What it is
Fibrous materials from connective tissue to engineered composites carry load through a hierarchy of structure. In the intervertebral discs of the spine, plywood-like layers of collagen fibres in the outer ring (the annulus fibrosus) are built from nanometre-scale fibrils of staggered molecules, and how an intact disc couples fibril-scale mechanics to fibre-scale organisation under load has been unresolved. The authors introduce TomoSAXS, a full-field three-dimensional small-angle X-ray scattering tomography that, combined with phase-contrast X-ray tomography, maps fibril-to-fibre mechanics across an intact tissue. On 3 intact rat discs imaged before and after a 50 μm compression, it reconstructed nanoscale structure for more than 55% of all fibres in the scanned volume. The authors report that the molecular pre-strain inside fibrils (the D-period stagger) is patterned by the disc's layers and tightly correlated with fibre strain, and that fibril strain fell as pre-strain rose (fibrils under initial tension tended to compress under load, those under initial compression to stretch), consistent, the authors say, with load-sharing through molecular unwinding; residual fibrillar strains ranged from −10% to +10%. Radial strain bridges and sharply curved zones where the outer ring meets the disc's gel core (the nucleus pulposus) emerge as critical regulators of local mechanics. The method also imaged 1 whole human disc, without loading.
Why it matters
It measures collagen strain at both the fibril and the fibre scale in one intact disc under load, so the way the two scales share stress can be seen directly. In one representative rat disc, fibre strain rose with fibre curvature (r² = 0.480), and the authors suggest this may partly explain why the posterior and posterior-lateral regions of the disc are most prone to herniation. TomoSAXS needs about 1% of the theoretical minimum number of scattering projections for a tomographic reconstruction, and the authors present it as a platform for imaging nanoscale-to-microscale matrix mechanics in biological and synthetic fibrous materials, with applications in ageing and disease, evaluating therapies and designing bio-inspired materials.
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Filed underAdvanced X-ray Imaging Techniques, Spine and Intervertebral Disc Pathology, Cellular Mechanics and Interactions
