Developing PIC-XRF to Quantify Surface Structural Changes Following Enamel Erosion

Objectives: (1) To assess the feasibility of using polarisation induced contrast-X-ray fluorescence (PIC-XRF) at the calcium-K edge to quantify hydroxyapatite(HaP) crystallite orientation of enamel surfaces; and (2) to develop methods to quantitatively characterise enamel surface structural changes associated with simulated short-term dietary acid exposures.
Methods: Paired cross-sectional and mid-buccal/palatal caries free enamel samples (n=6) were epoxy embedded, sectioned, machine polished and lasered with fiducial markers. The samples were interrogated using micro-focused synchrotron X-ray fluorescence (XRF) with a linearly polarised monochromatic source. Initial X-ray Absorption Near Edge Structure (XANES) mapping of calcium(Ca) was used to identify two key energies corresponding to key electron transitions in Ca (1s to 4p3/2 and 1s to 4p1/2). Subsequently samples were imaged using a 2µm spot size, adjusting the polarisation direction in 5° increments from 0° to 180°. Post-mapping, the samples underwent erosion in 0.3% citric acid at pH 2.7 for 10 min and were subsequently remapped. Data processing and analysis were conducted using Image J, MANTiS software, and Python. Processed data exploration included principle component and cluster analyses.
Results: XANES identified a 1.9eV difference in the post-edge position associated with the c-axis of HaP orientation extremes relative to the plane of X-ray polarisation. XRF measurements at 4049.2 and 5051.1 eV and varying angles relative to the polarisation plane facilitated composite image construction of primary HaP orientations in human enamel. Proof-of-principle measurements on human enamel discerned quantitative distinctions between uneroded and eroded samples. A notable enhancement in contrast was observed post-erosion, indicating the orientation specific selective dissolution of HaP crystallites.
Conclusions: PIC-XRF is a promising tool to study dental erosion at subprismatic scales, with minimal sample preparation compared with other synchrotron methods. It supports repeated and in-operando erosion measurements and could allow testing of remineralisation protocols in the future.