On the role of calcium fluoride in the cariostatic mechanism of fluoride

Abstract

The literature concerning the formation and stability of CaF₂ in the oral environment is reviewed. In early work the CaF₂ formed during topical application with fluoride was assumed to be beneficial. It was suggested that it could protect the enamel surface directly or provide free fluoride ions for subsequent incorporation into the hydroxyapatite lattice. However, McCann claimed, in 1968, that CaF₂ is soluble in saliva (12–15 mg/1), that it would be rapidly lost in the oral cavity, and that the clinical effect of fluoride was related to formation of firmly bound fluoride only. In this period many authors reported total loss of CaF₂ during 24 h after a topical application of fluoride. It has now been shown in several laboratories that calcium fluoride is stable in saliva at neutral pH owing to surface adsorption of HPO^2-₄ to the crystal surface and formation of a solubility-limiting phase. Extended exposure of saliva can cause formation of a fluorapatite layer on the CaF₂ crystals, restricting their dissolution further. Low pH (pH > 5) causes loss of the solubility-limiting adsorbed HPO^2-₄ and a slow dissolution of CaF₂. The CaF₂ crystals may thus serve as pH-controlled reservoirs of fluoride ions on the enamel or in plaque and release fluoride during caries challenges. It is suggested that calcium fluoride is an essential phase explaining important aspects of the mechanism of topically applied fluoride, contrary to what was assumed in the past.