Fluoridated Hydroxyapatite Solubility and Caries Formation

SINCE the early observations 1 on reduction of caries with increasing fluoride concentrations in drinking water, the tacit assumption was made that fluoride incorporated in dental enamel acts as a cariostatic agent. An appealing explanation for the possible mechanism involved is based on the nature of the enamel mineral of which hydroxyapatite (OHA), Ca 5 OH(PO 4 ) 3 , is considered to be the prototype. Replacement of fluoride for the hydroxyl would yield fluorapatite (FA) which has a lower solubility. Analyses of surface layers of enamel, however, show that the fluoride contents are far below the level expected for FA. It is relevant then to ascertain to what extent partial substitution of the fluoride for the hydroxyl group in HA affects its solubility.