Reconsidering the disc nucleus and its surroundings

Abstract

The disc nucleus is generally thought of as being an amorphous, gelatinous, semi-fluid substance which has little or no structural cohesion with its surroundings. However, a very simple experiment in which it was seen that the isolated nucleus could in fact support a substantial tensile load raised the question: is this really the case? Therefore, the general aim of this thesis was to investigate whether there is in fact any substantial structural organisation within the disc nucleus, and what, if any relationship exists between the nucleus and its surroundings, that is, the cartilaginous endplate and the inner annulus. The first study, which investigated axial connectivity in vertebra-nucleus-vertebra samples revealed that even with the annular fibres severed, and thus rendering them non load-bearing, there was a sufficient degree of structural cohesion within the body of the nucleus to transmit some load (on average 20 N) while undergoing tensile extension 2-5 times greater than that of the intact height of the disc. Microscopic examination revealed fibres inserting into the endplates and extending continuously from endplate to endplate in the central nuclear region. These nucleus fibres integrate with the fibres of the cartilaginous endplate via characteristic nodal insertions which were present throughout the nucleus region. The mechanism of connectivity at these nodes was investigated at the fibril level in the second part of this thesis. This revealed that the fibrils of the nucleus interweave with the fibrils of the cartilaginous endplate at the node tips. The effect of age on the axial mechanical properties and structure of the nucleus was also investigated in three additional age groups: newborn lambs, 3 month old lambs, and 12 month old lambs. The general morphology and mechanical properties of the disc were similar across all age groups. However, when the linear density of the nodes was examined it was found that there were considerably fewer nodes in the newborn lambs, suggesting that mechanical loading may play a role in the development of the nodal insertion network. Lastly, annulus-nucleus-annulus samples were prepared and subjected to mechanical testing and microstructural examination to investigate the transverse structural properties of the nucleus. Again, a characteristic failure pattern which consists of a smooth initial response of the sample, followed by progressive failure was observed. Examination of the structure of the samples shows that the normally highly convoluted nucleus fibres were drawn into alignment with the direction of loading. The annular lamellae direction was also reversed and drawn into the direction of loading, with the nucleus bundles drawing perpendicularly away from them. At higher magnification, the horizontally aligned nucleus fibres were seen to turn through approximately 90 in the transition region between annulus and nucleus, spreading out into the surrounding annular layers at a series of discrete attachment points. These results clearly show that the nucleus contains a convoluted but highly structured network of fibres of varying lengths which appear to integrate with the endplate and inner annulus and confer substantial axial and transverse interconnectivity which can be demonstrated mechanically. This new evidence makes it clear that the nucleus cannot be considered as a separate entity. At this time, it seems likely that this structural integration provides the nucleus with a form of tethered mobility that supports physiological functions distinct from the primary strength requirements of the motion segment.