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published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

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The complex microstructure and properties of dental enamel have been studied for decades using a variety of quantitative and qualitative techniques in order to gain a greater depth of understanding behind the chemical and physical processes that are associated with the formation and destruction of this biological apatite. Dental enamel is composed of highly ordered carbonated hydroxyapatite crystals which, together with its small organic component, are responsible for its mechanical strength, allowing it to serve its functional purpose. Environmental changes at any stage of the biomineralisation process or post eruption can disrupt the orientation and alter the structure and function, which can have detrimental clinical effects. The aim of this study is to understand and characterise the structural and crystallographic properties of disrupted enamel, and compare this to healthy unaffected tissue. Enamel affected by the genetic disorder, Amelogenesis Imperfecta, alongside enamel disrupted by dissolution and caries were studied using Synchrotron X-ray diffraction, 3D X-ray Microtomography, and Scanning Electron Microscopy techniques to relate these features to the clinically observed characteristics; to the chemistry; and to the known genetics of the tooth. Synchrotron radiation was used to map changes in preferred orientation, while the corresponding mineral density distributions were seen by using an in house developed, non-destructive microtomography system. Structural information on dental enamel at the crystallographic and micron length scales can benefit a variety of different disciplines. This project has the potential to inform early diagnosis, develop a tool for an early recognition of progressive or highly variable medical conditions, and design potential treatment regimes. The comparison of affected enamel to that of healthy enamel will provide a unique opportunity to identify the developmental pathways required for normal tooth development and give insights into the basic principles underlying mammalian biomineralisation.

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Precise timings and spatial progression of human enamel biomineralisation are still largely unknown due to scarcity of developing human enamel specimens available for investigation. This information is crucial for optimising emerging biomimetic regenerative and reparative dentistry routes. Five developing permanent incisors were obtained from an archaeological source and used alongside mature contemporary teeth for comparison. X-ray microtomography (XMT), synchrotron X-ray diffraction (S-XRD) and quantitative back-scattered electrons (qBSE) imaging were used to investigate the mineral density distribution, the crystallites texture magnitude and orientation and the nanostructure of dental enamel at various developmental stages, respectively. XMT revealed that there was a bi-directional mineralisation "front" that starts at the cusp tip and at the enamel-dentine junction (EDJ) travelling cervically and peripherally until the relative mineral density is uniform in the fully mature tooth (2.75 g/cm3 ± 0.01 g/cm3). S-XRD revealed that within one probed region, two populations of crystallite orientations exist simultaneously with an angular separation of 20-50°, with one population being more dominant than the other by a factor of approximately 3:7. Furthermore, one population displayed a higher degree of crystallite texture than the other. These phenomena were observed in all stages of tooth development. The crystallites in both populations were oriented approximately perpendicular to the EDJ regardless of development stage, indicating initial preferred directions of crystallites persist from early through to full maturation. The direction and magnitude of organisation within two distinct populations of crystallites within the developing and mature enamel has not been quantified previously. qBSE analyses suggested that the two observed populations are most likely due to prism decussation and revealed that mineralisation of prism cores precedes that of prism boundaries. These results provide new insights towards building a quantitative spatio-temporal model of human enamel biomineralisation in order to inform emerging biomimetic reparative/regenerative dental technologies.