Fluoride levels above 45ppm lead to the formation of insoluble Calcium Fluoride (fluorite).
[The following is taken from the abstract of the article]
Aim
The aim of this study was to monitor the fluoride-mineral phase formed on the enamel as a function of the concentration of fluoride ions [F–] in the demineralizing medium.
The secondary aim was to investigate fluorapatite formation on enamel in the mechanism of fluoride anti-caries efficacy.
Method
Enamel blocks were immersed into demineralization solutions of 0.1 M acetic acid (pH 4) with increasing concentrations of fluoride up to 2,262 ppm.
Results
At and below 45 ppm [F–] in the solution, F MAS-NMR showed fluoride-substituted apatite formation, and above 45 ppm, calcium fluoride (CaF2) formed in increasing proportions. Further increases in [F–] caused no further reduction in demineralization, but increased the proportion of CaF2 formed.
[Incremental information from the article ]
Calcium Fluoride is potentially detrimental
Calcium Fluoride is potentially detrimental to the structural integrity of enamel as its formation reduces the available Ca2+ required for remineralization of apatite. This, in turn, causes loss of phosphate.
Conclusion
The study confirms that the cariostatic effect of fluoride is due to the formation of fluorohydroxyapatite and calcium fluoride, depending on the fluoride in the solution.
Below 45 ppm, the fluoride ions from the solution mostly substitutes into the lattice of the enamel mineral as fluorohydroxyapatite
At 45 ppm fluoride ions, nearly equal amounts of fluorohydroxyapatite and calcium fluoride phases form
Above 45 ppm calcium fluoride is the main phase formed on the enamel surface
Authors :
Mohammed NR, Kent NW, Lynch RJ, Karpukhina N, Hill R, Anderson P.
Original Article Title
Effects of fluoride on in vitro enamel demineralization analyzed by 19F MAS-NMR.
Journal :
Caries Res. 2013;47(5):421-8. doi: 10.1159/000350171. Epub 2013 May 23.