Surface etching and subsurface demineralization of dental enamel induced by a strong acid

Intact enamel surfaces were exposed to 20 ml of 0.1% HCl for various periods. When the acid was unsaturated with respect to both hydroxyapatite and fluorapatite the surface enamel was etched away. With time the liquid close to the enamel became supersaturated with respect to fluorapatite but remained unsaturated with respect to hydroxyapatite. Under this condition subsurface lesions developed. These observations are discussed with reference to the use of diffusion-restricting agents in laboratory caries experiments.