Regional Variations in the Molecular Structure and Mechanics of the Lumbar Intervertebral Disc Annulus
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Despite the prevalence of lumbar intervertebral disc related pathologies, fundamental aspects of the structure and mechanics of healthy discs remain unexplored. Little is known about whether the collagen structure of the annulus varies between different circumferential locations at the sub-microscopic level. Moreover, while studies have explored regional variations in the tensile mechanics of the annulus, none have done this for the entire disc wall, with annulus/endplate/vertebrae integrations preserved. The objectives of this study were to investigate (i) whether molecular-level regional variations exist between the anterior and posterior regions of the lumbar intervertebral disc annulus, and (ii) whether potential differences in molecular structure are accompanied by differences in the tensile mechanics of the disc wall. Mature ovine lumbar spines were used as a model for the human lumbar spine. To assess collagen nanostructure of the anterior vs. posterior annulus, hydrothermal isometric tension analysis and differential scanning calorimetry were used. The mechanics of the disc wall at these regions were assessed using uniaxial tensile tests to failure on oblique sagittal bone-annulus-bone samples, prepared such that collagen fibres in half the lamellae were parallel to the direction of the applied load. Following rupture, samples were cryo-sectioned and light microscopy was used to determine the radial thickness of the annulus, confirm fusion of vertebral growth plates, and assess failure mode. HIT analysis revealed that collagen from the posterior annulus was significantly more thermally stable and indicated a presence of greater crosslinking density compared to the anterior annulus, from L5-6 to L1-2. In DSC, regional variations in thermal stability were less apparent than in HIT analysis. Despite an indication of greater crosslinking density, samples from the posterior annulus has significantly lower ultimate tensile strength compared to those from the anterior annulus. Alongside new contributions to disc structure and mechanics, the findings from this work suggest that the posterior annulus is optimized for a role other than strength, which could have important implications for therapies targeting annular repair.