SCANNING MICRORADIOGRAPHIC STUDIES OF RATES OF IN-VITRO DEMINERALIZATION IN HUMAN AND BOVINE DENTAL ENAMEL

The aim was to measure frequently and with precision the local integra ted mineral loss through small areas of the natural surface of human a nd bovine enamel during in vitro demineralization using an X-ray photo n-counting system (scanning microradiography). The method used was an adaptation of photographic longitudinal microradiography in which the attenuation of X-rays through the enamel is measured in the direction of acid attack, i.e., normal to the enamel surface. The mass of minera l (assumed to be hydroxyapatite) per unit exposed area was measured ov er 15 mu m dia. circles at a series of positions as a function of time in blocks of human and bovine enamel immersed in 0.1 mol/l acetic aci d buffered to pH 4.0 with NaOH. There was an initial period (approx. 4 5 h for human, approx. 75 h for bovine enamel) during which the minera l loss with time was sigmoidal, followed by a nearly linear loss for t he remainder of the experiment, in some cases up to 500 h. The initial sigmoidal period may be due to properties of surface enamel or be ass ociated with the development of a surface layer overlying subsurface d emineralization. The essentially constant rate of mineral loss after t he surface layer has formed confirms earlier observations and is consi stent with a rate-limiting process occurring at the dissolving enamel surfaces of the advancing front, and not by transport of ions within t he lesion. Small perturbations from a linear loss were seen, which wer e approximately periodic for human enamel. The slope of the linear per iod was rather constant within one human or bovine block, but variable between blocks without a clear distinction between human and bovine e namel. (C) 1998 Elsevier Science Ltd. All rights reserved.