Synchrotron X-ray Studies of the Structural and Functional Hierarchies in Mineralised Human Dental Enamel: A State-of-the-Art Review

Abstract

Hard dental tissues possess a complex hierarchical structure that is particularly evident in enamel, the most mineralised substance in the human body. Its complex and interlinked organisation at the Ångstrom (crystal lattice), nano-, micro-, and macro-scales is the result of evolutionary optimisation for mechanical and functional performance: hardness and stiffness, fracture toughness, thermal, and chemical resistance. Understanding the physical–chemical–structural relationships at each scale requires the application of appropriately sensitive and resolving probes. Synchrotron X-ray techniques offer the possibility to progress significantly beyond the capabilities of conventional laboratory instruments, i.e., X-ray diffractometers, and electron and atomic force microscopes. The last few decades have witnessed the accumulation of results obtained from X-ray scattering (diffraction), spectroscopy (including polarisation analysis), and imaging (including ptychography and tomography). The current article presents a multi-disciplinary review of nearly 40 years of discoveries and advancements, primarily pertaining to the study of enamel and its demineralisation (caries), but also linked to the investigations of other mineralised tissues such as dentine, bone, etc. The modelling approaches informed by these observations are also overviewed. The strategic aim of the present review was to identify and evaluate prospective avenues for analysing dental tissues and developing treatments and prophylaxis for improved dental health.

A novel image processing procedure for the quantitative evaluation of dental enamel prism arrangement

“…While nanocrystal orientation mapping of whole cross sections of dental enamel relies on several well established and state-of-the-art techniques like synchrotron X-ray diffraction (Al-Jawad et al, 2007;Free et al, 2022;Raue et al, 2012;Simmons et al, 2011) (for a recent review, see Besnard et al (2023)) or transmission electron back scattered diffraction (Koblischka-Veneva et al, 2018), the analysis of prism orientation mainly comprise qualitative evaluation of light microscopy or SEM images, there are only a few quantitative data regarding the prism pattern observed using these methods (e.g., Lynch et al, 2010;Rensberger, 1997). Enamel prisms, according to the movement of ameloblasts during amelogenesis, follow a wavy pathway from the DEJ to the enamel external surface (Osborn, 1965).…”

Advanced Time-Stepping Interpretation of Fly-Scan Continuous Rotation Synchrotron Tomography of Dental Enamel Demineralization

“…Enamel is an acellular tissue that forms the outer layer of tooth and is composed mainly of calcium and phosphorus minerals , formed from hydroxyapatite (HAp) crystallites arranged as a hierarchical structure from macroscopic down to the nanoscale. ,− Enamel has some remarkable mechanical properties, but it lacks resistance to acids (acid erosionor due to caries). ,− When exposed to acid, enamel undergoes dissolution of its structure to form voids that can be visualized at the macro scale down to the microscale (rods and inter-rods) as changes in structure and finally at the nanoscale with the modification of the crystallites and small-angle X-ray scattering (SAXS) signature. ,,− This demineralization process has been shown to be inhomogeneous, with the existence of preferential sites of dissolution, , and appears to be a dynamic process with loss, as well as redeposition, of ions. ,− …”

“…For studying smaller structures, such as enamel rods, one requires a fast acquisition time, high resolution, large photon flux, and certain flexibility in the positioning and motion of the sample within its environment. Synchrotron setups can be adapted to accommodate for these challenges. , Using the promising new approach and setup using continuous rotation and analysis algorithm applied at different stages of the process, the degradation of enamel was studied in this paper, with the resolution limited by the voxel size of the setup used. Here, a setup that allowed continuous rotation was used with a large field of view at the voxel size of 0.325 μm, as summarized in Figure . …”

The DIAD Approach to Correlative Synchrotron X-ray Imaging and Diffraction Analysis of Human Enamel

“…The correlative imaging paradigm aims to collect data using different modalities at matched locations to extract rich multimodal information. − However, the practical application of this idea presents significant challenges. , Synchrotron instruments are eminently well-suited for this type of study, − but the practical realization of the correlative approach requires both hardware and software implementation and the advancement of practical experimental techniques. The principal point of interest in conducting experiments at the recently commissioned Dual Imaging and Diffraction (DIAD) − beamline at Diamond Light Source (DLS) is to enable concurrent data collection in two modalities without changes to the beamline setup: the optics train, beam energies, and detector positions.…”