Abstract:
The intervertebral disc comprises three main regions — the nucleus, the annulus, and the endplates, the latter consisting of a thin layer of cartilage (cartilaginous endplate) over a layer of cortical bone (vertebral endplate). Annulus-endplate anchorage performs a critical role in the disc, creating a strong structural link between the compliant annulus and the stiff vertebrae. There has been a growing interest in endplate failure and its frequent involvement in disc herniation. Employing an ovine disc model and high-resolution multiscale imaging techniques, the aim of this thesis was to explore the structural principles that govern annulus-endplate anchorage and form the basis of its strength and its failure. The first study provides new insights into how anchorage is achieved in the mature disc. Microstructurally, annular bundles only penetrate through the shallow thickness of the cartilaginous endplate. Within this layer however, the bundles sub-divide to form a three-dimensional branched structure. This branching morphology provides an effective mechanism of integration — it increases the interface area between the two tissues over which forces are distributed and thus creates a stronger anchorage. Ultrastructural analysis provides new evidence of a fibril-based form of integration across the cement line, with fibrils from the annular sub-bundles penetrating beyond the cement line a short distance and in some cases blending with fibrils from the cartilaginous and vertebral endplates. These structural mechanisms, along with the presence of calcification (which accounts for a substantial depth of the cartilaginous endplate), presumably form the basis of the anchorage’s high tensile strength. Via a series of manual loading experiments followed by detailed structural analysis, the second study focuses on the mechanisms of anchorage failure. Two main modes were observed — failure at the tidemark (the calcified endplate/non-calcified annulus boundary) or at the cement line (the calcified endplate/bone interface). Samples subjected to axial tension typically contained more anchorage failures compared to those subjected to torsion and in-plane tension; this is consistent with the high frequency of endplate failure seen in flexion-induced herniation. Structural examination of the torsion-loaded discs indicates that this is probably due to acute fibre bending at the soft-hard tissue interface of the tidemark. The final study investigates the mechanism of annulus-endplate integration in immature discs to gain a better understanding of how the mature system develops. Despite significant changes in endplate morphology, the prevalence of branching from birth emphasises the critical role that this mechanism plays in strengthening annular-endplate anchorage. Further, the changing alignment of chondrons within the cartilaginous matrix provides new insights into endplate development and ossification.