Rates of mineral loss in human enamel during in vitro demineralization perpendicular and parallel to the natural surface.

Human enamel is a structurally anisotropic material. The aim of this study was to investigate whether this structural anisotropy is reflected in the demineralization behaviour of enamel. Kinetics of demineralization of in vitro caries lesions with the direction of acid attack perpendicular to the natural surface of dental enamel from human premolar teeth were compared with kinetics when demineralized parallel to this surface. Pairs of enamel samples from the same tooth were demineralized under identical conditions. Loss of mineral with time was very nearly linear for both directions (consistent with the rate-controlling step being reaction at the advancing front rather than transport processes), but the perpendicular rate was, on average, about 14% higher than the parallel rate. The rate of demineralization parallel to the surface increased from the natural surface to the enamel-dentine junction by 10-25%, depending on sample. The origin of fine structure and slight departures from linearity in the loss of mineral with time plots are discussed. Mineral masses per unit area were determined from absorption of a 15-microm diameter X-ray beam using photon (AgKalpha) counting methods.