Nanostructural evolution during carious and demineralisation process of human dentine using small angle X-ray scattering tensor tomography

Dental caries, one of the most prevalent non-communicable diseases worldwide, is characterised by the progressive deterioration of the structure and mechanical properties of dental hard tissues. In human teeth, dentine is the most abundant mineralised tissue, forming the primary support material. To assess changes in the mechanical properties of dentine caused by dental caries and acid erosion, it is crucial to understand the relationship between organic and inorganic dentine components and their organisation into a 3D anisotropic structure at the nanoscale. Over the past 20 years, alterations in dentine structure caused by caries and artificial demineralisation have been reported using conventional microscopy techniques. However, due to the limited spatial resolution of these techniques, the 3D structural organisation including orientation and degree of alignment of mineralised collagen fibrils at the nanoscale, has not been fully explored. This study investigated alterations in the 3D structure of normal, carious and artificially demineralised dentine using SAXS tensor tomography (SASTT). This technique enabled the observation of differences in the local orientation of organic and inorganic components, as well as variations in local scattering intensity, resulting from natural caries and artificial demineralisation. In comparison to normal dentine, caries caused minor orientational differences of both components but had a major impact on the local X-ray scattering intensity. After artificial demineralisation of the dentine, most of the mineral was lost in the outer layers, resulting in a greater reduction in scattering intensity than that caused by caries. Significance: The remarkable mechanical properties of human dentine arise from its complex hierarchical 3D structure. In this article, we have investigated the 3D structural alterations in dentine, caused by caries and artificial demineralisation. For this detailed investigation, SAXS tensor tomography (SASTT) has been implemented on the I22 beamline at Diamond Light Source, UK. SASTT is a technique that can probe the nanostructure of dentine, yielding orientation and degree of alignment of the mineralised collagen fibrils, while also providing a 3D reciprocal space map to investigate the detailed non-uniform scattering intensity distribution in all directions. The initial SASTT data provide insights into dentine structural alterations caused by caries and artificial demineralisation, facilitating further exploration of structure–mechanical property relationships, which may lead to improve the development of novel biomimetic materials for dental applications.