Jaap J. ten Bosch

REGRESSION OF WHITE SPOT ENAMEL LESIONS - A NEW OPTICAL METHOD FOR QUANTITATIVE LONGITUDINAL EVALUATION IN-VIVO

This article describes a new nondestructive optimal method for evaluation of lesion regression in vivo. White spot caries lesions were induced with orthodontic bands in two vital premolars of seven patients. The teeth were banded for 4 weeks with special orthodontic bands that allowed plaque accumulation on the buccal surface. The teeth were left in the dentition for 2 or 4 weeks after debanding. Regular oral hygiene with a nonfluoridated toothpaste was applied during the entire debanding. Regular oral hygiene with a nonfluoridated toothpaste was applied during the entire experimental period. The optical scattering coefficient of the banded area was measured before banding and in 1-week intervals thereafter. The scattering coefficient returned to the sound value in an exponential manner, the half-value-time for left teeth being 1.1 week, for right teeth 1.8 weeks, these values being significantly inequal (p=0.035). At the start of the regression period, the scattering coefficient of left teeth lesions was 2.5 as high as of right teeth lesions, values being inequal with p=0.09. It is concluded that regression of initial lesions in the presence of saliva is a relatively rapid process. The new optical method may be of clinical importance for quantitative evaluation of enamel lesion regression developed during fixed appliance therapy.

Optical path-length spectroscopy of incipient caries lesions in relation to quantitative light-induced fluorescence and lesion characteristics

A basic understanding of the light-scattering processes that take place inside the dental tissue (either sound or carious) is obtained both with measurements of the photon path-length distribution of light inside such media and with Monte Carlo simulations. Furthermore, the following is investigated: the correlations between different momenta of the photon path-length distribution of light inside caries lesions, the fluorescence loss determined with quantitative light-induced fluorescence, and/or the demineralization and depth of caries lesions determined with transversal microradiography. It is concluded that (i) the light paths inside both carious and sound enamel are considerably influenced by the refractive-index contrast at the tooth surface; (ii) contrary to a previous hypothesis, the fluorescence loss is larger in lesions in which the average photon path length is longer; (iii) very good correlations are obtained between the optical characteristics and the physical parameters of lesions when the optical measurements are performed such that there is high refractive contrast at the tooth surface.

Two dimensional patterns of Fraunhofer diffraction by dental enamel

Light transmitted by thin slabs of human dental enamel is scattered and Fraunhofer diffracted. At three wavelengths we used the two dimensional transmitted Fraunhofer diffraction patterns to determine the periodicity of the prism structure and the curvature and misalignment of the prisms. This was performed by fitting theoretical curves to measured intensity functions. We found for each sample a mean value of the periodicity and its standard deviation. The deviation between the mean values found at each wavelength was much smaller than the deviation of the size distribution in each sample. The measured overall mean value of the periodicity and its standard deviation was d equals 4.9 plus or minus 0.6 micrometer. The mean angle of curvature and misalignment was (Delta) (phi) equals 32 degrees plus or minus 22 degrees.

ANALYTICAL CLOSE-TO-SOURCE INVESTIGATION FOR AN ISOTROPIC POINT SOURCE IN AN UNBOUNDED, ANISOTROPICALLY SCATTERING MEDIUM

The diffusion approximation, which is often used to describe the propagation of light in biological tissues, is only good at a sufficient distance from sources and boundaries. Light- tissue interaction is however most intense in the region close to the source. It would therefore be interesting to study this region more closely. Although scattering in biological tissues is predominantly forward peaked, explicit solutions to the transport equation have only been obtained in the case of isotropic scattering. Particularly, for the case of an isotropic point source in an unbounded, isotropically scattering medium the solution is well known. We show that this problem can also be solved analytically if the scattering is no longer isotropic, while everything else remains the same.

Optical pathlengths in dental caries lesions

The average pathlength of light inside dental enamel and incipient lesions is measured and compared, in order to quantitatively confirm the prediction that incipient lesions have higher scattering coefficients that sound enamel. The technique used, called optical pathlength spectroscopy provides experimental access to the pathlength distribution of light inside highly scattering samples. This is desirable for complex biological materials, where current theoretical models are very difficult to apply. To minimize the effects of surface reflections the average pathlength is measured in wet sound enamel and white spots. We obtain values of 367 micrometers and 272 micrometers average pathlength for sound enamel and white spots respectively. We also investigate the differences between open and subsurface lesions, by measuring the change in the pathlength distribution of light as they go from dry to wet.

An Instrument To Measure The Color-Determining Properties Of Bulk Translucent Materials

A pencil beam incident on a translucent material causes a luminous circular spot of volume-reflected light. Its spectral radiance L (r) decreases with radius r. Lc, the value close to the centre is mainly determined by the scattering parameter s(1-g) of material. Le, the value at the edge of the spot, is determined by s(1-g) and the absorption coefficient a. The instrument (the CTM) employs three fibre bundles to measure Lc and Le, both as a function of λ (400-700 nm). A central bundle (2 mm p) of - 1300 fibres each of 50 um g is randomly divided in a bundle for illumination and a bundle for measurement of Lc. A concentric ring (i.d. 4 mm, o.d. 5 mm) of - 2500 fibres is used to measure Le. This instrument was tested with aqueous suspensions of latex and a dissolved, non-latex adsorbing, red dye. Thus s(1-g) (scattering) and a (absorption) could be independently varied. For a=0, both Lc and Le increased with s(1-g), reached a maximum and decreased. The maxima for Lc and Le were at s(1 g) = 1 and 0.06 mm -1, respectively. At a constant scattering, increase of absorption decreased Le much stronger than it did Lc. This absorption-caused decrease depended only weakly on the scattering coefficient: a variation of scattering of a factor 20 caused only a few percent change in absorption-caused decrease of Lc and only a factor 2 in absorption-caused decrease of Le. At s(1-g) = 0.3 mm-1, Le depended much more strongly on a than did the overall regular reflection spectrum of the suspension as measured with a Hunter spectrophotometer under 0°/45° geometry. The readings of Lc and Le with this instrument can be used to determine s(1-g) and a and the reflection spectrum. Only small samples are needed in comparison to the regular reflection spectrometry. To obtain absolute values the instrument has to be calibrated on the specific type of material under investigation.

The effect of ultraviolet induced fluorescence on visually perceived tooth color under normal light conditions

OBJECTIVE Restorative and prosthetic materials should provide an appearance similar to natural teeth under all light conditions, including UV-rich environments and daylight. Various studies claim that UV-induced fluorescence makes teeth whiter and brighter in daylight. The aim of this paper is to determine experimentally the significance of tooth fluorescence in natural sunlight on perceived tooth color. METHODS A total of 35 extracted, hydrated teeth without restorations or endodontic treatments were evaluated in an experimental setup. A UV/VIS spectrometer using a reflectance/backscattering probe was used to collect the reflected spectrum. Unfiltered and filtered sunlight was used for irradiation of the samples so as to use the combined ultraviolet and visible spectrum (UV/VIS) and the visible spectrum (VIS) exclusively. Color coordinates for each group were measured using the CIE L*a*b* 1976 system, averaged, and compared. RESULTS The average color difference between both groups (UV/VIS and UV) was ΔE* 0.527. The average tooth color for the VIS group was L*VIS 72.21, a*VIS -2.42, and b*VIS 22.35, and for the UV/VIS group was L*UV/VIS 72.00, a*UV/VIS -2.47, and b*UV/VIS 22.44. SIGNIFICANCE UV induced fluorescence from sunlight does not make teeth whiter and brighter.

New method to calculate back-reflected radiance for isotropic scattering

We present a method to determine the back reflected radiance from an isotropically scattering halfspace with matched boundary. The bonus of this method lies in the fact that it is capable, in principle, to handle the case of narrow beams, something which, to our knowledge, no other analytic method can do. Essentially, the method derives from a mathematical criterion that effectively forbids the existence of solutions to the transport equation which grown exponentially as one moves away from the surface and deeper into the medium. Preliminary calculations for infinitely wide beams yield results which agree well with what is found in literature.

Obtaining high g-values with low degree expansion of the phasefunction

Analytic theory of anisotropic random flight requires the expansion of phase-functions in spherical harmonics. The number of terms should be limited while a g value should be obtained that is as high as possible. We describe how such a phase function can be constructed for a given number N of spherical components of the phasefunction, while obtaining a maximum value of the asymmetry parameter g.

Measurement of color and scattering phenomena of translucent materials

Scattering and absorption phenomena in translucent materials are usually studied by spectroreflectometric measurements, but can also successfully be investigated by employing a color monitor for translucent materials (CTM). A CTM is a fiber optical instrument which separately detects light which traveled short (Lc) and long distances (Le) in the material under consideration. In this study, the authors qualitatively interpret experimental results by using a Monte Carlo simulation of photon pathways in the material. In this simulation, photons are assumed to be absorbed along the ray path between two scattering events. Scattering is simulated by changes in the photon direction, taking into account the exact phase function for scattering on spherical particles. The boundary conditions imposed are analogous to the experimental situation. Results are presented as the radial distribution of reflected photons, i.e., photons emerging at the plane of incidence. In order to transfer the simulation results to the experimental situation with several more or less randomly oriented fibers, the fiber distribution in the measuring head of the CTM has been characterized by automated image analysis. This fiber distribution has been used to calculate a specific transfer function which enables a theoretical prediction of CTM measurements. Experimental results with monodisperse latex suspensions as well as the model simulations demonstrate the existence of an optimum in the detected Le signal at a specific concentration of the suspension. Discrepancies, however, exist between the experimentally determined and simulated location of the optimum. The simulation model furthermore predicts a highly increasing sensitivity to absorption with increasing suspension concentration. It is concluded that Monte Carlo simulation can be a valuable tool for predicting fiber optical measurements of reflection and absorption phenomena in bulk translucent materials, at least for the Lc signal.