In situ Tracking of Individual Collagen Fibre Bundles in Intact Loaded Intervertebral Discs Exposes Damage-Susceptible Collagen Organisations

Many soft tissues, such as the intervertebral disc (IVD), have a hierarchical fibrous composite structure which suffers from regional damage. We hypothesise that clinical injury patterns in these tissues occur in localised regions where there is an integrated requirement for both marked compliance and significant load transfer. Here we used synchrotron computed tomography (sCT) to resolve collagen fibre bundles (~5μm width) in 3D throughout an intact native rat lumbar IVD under increasing compressive load. Using intact samples meant that tissue boundaries (such as endplate-disc or nucleus-annulus) and residual strain were preserved; this is vital for characterising both the inherent structure and structural changes upon loading in tissue regions functioning in a near-native environment. Nano-scale displacement measurements along >10,000 individual fibres were tracked, and fibre orientation, curvature and strain changes were compared between the failure-prone posterior-lateral region and the more robust anterior region. These methods can be widely applied to other soft tissues, to identify fibre structures which cause tissue regions to be more susceptible to injury and degeneration. Our results demonstrate for the first time that highly-localised changes in fibre orientation, curvature and strain indicate differences in regional strain transfer and mechanical function (e.g. tissue compliance), correlating directly with locations clinically at risk of damage. This included decreased fibre reorientation at higher loads, specific tissue morphology which reduced capacity for flexibility and high strain at the disc-endplate boundary.