  Structural and chemical changes that arise from a fluoridation of synthetic
hydroxyapatite in neutral and acidic fluoridation agents are investigated. For
synthetic hydroxyapatite (Ca5(PO4)3OH = HAp), the elemental depth profiles were
determined by X-ray photoelectron spectroscopy (XPS) to reveal the effect of
fluoridation in nearly neutral (pH = 6.2) and acidic agents (pH = 4.2). Due to
the high surface sensitivity of the technique, the depth profiles have a
resolution on the nm scale. With respect to the chemical composition and the
crystal structure XPS depth profiling revealed very different effects of the
two treatments. In both cases, however, the fluoridation affects the surface
only on the nm scale which is in contrast to recent literature, where a
penetration depth up to several microns was reported. Moreover, evidence is
given that the actual fluoridation depth depends on the pH value. Recently, a
qualitative three layer model was proposed for the fluoridation of HAp in an
acidic agent. In addition to the elemental depth profile, as presented also by
various other authors, we present a quantitative depth profile of the compounds
CaF2, Ca(OH)2 and fluorapatite (FAp) that are predicted by the three layer
model. The analysis of our experimental data exactly reproduces the structural
order of the model, however, on a scale that differs by nearly two orders of
magnitude from previous predictions. Our results also reveal that the amount of
Ca(OH)2 and FAp is small as compared to that of CaF2. Therefore, it has to be
questioned whether such narrow Ca(OH)2 and FAp layers really can act as
protective layers for the enamel.
