Fluoride-cation interactions in the formation and stability of apatites

The effect of fluoride, F-, on the formation and stability of the apatites, with and without the simultaneous presence of some cations M* (M* = Al3+, Mg2+, Sr2+ or Zn2+) in the solution were investigated. The preliminary findings were: (a) the cations, M*, when present in low levels in solution, had negative effects on the properties of apatite e.g., caused lower crystallinity, lower Ca/P ratios, higher HPO4/PO4, and higher extent of dissolution in an acid buffer; (b) when only F- ions were present in the precipitation solution, F in the apatite formed had positive effect: greater crystallinity, higher Ca/P, lower HPO4/PO4, lower extent of dissolution; and (c) the simultaneous presence of F with the cations, M*, minimized their negative effects and suppressed the formation of Mg-substituted β-TCP. These preliminary results suggest that in biological systems, the simultaneous presence of F- ions with aluminum, magnesium, strontium or zinc ions will minimize the otherwise negative effects of these cations on the formation and stability of the biological apatites (enamel, dentin, bone mineral).