High Diffusivity Pathways in Human Dental Enamel

Tooth enamel is an acellular, mineralized tissue with limited regenerative potential. Loss of enamel integrity and function, whether congenital, acquired, or environmental in origin, is irreversible, ubiquitous, and imposes great burdens on society [1]. Ions such as F^-, CO₃^2-, Na⁺, Ag⁺, Sr²⁺, Fe³⁺, and Mg²⁺, are important determinants of enamel solubility; some are widely used in the prevention or treatment of defects. However, their mechanism of action and mode of transport in enamel remain poorly understood. This is at least in part due to the fact that the complex, hierarchical architecture of enamel was not taken into account in prior work studying diffusion in enamel under physiological conditions [2].<br/><br/>At the microscale, human enamel consists of rods that run from the dentino-enamel junction towards the external enamel surface and interrod enamel. Both are comprised of crystallites of hydroxylapatite (OHAp) that are highly elongated along the c-axis direction but display typical cross-sectional edge lengths of only 25-50 nm. Atom probe tomography (APT) and synchrotron X-ray microdiffaction revealed that crystallites are cemented together by an amorphous intergranular phase (AIGP) [3], are compositionally graded with a core enriched in minority components and a depleted shell [4], and that their average composition differs between rod and interrod enamel [5]. In the context of these recent insights, our long-term goal is to improve the understanding of transport processes in healthy enamel and enamel lesions as an integral step towards understanding the mechanisms by which lesions form and therapeutics act.<br/><br/>Herein, we describe a multi-scale correlative approach using depth profiling time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photo-electron spectroscopy (XPS) to determine average concentration profiles for multiple rods and interrod enamel (~2500 µm²) and APT to systematically determine effective diffusivities of F^-, Na⁺, Ag⁺, Sr²⁺, Fe³⁺, Zn²⁺, and Mg²⁺in the intergranular phase and, by extension, in rod and interrod enamel. For this purpose, ground and polished sections roughly parallel to the external surface of human premolars were treated with aqueous solutions of selected ions (0-1000 ppm, pH 7, 25 C, 24h). Concentration depth profiles (normal to the surface) were determined using ToF-SIMS and/or XPS. Chemical diffusivities were determined by fitting concentration profiles. Preliminary analysis suggest that diffusivities in rod and interrod enamel may differ by a factor of up to 30. Effective bulk diffusivities for both rods and interrods fell into the range between 10^-16 to 10^-19 cm²/s and decreased in the following order: Sr²⁺ = F^- &gt; Mg²⁺= Na⁺&gt; Cl^- &gt;&gt; Ba²⁺&gt;&gt; K⁺. Effective and absolute diffusivities at nanoscale level were acquired by fitting the APT reconstruction along with ion profiles from TOF-SIMS or XPS. APT analysis has been conducted on enamel treated with Na⁺, Mg²⁺, Ag⁺, F^-, and Sr²⁺ compared with the original untreated enamel. Na⁺and F^- uptake by the AIGP can differ from crystallites by a factor of up to 100; Mg²⁺ and Sr²⁺ uptake by the AIGP was less than 10 times than from crystallites; Ag⁺ barely infiltrates both crystallites and AIGP.<br/><br/>[1] N. Kassebaum, E. Bernabé, M. Dahiya, B. Bhandari, C. Murray, W. Marcenes, Journal of dental research, 94 (2015) 650-658.<br/>[2] V.K. Kis, A. Sulyok, M. Hegedus, I. Kovács, N. Rózsa, Z. Kovács, Acta Biomaterialia, 120 (2021) 104-115.<br/>[3] L.M. Gordon, M.J. Cohen, K.W. MacRenaris, J.D. Pasteris, T. Seda, D. Joester, Science, 347 (2015) 746-750.<br/>[4] K.A. DeRocher, P.J. Smeets, B.H. Goodge, M.J. Zachman, P.V. Balachandran, L. Stegbauer, M.J. Cohen, L.M. Gordon, J.M. Rondinelli, L.F. Kourkoutis, Nature, 583 (2020) 66-71.<br/>[5] R. Free, K. DeRocher, V. Cooley, R. Xu, S.R. Stock, D. Joester, Proc Natl Acad Sci U S A, 119 (2022) e2211285119.