STUDIES ON THE INFLUENCE OF FLUORIDE ON THE EQUILIBRATING CALCIUM-PHOSPHATE PHASE AT A HIGH ENAMEL ACID RATIO

Data obtained in a previous study suggested that brushite is the solub ility-determining phase when enamel is first exposed to acid solution in a series of repeated equilibrations. Fluoride in solution might be expected to inhibit brushite formation, and experimental studies at lo w solid/solution ratio support this. We have now re-examined the effec t at a very high ratio, in an attempt to mimic what happens in an enam el caries lesion. Powdered enamel was repeatedly exposed to HCl soluti on, 10-70 mmol/l, containing 2 ppm F, for 24 h, initially in a ratio o f 1 g/3 mi. Ion activities were determined after 20 min and 24 h and p otential plot diagrams constructed. In early repetitions the -log (Ca2 +) (OH-)(2) vs. -log (H+)(3)(PO43-) points tended to follow the brushi te line, rather than the hydroxyapatite (HAp) line which one would exp ect if enamel behaved as pure HAp. Solution F was below measurable lim its after 20 min and F then had little influence on the brushite equil ibrating phase. In later (>13) repetitions, points fell closer to the HAp line, with or without F added to the acid solution. However, added F, which was not then completely removed from solution, caused the sl ope of the regression line through the points to approach the Ca/P rat io of HAp, and therefore may have had a small effect in reducing the b rushite phase. It is concluded that high solid/solution ratio, a previ ously neglected factor in enamel dissolution studies, has a profound e ffect in increasing the manifestation of a brushite surface phase and reducing the inhibitory effect off on this phase.