EFFECT OF FLUORIDE INCORPORATION INTO HUMAN DENTAL ENAMEL ON ITS DEMINERALIZATION IN-VITRO

Coronal surfaces of extracted human teeth were ground to a depth of ab out 1 mm and then cut in half labiolingually. One half was used as a c ontrol; the other half was exposed, for 3 days, to a fluoride-enrichin g buffer (0.1 mol/l lactic acid, 19.7 mmol/l CaCl2, 10.8 mmol/l KH2PO4 , 3 mmol/l NaN3; Ph adjusted to 4.68 with KOH) having fluoride concent rations from 0.0002 to 2.2 parts/10(6). This exposure resulted in an u ptake of fluoride by the enamel to a depth of 2 mum without any appare nt demineralization. The fluoride uptake was proportional to the F con centration of the enriching solution, reaching concentrations of about 8000 parts/10(6) within the first micrometre of enamel exposed to the highest F concentration; the controls had uniform F concentrations no t exceeding 50 parts/10(6) along the 2.5 mum of enamel depth sampled. Thin sections (140-160 mum) were cut perpendicularly to the lingual su rface, coated with protective resin except for a window about 1 mm lon g on the ground lingual surface, and exposed to a demineralizing buffe r. The mineral losses of the sections were followed over 5 days by mic roradiography and image analysis. Fluoride enrichment resulted in redu ced demineralization and the reduction was inversely related to the en amel fluoride content. The controls displayed a uniform erosion of the surface enamel whereas all the treatments below 1.5 parts/10(6) in th e enriching solutions developed typical subsurface 'lesions'. The mine ral content of the surface layer increased with increasing time of exp osure to the demineralizing buffer. In enamel having 5000-6000 part/10 (6) F within the first 1-2 mum from the surface, no detectable loss of enamel mineral was observed after the 5-day exposure to the demineral izing buffer. Incorporation of fluoride into enamel is explained by a process of dissolution and reprecipitation yielding a mineral that is more resistant to acid demineralization.