3D Multi-modal Imaging of demineralised dentine using combined synchrotron micro-XRD-CT and STXM-CT.

Dental caries is the most prevalent oral disease that causes structural and compositional changes of the dental hard tissues due to a chronic demineralisation (combined with possible phases of remineralisation) process. Though considerable efforts have been directed at studying natural and artificial carious lesions, most characterisations remain either constrained to 2D analyses or have been unable to achieve fine resolution in 3D due to limited field of view. To overcome this challenge, the present study combined X-ray diffraction (XRD) and scanning transmission X-ray microscopy (STXM) tomography techniques to analyse the mineral density, scattering intensity, and crystallite size in normal, carious, 30 % artificially demineralised, and 50 % artificially demineralised dentine. Combined XRD and STXM tomography was performed on the I18 beamline at Diamond Light Source, using a 15 keV monochromatic beam with 2 x 2 microm spotsize and scanning with translation steps of 2 microm, providing a reconstructed voxel size of 2 x 2 x 2 microm. Natural carious dentine showed a reduction in hydroxyapatite (HAp) crystallite size due to chronic demineralisation. This was unlike artificially demineralised dentine samples that underwent short, continuous demineralisation, which created a zone of fully demineralised dentine, near the sample surface, and a zone of partially demineralised dentine that had a reduced mineral density but an increased average crystallite size.