Abstract
The detailed anatomy of interconnectivity of intervertebral disc annular fibre layers remains unclear and a structural survey of interlammellar connectivity is required to understand this anatomy and mechanical behavior. The subsequent failure modes of the annulus under hydrostatic loading require definition to understand genesis of annular tears and disc herniation.
Interlamellar Connectivity. We imaged anterior annular sections from ovine lumbar discs. Using differential interference contrast microscopy we were able to reconstruct a three-dimensional image of the interconnecting bridging network between layers. Annular Disruption. The nuclei of ovine lumbar discs were gradually pressurised to failure by injecting a viscous radio-opaque gel via their inferior vertebrae. Investigation of the resulting annular disruption was carried out using micro-computed tomography and DIC microscopy. This allowed analysis of annular failure patterns and herniation, with analysis of the pathway of nuclear movement during prolapse in relation to annular fibre separation within and between fibre layers.
Interlamellar Connectivity. A high level of connectivity between apparently disparate bridging elements was revealed. The extended form of the bridging network is that of occasional substantial radial connections spanning many lamellae with a subsidiary fine branching network. The fibrous bridging network is highly integrated with the lamellar architecture via a collagen-based system of interconnectivity. In particular this bridging network appears to have a major role in anchoring leading edges of incomplete annular lamellae. Annular Disruption and Disc Herniation. Gel extrusion from the posterior annulus was the most common mode of disc failure. Unlike other regions of the annular wall, the posterior region was unable to distribute hydrostatic pressures circumferentially. In each extrusion case, severe disruption to the posterior annulus was observed. While intralamellar disruption occurred in the mid annulus, interlamellar disrupt ion occurred in the outer posterior annulus. Radial ruptures between lamellae always propagated in the mid-axial plane.
The interlamellar architecture of the annulus is far more complex than has previously been recognised and this paper further defines the microanatomy of the disc wall. The hydrostatic pressure failure mode of the posterior annulus mirrors clinic al sites of annular tear and disc prolapsed in the neutral loading position.
Correspondence should be addressed to Associate Professor N. Susan Stott, Orthopaedic Department, Starship Children’s Hospital, Private Bag 92024, Auckland, New Zealand.
