Fluoride penetration into enamel was studied in vitro immediately after application of 0.8 M NaF, 1.5 M KF or 1.5 M NH4F. Formation of apatitically bound fluoride was determined immediately after application and after 24 h incubation at 100% humidity and 37 °C. The treatment solutions were either neutral with or without orthophosphate (0.05 M in the case of NaF, 1.5 M in the case of KF or NH4F) or acidic with or without 0.05 M acetate in the case of NaF and with or without 1.5 M orthophosphate in the case of KF and NH4F. Initial experiments with NaF involved application times of 5 min, 1, 6 or 24 h and in subsequent experiments 5 min and 6 h only. More fluoride penetrated in depth with increased application time. Fluoride penetration was greater from acidic than from neutral solutions after 5-min treatments. The presence of 1.5 M orthophosphate decreased fluoride penetration in depth. More fluoride penetrated into the enamel from NH4F than from KF and NaF solutions. Increased application time or introduction of an incubation period resulted in a flattening of the concentration gradient of firmly bound fluoride. 6-hour treatments effected much greater deposition of firmly bound fluoride in depth than 5-min treatments. Of the 5-min treatments, neutral KF with orthophosphate and acidic NaF deposited most firmly bound fluoride at 20 μm depth. The presence of orthophosphate tended to enhance the deposition of firmly bound fluoride. The highest concentrations of firmly bound fluoride resulted from 6-hour treatments with NH4F. The findings show that it is possible to affect fluoride penetration and fluorapatite formation in enamel from a topically applied fluoride solution by modifying the solution composition.

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