Nature and Role of Loosely Bound Fluoride in Dental Caries

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Abstract
This paper discusses loosely bound fluoride and its role in dental caries and prevention. Loosely bound fluoride (abbr. by F) is fluoride adsorbed onto enamel mineral crystallites. Several recent studies indicate that a high total level of fluoride in enamel does not guarantee protection against caries. This leads to the conclusion that a major part of fluoride present in the solid enamel is not active in prevention. The adsorption of Fa to the mineral under acidic conditions is described. Most likely there is a dynamic equilibrium between fluoride in solution and adsorbed Fa at the crystal surface interface. When the crystallite is completely covered by adsorbed Fa, there is a maximum inhibition of dissolution. The rate of dissolution of mineral depends on pH, the actual concentrations of calcium and phosphate in the liquid in contact with the crystallites, and on the fraction of the surface covered by adsorbed fluoride. The fluoride, Fs, localized in the inner part of the crystallites is relatively unimportant. “CaF2-like” material can be formed on and in enamel depending on conditions. The in vivo-formed globular “CaF2-like” material is not pure CaF2 and releases F~ ions when dissolving; these ions will also be partly adsorbed as Fa in and on enamel. Presently, the amount and importance of Fa originating from in vivo-formed “CaF2-like” material are not known. The level of fluoride, Fa, necessary for strong inhibition of enamel demineralization in vitro is estimated to correspond to a fluoride concentration, FL, in the liquid phase of 1 ppm or 50 μmol/L fluoride ions. The concentration, FL, needed in vivo may be substantially greater, whereas F, has a maximum value, of the order 2 μmol/m2, corresponding to the concentration of hydroxyl sites in the surface of the crystallites. The present paper indicates that frequent low-level applications of fluoride are most likely more beneficial in caries prevention than few treatments with solutions having a high fluoride concentration.