A scanning electron-microscopic study of solubility variations in human enamel and dentine

Cut, polished surfaces of human teeth were exposed to acetate buffers, pH 5.2, with a background electolyte composition similar to plaque fluid and with - log10 (ion-activity product) for hydroxyapatite, pIHA, ranging from 55 to 62, at 37 degrees C under a 5% v/v CO2 atmosphere. Demineralization was assessed by scanning electron microscopy of the treated surfaces and of methacrylate replicas of subsurface pores. Intertubular dentine was slightly soluble at pIHA 55, while peritubular dentine was less soluble, dissolving at pIHA > or = 58. In enamel, dissolution of prism junctions occurred at pIHA > or = 55, but was slight in middle enamel. Demineralization of intraprismatic enamel was observed at pIHA > or = 56 in inner enamel and in a few places in outer enamel but did not occur in middle enamel and most of the outer enamel at pIHA < or = 58. The results support the conclusion that the bulk of enamel has the solubility properties of a slightly defective form of hydroxyapatite. The correlations between structure and solubility are likely to have a strong influence on the pattern of caries lesion formation.