radiometry and SEM micrographs indicate that ablation is initiated at temperatures well below the melting and vaporization temperatures of the carbonated hydroxyapatite mineral component (1200 °C). Ablation occurred at lower surface temperatures and at lower fluences for Er:YAG than for Er:YSGG laser irradiation: 400 °C vs. 800 °C and above 7 J/cm2 vs. 18 J/cm2, respectively. However, the measured surface temperatures were higher at l=2.79 mm than at l=2.94 mm during low fluence irradiation (<7 J/cm2). Spatially dependent absorption in the enamel matrix is proposed to explain this apparent contradiction.

Permanent and transient changes in the reflectance of CO2 laser‐irradiated dental hard tissues at λ = 9.3, 9.6, 10.3, and 10.6 μm and at fluences of 1–20 J/cm2

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.

Thermal response of hard dental tissues to 9‐ through 11‐μm CO2‐laser irradiation

The morphology and the chemistry of dental enamel and dentin can be modified by irradiation with a CO2 laser to increase the acid resistance of the intrinsic mineral. The changes induced in hard dental tissues after laser irradiation are predominately determined by the photothermally induced temperature rise in the tissue. Therefore the temperature rise in the irradiated enamel and dentin must be determined under controlled laser conditions. Surface and subsurface temperatures were monitored after multiple-pulse CO2-laser irradiation at ?=9.3, 9.6, 10.3, and 10.6 µm with 1- to 20-J/cm2 pulses of 50- to 500-µs duration using radiometry and microthermocouples. Surface temperatures were significantly higher after 9.3- and 9.6-µm irradiation than for the more commonly utilized 10.6-mm CO2-laser wavelength. Permanent changes in the temperature response of enamel and dentin were observed at fluences greater than 2 J/cm2 and 100-µs duration for dentin and 5 J/cm2 for enamel. CO2-laser irradiation changes the thermal and the optical properties of these tissues, substantially changing the energy deposition for subsequent laser pulses. These changes affect both the amount of energy absorbed and the depth of absorption. The more efficient absorption at ?=9.3 and 9.6 µm may be advantageous for both cariespreventive treatments and ablation of exposed hard dental tissues while minimizing heat deposition in the tooth.

Multiple-pulse irradiation of dental hard tissues at CO2 laser wavelengths

Surface temperatures were monitored using pulsed photothermal radiometry (PPTR) during multiple pulse carbon dioxide laser irradiation ((lambda) equals 9.3, 9.6, 10.3 and 10.6 micrometers ). Permanent changes in the optical properties (reflectance and absorption) were observed at fluences greater than 2 J/cm2 for dentin and 5 J/cm2 for enamel. The laser irradiation changes the thermal and the optical properties of these tissues, substantially changing the energy deposition for subsequent laser pulses. The temperature response of enamel and dentin and the reflectance of dentin changed considerably with successive laser pulses. After 10 to 50 pulses the surface stabilized and no further changes were noted. Scanning electron micrographs of the laser conditioned surfaces showed large crystals of modified hydroxyapatite (approximately equals 500 nm) devoid of the organic matrix. Presumably, the water and the interwoven biopolymer matrix had been carbonized nd vaporized. Caries inhibition measurements after multiple pulse irradiation of enamel indicate that the stable laser conditioned surface is more resistant to acid dissolution than untreated enamel.

Light-scattering properties of dentin and enamel at 543, 632, and 1053 nm

The angular distribution of scattered laser light was measured at 543 nm (HeNe), 632 nm (HeNe), and 1053 nm (YLF) through 50 to 100 micrometers sections of enamel and dentin. The scattering distributions were strongly forward peaked at all three wavelengths, indicating that all samples were highly transmissive. The scattering distributions are very sensitive to surface scattering, and a large percentage of the incident light is internally reflected at the exit surfaces of these materials, masking the true scattering distributions. True bulk scattering distributions were measured in a bath of index-matching fluid. For 1 micrometers radiation there was no measurable absorption and only minimal highly forward-directed scattering in 100 micrometers enamel sections. In dentin there was somewhat more forward-directed scattering with less than 10% absorption in 100 micrometers sections. This information should be taken into consideration in any laser irradiation of dental surfaces at these wavelengths.