We have launched a systematic series of studies to establish the mechanism by which F-varnish affords long-term caries protection and to identify the optimal fluoride concentration for anticaries efficacy of the F-varnish. Our in vitro studies show that F-varnish saturates the fluoride binding sites of enamel by flooding the tooth surface with high concentrations of fluoride ion for several hours. Our study also shows that very high concentrations of fluoride in contact with the tooth for extended time are required to overcome the slow rate of fluoride binding to enamel binding sites. Figure 6 shows the fluoride uptake into hydroxyapatite discs (a mineral model for tooth enamel) as a function of the fluoride concentration in F-varnish at 3 h exposure. We measured both tightly bound fluoride, which gives long term protection by preventing dissolution and loosely bound fluoride, which gives short term protection by inducing surface stability and through antimicrobial mechanisms. Both the loosely bound and tightly bound F uptake into the hydroxyapatite disc is a non-linear function of NaF concentration in the varnish. The fluoride saturates the binding sites at 2.5% NaF, thus the uptake does not increase above this concentration. In our study we have found that the number of fluoride binding sites on the hydroxyapatite disc is limited and there is an optimal NaF concentration at 2.5% that saturates the binding sites. Saturation of fluoride binding sites gives maximal protection against demineralization. 18