Acid susceptibility at various depths of pH-cycled enamel and dentine specimens

pH-cycling and in situ studies have shown that fluctuations in de-/remineralisation conditions or in fluoride usage can lead to laminations inside enamel or dentine lesions. Layers with different mineral content are thought to reflect the history of fluoride administrations. Studying the dissolution properties of such lesions at various depths -- using bulk specimens -- is presumably hampered by limited diffusion of acids through the lesion pores. Therefore, in this study the acid susceptibility of enamel and dentine lesions and the underlying sound tissues was studied by exposing sections to acid buffers from the cut rather than from the external surface. Specimens were obtained from a previous study of the effects of high-fluoride (0, 1,000, 2,000, 3,000/5,000 ppm F) toothpastes on enamel and dentine de-/remineralisation. Sections were subjected to acid buffers for 3 and 7 days and the changes in mineral content were monitored by contact microradiography. For enamel lesions a significant difference in dissolution over depth was observed between the groups subjected to the different fluoride schemes. At 7 days a dose response was found between the different fluoride groups and the lesion parameters. In the no-fluoride group dissolution in the original lesion and the sound tissue were similar. All dentine lesions which had been treated with fluoride showed inhibition of dissolution, but the inhibition did not increase with higher fluoride concentrations. Deeper into the dentine tissue there was some protection, but it was not statistically significant. We conclude that penetration of fluoride through the lesion pores determines the dissolution pattern of a lesion at various depths.