The Hierarchical Structure of Carious Tooth Enamel: In Vivo Rodent Models and Novel X-ray Microdiffraction Approaches

Free, Robert  D

doi:https://doi.org/10.21985/n2-f72k-1k43

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The Hierarchical Structure of Carious Tooth Enamel: In Vivo Rodent Models and Novel X-ray Microdiffraction Approaches

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Tooth enamel, the outermost layer of human teeth, is a complex, hierarchically structured biocomposite. The details of this structure are important in multiple human health contexts, from understanding the progression of dental caries (tooth decay) to understanding the process of amelogenesis (enamel formation) and related developmental defects.While much is known about the structural and compositional heterogeneities of enamel across multiple length scales, new findings at the nanoscale have recently been made possible by a combination of advancements in sample preparation and characterization techniques. Specifically, the application of atom probe tomography (APT) to enamel has established the existence of amorphous intergranular phases (AIGPs) encasing the hydroxylapatite nanowires that compose enamel1. Furthermore, the nanoscale distribution of impurity elements within these phases, including magnesium, fluoride, and iron, have been shown to strongly influence the tissue’s acid susceptibility in vitro. Building on these discoveries and developing additional tools to further characterize enamel’s hierarchical structure, specifically in the context of dental caries, is the focus of this thesis. First, to explore the role that AIGPs play during in vivo caries, a rodent caries model was employed to generate a large pool of early-stage subsurface lesions, X-ray microtomography was applied to non-destructively map the lesion’s 3D structure, and targeted volumes of enamel were extracted for characterization via APT. Significantly, the tomographic datasets established that rodent caries is a strong structure analogy to the human case, a previously unverified assumption in many caries studies. Furthermore, the first successful APT of caries-affected enamel suggest that there is a significant restructuring of the enamel nanostructure during early stage caries, including a redistribution of AIGP-associated minority elements into small disconnected nanoclusters, an overall reduction in the concentration of AIGP-associated species, and an increase in organic signals. The second half of the thesis describes the application of a novel synchrotron-X-ray-based approach that elucidates the crystallographic variations across the human enamel microstructure. Combining a sub-micron scale X-ray probe with specially prepared thin enamel sections enables 2D diffraction patterns to be collected from small, well-separated volumes within the enamel microstructure, but still probes enough crystallites to extract population-level statistics on crystallographic features like lattice parameter, crystallite size, and orientation distributions. Furthermore, newly developed analytical approaches described herein allowed these measurements to be correlated to regions within the enamel microstructure, revealing systematic variations in lattice parameter and crystallite size with implications for enamel formation and its response during carious attack.

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