Ceramics-Silikáty 57, (1) 1 - 5 (2013)

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 > 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 8.4x10^-5 mol.dm^-3 (+/- 1.0x10^-5 mol.dm^-3) and 9.3x10^-5 mol.dm^-3 (+/- 1.8x10^-5 mol.dm^-3) respectively. These were not significantly different and had a mean of 8.8x10^-5 mol.dm^-3 (+/- 1.2x10^-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.