Lumbar Intervertebral Disc Disruption and Herniation: A Biomechanical and Multiscale Structural Investigation

Abstract

The intervertebral disc is a compliant connective tissue integrating the spinal vertebrae mechanically and facilitates movement and flexibility. Disruption and failure of the intervertebral disc is associated with herniation, sciatica and lower back pain, with these latter two conditions resulting in a significant burden of disability globally. It is therefore essential to have an in-depth understanding of the relation between the detailed structure of the disc and its associated endplates and those mechanical factors leading to their disruption. Using the ovine lumbar motion segment model and employing a series of multiscale structural studies in combination with mechanical testing, the research reported in this thesis explores a number of lesser-known areas associated with intervertebral disc/endplate disruption and herniation. Firstly, structural integration across the cartilage-vertebral endplate junction was investigated in order to understand why fragments of bone or cartilage are often attached to extruded herniation material. To unravel the structural complexity of the cartilage-vertebral endplate junction a novel method of selective demineralisation was used to induce partial junction failure. The various fibril intermingling morphologies observed using ultrastructural imaging at this junction suggests a mode of robust integration analogous to that in steel-reinforced concrete, where the steel elements are brought only into close proximity, but then fully integrated via the concrete matrix. In the second study it was shown that regardless of whether or not the facet joints are defunctioned, a flexed-compressed disc tends to fail with the nucleus first breaking through the lateral annulus and then tracking down into the posterolateral annular regions and subsequently appearing as a contained or uncontained herniation. This failure morphology was suggested to be a result of a small component of anteriorly directed shear which has the effect of over-loading the lamellae whose fibre sets are tilted anteriorly, but at the expense of under-loading those alternate lamellae whose fibre sets are tilted posteriorly. The third and final study provided further evidence for the above described concept of differential fibre set recruitment leading to localised annular disruption. This study showed that when an increasing amount of forward shear was generated due to the incorporation of a small segmental slope without any flexion, the nucleus was able to extrude directly through the lateral annulus.