ISSLS PRIZE IN BASIC SCIENCE 2020: Beyond microstructure-circumferential specialization within the lumbar intervertebral disc annulus extends to collagen nanostructure, with counterintuitive relationships to macroscale material properties

Purpose To determine whether the annulus of lumbar intervertebral discs contains circumferential specialization in collagen nanostructure and assess whether this coincides with functional differences in macroscale material properties. Methods Anterior and posterior disc wall samples were prepared from 38 mature ovine lumbar segments. Regional differences in molecular thermal stability and intermolecular network integrity of the annulus' tension-bearing collagen fibres were examined with hydrothermal isometric tension (HIT) analysis, with and without preceding NaBH4 treatment to stabilize labile crosslinks. Energetics of collagen denaturation were studied by differential scanning calorimetry (DSC). Tensile mechanics of annular lamellae were studied using oblique sagittal bone-disc-bone samples loaded to rupture. Annular failure characteristics of the ruptured test segments were compared via microscopy of serial sections. Results HIT showed that tension-bearing collagen fibres of the posterior annulus were composed of collagen molecules with significantly greater thermal stability and intermolecular network integrity than those of the anterior annulus. NaBH4 treatment confirmed that labile intermolecular crosslinks did not significantly contribute to network integrity in either region. Regional differences seen in DSC were smaller than those observed in HIT, indicating structural similarities in annular collagen outside of the main fibre bundles. Mechanical testing showed that the posterior annulus was significantly weaker than the anterior annulus. For both regions, ultimate tensile strengths of annular fibres were significantly greater than those previously reported. Ruptures in both regions were predominantly due to annular failure. Conclusion Specializations in collagen nanostructure exist between different circumferential regions of the annulus and coincide with significant differences in material properties. Graphic abstract These slides can be retrieved under Electronic Supplementary Material.