A study of the microstructure of fluoride globuli formed after AmF and NaF treatment of enamel surfaces

It is generally accepted that a topical treatment of tooth surfaces with fluorides contributes to the decline in dental caries prevalence. Fluorides can cause the formation of calcium fluoride-containing globular precipitates at the tooth surface. During dissolution of these precipitates free fluoride ions are formed influencing the de- and remineralisation processes in enamel hydroxyl apatite. However, the solubility and fluoride release of the precipitates depend on their particular structural properties. The aim of the presented paper is to study the microstructure and the chemical composition of the fluoride containing globuli using transmission electron microscopy (TEM) in combination with energy dispersive X-ray analysis (nanospot-EDX with a spatial resolution of about 5 nm) and high resolution scanning electron microscopy using a field emission cathode (FE-SEM). The study was focused on fluorides typically used during dentrifice, such as amine fluoride (AmF) and NaF, using different treatment conditions. It was found that the microstructure of the globuli depends on fluoride type, treatment time, fluoride concentration and pH. Electron diffraction showed that the microstructure is polycrystalline with no indication of an amorphous matrix. The crystallographic parameters are in agreement with those of CaF2 while the grain size ranges from 2 nm (pH 3.5 - 4.5) up to 20 nm (pH 6.0). The fluoride content of the globuli is 40 - 60 wt% F in dependence on pH and treatment time. The data are in accordance with the results of preliminary studies but give more details of the microstructure of the fluoride precipitates.