INTERNAL DISC DISRUPTION: THE ROLE OF STRESS GRADIENTS

Background In the annulus fibrosus of degenerated intervertebral discs, disruption to inter-lamellar cross-ties appears to lead to delamination, and the development of anulus fissures. We hypothesise that such internal disruption is likely to be driven by high gradients of compressive stress (i.e. large differences in stress from the nucleus to the mid anulus). Methods Eighty-nine thoracolumbar motion segements, from T7/8 to L4/5, were dissected from 38 cadavers aged 42-96 yrs. Each was subjected to 1 kN compressive loading, while intradiscal compressive stresses were measured by pulling a pressure transducer along the disc9s mid-sagittal diameter. Measurements were repeated in flexed and extended postures. Stress gradients were measured, in the anterior and posterior anulus of each disc, as the average rate of increase in stress (MPa/mm) between the nucleus and the region of maximum compressive stress in the anulus. Average nucleus pressure (IDP) was also recorded. Results Stress gradients increased with grade of disc degeneration, especially in the posterior anulus (p Discussion Stress gradients are highest in the region of the disc (the middle posterior anulus) that is most disrupted by the degenerative process. Unlike the overall peak stresses, or IDP, stress gradients remain high in severely degenerated discs, and are not reduced when load-bearing is transferred to the neural arch. These results suggest that stress gradients play a major role in the internal disruption of degenerated human discs. Acknowledgements M Stefanakis would like to thank the Greek Institute of Scholarships (I.K.Y) for financial support.