[Possible limitations in the caries preventive effect of fluorides?].

The cariostatic effect of fluoride at different levels of pH in the plaque fluid is discussed. At the pH level 5.5 to 4.5 the plaque fluid is undersaturated with respect to hydroxyapatite and supersaturated with respect to fluorapatite (3). The hydroxyapatite of the enamel then dissolves. With fluoride present in the liquid phase a fluoridated apatite is precipitated in the surface zone of the lesion. In acidic, old plaque the plaque fluid is very likely undersaturated also with respect to fluorapatite (pH less than 4.5) (11). When the liquid phase is undersaturated with respect to fluorapatite no redeposition of mineral lost can occur. In due time an erosion will develop. It is speculated that one reason for the minor effect of fluoride in some caries active patients and in fissures as well is that the plaque fluid is undersaturated with respect to fluorapatite for extended periods.