"CaF₂" on enamel from total or soluble fluoride in varnish

Objective: Fluoride varnish Duraphat® contains a total of 22,600 ppm F of which approximately 20% is soluble for an immediate reaction with enamel. This reaction forms calcium fluoride-like products (“CaF₂”), which are responsible for the anti-caries effect of professional fluoride application. It has been recommended that the varnish should not be removed soon after application and during this time the insoluble NaF particles in varnish suspension could be dissolved by saliva increasing the amount of “CaF₂” formed on enamel, but this mechanism has not been properly tested. Thus, the aim of this study was to assess the kinetics of “CaF₂” formation on enamel from total or soluble fluoride in varnish. Methods: Ninety-six bovine enamel slabs were randomly allocated to be treated with either Duraphat® (whole-varnish), containing soluble and insoluble fluoride, or the supernatant of centrifuged Duraphat® (5 min, 16.000 g, centrifuged-varnish), which contained only soluble fluoride. Additionally, eight untreated slabs were used as control (baseline “CaF₂” concentration). The treated slabs (n=8/group/time) were immersed in artificial saliva under continuous flow (0.5 mL/min/slab) during 5min, 30min, 2h, 6h, 12h or 24h. Afterwards, the varnish was removed with acetone. “CaF₂” was extracted with KOH and fluoride ion was determined with ion-specific electrode. Data were analyzed by two-way ANOVA. Results: For the whole-varnish group, “CaF₂” concentration (Avg±SD;n=8) on enamel increased linearly with time (p<0.05) reaching 22.0±4.5 µg F/cm² at 24h. In the centrifuged-varnish group, a maximum concentration of 3.2±0.8 µg F/cm² was found after 6h. “CaF₂” on enamel treated with whole-varnish was higher (p<0.05) than on those treated with the centrifuged-varnish at all times. Conclusion: The results suggest that insoluble fluoride in Duraphat® is dissolved during intraoral exposure, increasing fluoride reactivity with enamel according to time of contact and this fact should be clinically considered, delaying the removal of the varnish applied.