The spinal motion segment relies critically on there being a mechanically robust integration between the compliant disc tissues and the rigid vertebral bone. Achieving such integration represents a major structural challenge. This study explores in detail the microstructural mechanisms involved in both the nucleus-endplate and annulus-endplate regions.

Vertebra-nucleus-vertebra samples were obtained from mature ovine lumbar motion segments and subjected to a novel ring-severing technique designed to eliminate the strain-limiting influence of any remaining annular elements. These samples were loaded in tension and then chemically fixed in order to preserve the stretched fibre arrangement, and then decalcified. Annulus-vertebra samples were similarly treated but without any loading prior to fixation. Differential interference contrast optical microscopy was then used to image at high resolution cryosectioned slices of the still integrated disc-vertebral endplate regions while maintained in their fully hydrated state.

Structural continuity across the nucleus-endplate junction was sufficient for the samples to support, on average, 20 N before tensile failure occurred. Microscopic examination revealed fibres inserting into the endplates and extending continuously from vertebra to vertebra in the central nuclear region. While the fibres in the nucleus possess a significant level of structural integration with the endplates their role is not primarily a tensile one: rather, in combination with their convoluted geometry, they confer on the nucleus a form of ‘tethered’ mobility. This permits a high degree of shape change in the nucleus during normal disc function in which hydrostatic loading plays an essential role. The annular fibre bundles on entering the endplate are shown to subdivide into sub-bundles to form a 3-D multi-leaf morphology with each leaf separated by cartilaginous endplate matrix. This branched morphology increases the interface area between bundle and matrix in proportion to the number of sub-bundles formed.

Our study challenges previously published views on nucleus-endplate relationships. We also show that the robust integration of the annular fibres in the endplate is achieved via a branched morphology exploiting a mechanism of shear-stress transfer, with the anchorage strength optimised over a relatively short endplate insertion depth.