Fluorine gradients in outermost surface enamel after various forms of topical application of fluorides in vivo

Fluorine gradients were analysed in sound facial enamel of 58 pairs of homologous contralateral premolar teeth. A macroscopic stepwise acid etching technique was used to determine the F-concentration to the depths of 40-200 microns. Usually three successive enamel layers (10, 10, and 20 microns) from enamel surface and inwards were removed. Eight pairs of teeth (control group) which had not been subjected to any form of fluoride treatment in the study, were used to assess the difference between teeth extracted at three-week-interval. The F-gradients were almost identical, with only small differences between the teeth. F-concentrations usually decreased from about 900 ppm within the first 10 microns to about 200 ppm at a depth of 40-50 microns. In ten experimental groups (5 pairs of teeth in each group) the F-uptake was studied after various forms of topical application. Contralateral teeth served as control teeth. The highest F-uptake was observed when a fluoride varnish, Duraphat (5% NaF) was applied three times at one weeks' interval. The F-concentrations were about 2000, 900 and 550 ppm, respectively, in the first, second and third enamel layer. The increase in F-concentration in the first enamel layers analysed was significant, but not in the deepest. When testing the effect of paintings procedures with 2% NaF (pH = 7 and 5); 8% SnF2; 5% Na2PO3F; 2% APF-gel (Flura-Gel) in a similar way, a moderate F-uptake was found (100-450 ppm). The uptake of F was limited to the first 10 microns of the enamel. Only 2% NaF (pH = 5) And the APF-gel gave a significant F-uptake within the first layer. Daily mouthwashing procedures during 21 days resulted in insignificant uptake of F when 0.025% and 0.05% NaF solutions were used. Not even rinsing with 0.025% NaF solution for more than 100 days and rinsing with 0.2% NaF solution once a week for three weeks resulted in any significant uptake. The results showed that the F-uptake in vivo from conventional topical application is small and limited to the first few microns of the enamel surface. However, by prolonging the exposure of the enamel surfaces to fluorides in varnishes with high F-concentrations it appears possible to highly raise the F-concentration of the enamel surface and in deeper layers.