Uptake and retention of fluoride by intact and etched enamel

180 fluoride biopsies were taken in 30 children from the labial surfaces of the 6 maxillary anterior teeth. The average thickness of the biopsied surface enamel was 8.35+/-0.75 mjm. Intact, not treated enamel had an average F-content of the integral of 550 ppm. 7 days after one 3-min topical application of amine fluoride on intact enamel the average F-concentration was the integral of 1150 ppm. 7 days after one amine fluoride application on enamel previously etched with pyruvic or orthophosphoric acid, the average F-content was the integral of 3400 ppm and the integral of 2800 ppm respectively.