The regional susceptibility of the intervertebral disc to mechanically induced disruption and herniation

Abstract

Internal disc disruption and herniation are associated with low back pain and while it is widely accepted that the posterior and posterolateral regions of the disc are the primary herniation sites, much less is known about mechanisms of herniation operating in other regions of the disc. The aim of this thesis was therefore to investigate whether other regions of the disc might be implicated in mechanically induced disruption and herniation. This thesis reports the findings of three interrelated studies. The first study investigated mechanically induced herniations in flexed and compressed ovine lumbar motion segments by employing a progressive transverse sectioning technique, enabling examination of the entire disc volume. Disruption in the lateral annulus was commonly observed, associated with circumferential tracking of nucleus within the annulus towards the posterolateral and posterior regions. The vulnerability of the lateral annulus to disruption was thought to arise from the overloading of its differentially recruited oblique/counteroblique fibre sets, resulting from induced anterior shear. The second study investigated whether localised damage created by a needle puncture, as performed during discography, provides a preferred passage for nuclear material through the annulus, independent of any longer-term degenerative changes. Herniations were mechanically induced in discs that had been punctured with either a 25-gauge or a larger 18-gauge needle. Transverse sectioning of the whole disc volume revealed that there was no association between the 25-gauge puncture and disc disruption/herniation. In contrast, nuclear material was observed to migrate through the 18-gauge puncture. Further, disruption in the lateral annulus was commonly observed independent of the presence of a puncture thereby reinforcing the fact that this previously unreported site of initial disc disruption is of fundamental importance for the development of a more rigorous understanding of disc failure. The final study investigated the response of the annulus to anterior shear and flexion. Micro-structural analysis of motion segments fixed in anterior shear or flexion demonstrated that in the lateral regions anterior shear differentially recruits the oblique and counteroblique fibres, while flexion recruits these fibres more equally.