Time Dependence of Fluoride Uptake in Hydroxyapatite

Fluoridation of enamel is believed to provide an effective tool to protect teeth from caries, but there is still little information on the time scale of fluoride uptake. In this study, highly compressed pellets of hydroxyapatite are used as first-order model systems to approximate the mineral component of natural enamel for investigations on the time-dependence of fluoride uptake. We found that both the overall amount of fluoride as well as the mean thickness of the fluoridated surface layer cannot be extended to any values just by increasing the application time of a fluoride containing agent. Instead, both parameters start to become constant on a time scale of about 3 min. The present results as obtained on a synthetic model "tooth" show that the time scale to provide the maximum amount of fluoride possible is of the same order of magnitude as that in usual daily practice in dental care when applying toothpastes or mouth rinses.