Relationship between F-release and F-uptake into HAp from F-varnishes

F-uptake, (loosely-bound and tightly-bound) into the tooth is considered to be the cause of caries protection efficacy from fluoride releasing varnishes (F-varnish).  The F-release profile of F-varnish into solution has been used to imply superiority between F-varnish products.  This assumes that F-release is predictive of F-uptake. Objectives: Test the assumption that F-release is predictive of F-uptake by evaluation of several commercially available F-varnish products for F-release over 3h and F-uptake (loosely- and tightly-bound) into hydroxyapatite (HAp). Methods: Five commercially available F-varnishes were used in this study.  Four contain 5% NaF, some with ingredients that may affect F-release and/or F-uptake.  They were NaF-only (V_NaF), NaF-flavored (V_Flav), NaF-ACP (V_ACP), NaF-TCP (V_TCP); and one contained Fluorsilane at 0.1% Fluoride (V_Sil).  F-varnish (0.01-0.02g) was applied to one side of a HAp disc.  F-release into a continuous flow of 30mmol/L KCl at 1 mL/min was measured over 3h. The HAp discs were then cleaned and loosely-bound fluoride extracted overnight in KOH (1.0mL, 1.0mol/L).  Tightly bound fluoride was extracted via serial extractions in HClO₄ (1.0mL, 0.5mol/L).  The extraction solutions were neutralized and analyzed via fluoride ion-selective electrode. Results: The cumulative F-release profile for all products were hyperbolic, with some products releasing significantly more F than others (V_ACP>>V_NaF>V_TCP≈V_Sil≈V_Flav) at 3h (p<0.05). The time for 50% of the 3h-cumulative F-release did not follow the same pattern (V_Flav>>V_ACP>V_NaF>V_TCP≈V_Sil). F-uptake for loosely-bound (V_NaF>V_Flav>V_TCP>>V_ACP≈V_Sil) and for tightly bound (V_NaF>V_Flav≈V_Sil>V_ACP≈V_TCP) were significantly correlated (r²=0.7536, p<0.05). Neither loosely- or tightly-bound F-uptake correlated with the cumulative F-released at 3h (r² <0.15). Conclusions: The continuous flow method generates F-release profiles and F-uptake data from a variety of products.  The 3h-cumulative F-release is not predictive of the F-uptake (loosely- or tightly-bound) for these products.  Further studies are needed to establish quantitative measures that are predictive of the protective efficacy for F-varnish products.