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.

Most researchers have employed conventional histological and related methods to investigate the complex architecture of the IVD. Recognizing the inherent limitations of these methods we have pioneered new microstructural and micromechanical techniques that have greatly enhanced our understanding of the 3-D architecture of the IVD. Using sectioning planes that take full account of the oblique fibre angles in the annular wall, combined with specialized optical imaging techniques that provide high resolution structural images of fully hydrated thick sections we have described new levels of structural complexity that are clearly implicated in the biomechanical function of this highly complex connective tissue organ.

The primary regions of structural interest are the annulus, the annular-endplate junction and the nucleus-end-plate junction. Within the complex multilayered annular wall we have identified a system of collagen-rich bridging structures that both integrate proximate oblique and counter-oblique layers as well as providing long-range radial continuity across many layers. We argue that this system has an important biomechanical role of lashing alternate ‘like’ layers together whilst providing for some freedom of fibre angle change between immediately adjacent layers coursing in counter oblique directions. Thus, under the deformations generated by direct compressive, bulging, flexion and minor rotational forces, the structural integrity of the annulus is maintained.

We have also clarified important features of both annular/endplate and nucleus/endplate structural integration. Our very recent structural studies of the lumbar motion segment suggest that the current models of disc/endplate integration require substantial revision. This presentation will describe new experimental evidence in support of a more appropriate model of structural integration.