Abstract
Dental caries is the most prevalent disease encountered by people of all ages around the world. Chemical changes occurring in the oral environment during the caries process alter the crystallography and microstructure of dental enamel resulting in loss of mechanical function. Little is known about the crystallographic effects of demineralization and remineralization. The motivation for this study was to develop understanding of the caries process at the crystallographic level in order to contribute towards a long term solution. In this study synchrotron X-ray diffraction combined with scanning electron microscopy and scanning microradiography have been used to correlate enamel crystallography, microstructure and mineral concentration respectively in enamel affected by natural caries and following artificial demineralization and remineralization regimes. In particular, the extent of destruction and re-formation of this complex structure has been measured. 2D diffraction patterns collected at the European Synchrotron Radiation Facility were used to quantify changes in the preferred orientation (crystallographic texture) and position of the (002) Bragg reflection within selected regions of interest in each tooth slice, and then correlated with the microstructure and local mineral mass. The results revealed that caries and artificial demineralization cause a large reduction in crystallographic texture which is coupled with the loss of mineral mass. Remineralization restores the texture to the original level seen in healthy enamel and restores mineral density. The results also showed that remineralization promotes ordered formation of new crystallites and growth of pre-existing crystallites which match the preferred orientation of healthy enamel. Combining microstructural and crystallographic characterization aids the understanding of caries and erosion processes and assists in the progress towards developing therapeutic treatments to allow affected enamel to regain structural integrity.
Highlights
Dental enamel is a unique, highly mineralized biological hard tissue comprised of 96 wt% mineral present in the form of substituted carbonated hydroxyapatite (HA) [Ca10-x(PO4)6x(CO3)x(OH)2] [1], with traces of organic material and fluid
Since this is the long axis of the needle-like enamel crystallites, evidence of preferred orientation is most strongly seen as variations in the intensity around the Debye-ring of (002) Bragg reflection [32]
Sharp intense peaks show the presence of strong preferred orientation, whereas broad peaks would have indicated a more randomly distributed orientation of crystallites [36]
Summary
Dental enamel is a unique, highly mineralized biological hard tissue comprised of 96 wt% mineral present in the form of substituted carbonated hydroxyapatite (HA) [Ca10-x(PO4)6x(CO3)x(OH)2] [1], with traces of organic material and fluid. At the nanometre length-scale, the apatite crystallites themselves grow into a needle-like shape which displays strong preferred orientation along the c axis (growth direction). This intricate multiple length-scale organized structure allows enamel to fulfil its functionality, resulting in a material with oriented strength and hardness for the compressive and shear forces it undergoes during mastication [3]. Early-stage tooth decay (dental caries) is an important pathological disease of enamel which is a worldwide problem affecting 60–90% of the population [4] It results in the loss of dental tissue, causing the strength and structural integrity to become compromised. Acidic buffers (for example, acetic acid or lactic acid [8]) can be used to create in vitro artificial caries-like lesions but may not necessarily produce a subsurface lesion (as it is isolated from any sort of remineralizing conditions), in which the surface of the enamel is etched away, displaying a steady gradient of dissolution from the enamel surface towards the bulk of the enamel [9]
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