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bioRxiv · 10.1101/2021.10.01.462427

Fibrillar-Level Strain Gradients Across the Calcified Bone-Cartilage Interface

Badar, W.·Ali, H.·Brooker, O. N.·Newham, E.·Snow, T.·Terrill, N. J.·Tozzi, G.·Fratzl, P.·Knight, M. M.·Gupta, H. S.
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Abstract

The bone-cartilage unit (BCU) is a universal feature in diarthrodial joints, which is mechanically-graded and subjected to shear and compressive strains. Changes in the BCU have been linked to osteoarthritis progression. Here we report existence of a physiological internal strain gradient (pre-strain) across the BCU at the ultrastructural scale of the extracellular matrix constituents, specifically the collagen fibril. We use X-ray scattering that probes changes in the axial periodicity of fibril-level D-stagger of tropocollagen molecules in the matrix fibrils, as a measure of microscopic pre-strain. We find that mineralized collagen nanofibrils in the calcified plate are in tensile pre-strain relative to the underlying trabecular bone. This behaviour contrasts with the previously accepted notion that fibrillar pre-strain (or D-stagger) in collagenous tissues always reduces with mineralization, via reduced hydration and associated swelling pressure. Within the calcified part of the BCU, a finer-scale gradient in pre-strain (0.6% increase over [~]50m) is observed. The increased fibrillar pre-strain is linked to prior research reporting large tissue-level residual strains under compression. The findings may have biomechanical adaptative significance: higher in-built molecular level resilience/damage resistance to physiological compression, and disruption of the molecular-level pre-strains during remodelling of the BCI may be potential factors in osteoarthritis-based degeneration.

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Badar, W., Ali, H., Brooker, O. N., Newham, E., Snow, T., Terrill, N. J., Tozzi, G., Fratzl, P., Knight, M. M., Gupta, H. S.. 2021-10-02. Fibrillar-Level Strain Gradients Across the Calcified Bone-Cartilage Interface. https://doi.org/10.1101/2021.10.01.462427

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