Abstract
Introduction
Delamination of the annulus fibrosus is an early feature of disc degeneration, and it allows individual lamellae to collapse into the nucleus, or to bulge radially outwards. We hypothesise that delamination is driven by high gradients of compressive stress in the annulus.
Methods
102 thoracolumbar motion segments (T8-9 to L5-S1) were dissected from 42 cadavers aged 19–92 yrs. Each specimen was subjected to 1 kN compression, while intradiscal compressive stresses were measured by pulling a pressure transducer along the disc's mid-sagittal diameter. Stress gradients were measured, in the anterior and posterior annulus, as the average rate of increase in compressive stress (MPa/mm) between the nucleus and the region of maximum stress in the annulus. Average nucleus pressure was also recorded. Disc degeneration was assessed macroscopically on a scale of 1–4.
Results
Compared to grade 2 discs, moderately degenerated grade 3 discs showed increased stress gradients in the annulus, especially in the posterior annulus where they increased by an average 106%. Nucleus pressure showed minimal changes. However, comparing grade 3 discs with severely degenerated grade 4 discs showed that nucleus pressure fell by 47%, while stress gradients showed little or no further change.
Discussion
The results support our hypothesis. In early disc degeneration, a minor reduction in nucleus pressure is sufficient to generate high stress gradients in the annulus. These shear adjacent lamellae, causing delamination and allowing internal displacement of nucleus. As disc degeneration progresses, nucleus migration causes severe decompression, and compressive loading is transferred increasingly to the neural arch.
Conflicts of Interest
None
Source of Funding
None
This abstract has not been previously published in whole or in part; nor has it been presented previously at a national meeting.
