The Interaction of stannous fluoride with synthetic hydroxyapatite: modelling the anticaries effect

The anti-caries mechanism of stannous fluoride has been investigated by determining its behaviour in aqueous solution and in contact with hydroxyapatite powder. Free fluoride levels in solutions of SnF2 at total fluoride concentrations of 500 and 1000 ppm were measured using an ion-selective electrode. From the results, the value of the equilibrium constant for the process SnF2 D SnF+ + F– was determined. Solutions were then placed in contact with synthetic hydroxyapatite powder at 5 cm3 to 0.100 g and free fluoride determined after 1 minute, then 5 minute intervals for 1 hour. All determinations were in triplicate. Free fluoride was 38.4 (± 4.7) ppm and 57.7 (± 11.4) ppm for the nominal 500 and 1000 ppm solutions respectively. Equilibrium constants were 8.4×10-5 mol.dm-3 (± 1.0×10-5 mol.dm-3) and 9.3×10-5 mol.dm-3 (± 1.8×10-5 mol.dm-3) respectively. These were not significantly different and had a mean of 8.8×10-5 mol.dm-3 (± 1.2×10-5 mol.dm-3). Free fluoride in solutions of SnF2 exposed to hydroxyapatite powder increased rapidly, equilibrating within 20 minutes. Uptake of tin by hydroxyapatite was confirmed from EDAX (SEM) on the recovered hydroxyapatite. We conclude that SnF+ is taken up by hydroxyapatite and is responsible for the anti-caries effect in vivo. Further work is required to determine the nature of this uptake, i.e. surface adsorption or incorporation into the hydroxyapatite lattice during remineralization.