Broad-beam PIXE and µ-PIXE analysis of normal and in vitro demineralized dental enamel

Dental enamel has been widely studied by particle-induced x-ray emission (PIXE), but less attention was paid to its demineralization, which leads to caries formation. Using broad-beam PIXE and mu-PIXE, we investigated normal enamel and the in vitro formation of pre-carious lesion in lactic acid solution, aiming also to evaluate intercusp differences within the same tooth. Broad-beam PIXE was performed using 3.0 MeV protons, and µm-PIXE maps of Ca, Fe and Zn were collected with 3.1 MeV protons at similar to 4 mu m resolution. In normal enamel a differentiated Ca-rich surface layer was observed, where Fe and Zn reached their highest levels. In deeper layers, Fe and Zn evidenced quasiperiodic patterns of maxima, possibly due to coupled diffusion-reaction catalytic processes involved in the enamel growth. Both Fe and Zn appeared to be located in a few distinct types of pools. Near the surface, demineralization induced an increase of Fe, Cu, Zn, Sr and Pb with respect to Ca, attr!
ibuted to partial hydroxyapatite dissolution and/or to chelate extraction and concentration of trace metals. Ca maps revealed limited changes in the surface layer and a massive loss in the inner enamel; here, Fe was almost depleted and Zn partially removed. The maps of Ca, Fe and Zn demonstrated major intercusp variations in both normal and altered enamel. Thus, broad-beam PIXE and mu-PIXE, which do not require (semi)thin sectioning of the tooth as the conventional methods, provide compositional and structural insight of normal dental enamel, of its intercusp variability and of the alterations produced by ill vitro demineralization, largely not accessible to the current techniques, and highly relevant for understanding the incipient caries formation.