An electron microscope study of the in situ interaction between CaF2-like precipitates and dental enamel surfaces
The aim of the present investigation was to study the effect of fluoride released under in situ conditions from CaF2-like precipitates formed after topical treatment on dental enamel microstructure. Sound human enamel slabs of three different patients were treated in vitro with a fluoride-containing solution (Olaflur; 1000 ppm fluoride; pH 4.7; 60 min) resulting in an intensive CaF2 precipitation at the sample surface. These fluoridated specimens were attached to orthodontic appliances and worn in situ by four different volunteers without further fluoride treatment. After 1, 3 and 10 days the sample surface microstructure was investigated using Transmission Electron Microscopy (TEM) and nanospot x-ray analysis in cross-section. Un-treated samples and fluoridated specimens treated in vitro by demineralization/remineralization cycling served as control groups. For samples worn in situ for three and ten days, the TEM investigations revealed the existence of a fluoride-affected enamel surface layer with about 1 µm depth immediately beneath the remaining CaF2 surface deposits. Similar to remineralization effects in demineralized lesions, the enamel surface layer consisted of transformed apatite crystals with distinctly increased grain sizes but showed also a high fluoride content of up to 10 %. This effect was absent for the control specimens. The model used illustrates directly the efficacy of CaF2-deposits formed after topical treatment as a fluoride-releasing reservoir and its effect on enamel microstructure under oral conditions. The observed formation of a F--rich enamel surface layer can contribute to understand the role of topical fluoride treatments for caries prevention and protection against erosion in more detail. Supported by GABA International AG.