Abstract
Introduction
The feature of disc degeneration most closely associated with pain is a large fissure in the annulus fibrosus. Nerves and blood vessels are excluded from normal discs by high matrix stresses and by high proteoglycan (PG) content. However, they appear to grow into annulus fissures in surgically-removed degenerated discs. We hypothesize that anulus fissures provide a micro-environment that is mechanically and chemically conducive to the in-growth of nerves and blood vessels.
Methods
18 three-vertebra thoraco-lumbar spine specimens (T10/12 to L2/4) were obtained from 9 cadavers aged 68-92 yrs. All 36 discs were injected with Toluidine Blue so that leaking dye would indicate major fissures in the annulus. Specimens were then compressed at 1000 N while positioned in simulated flexed and extended postures, and the distribution of compressive stress within each disc was characterised by pulling a pressure transducer through it in various planes. After testing, discs were dissected and the morphology of fissures noted. Reductions in stress in the vicinity of fissures were compared with average pressure in the disc nucleus. Distributions of PGs and collagen were investigated in 16 surgically-removed discs by staining with Safranin O. Digital images were analysed in Matlab to obtain profiles of stain density in the vicinity of fissures.
Results
Fifteen circumferential or radial annulus fissures were identified. Focal compressive stress within the fissures was lower than nucleus pressure, by 11-26% in normal discs, and by 47-63% in degenerated discs, depending on posture. Stress reductions within fissures were inversely related to average nucleus pressure (p<0.05). The edges of fissures were usually depleted of PGs, leaving a collagenous scaffold, and PG stain density remained depleted (less than 65% of the disc average) for an average 450 micrometers from the fissure edge.
Discussion
Matrix compressive stresses are particularly low within annulus fissures if the disc nucleus is degenerated and decompressed. Nucleus pressure normally tensions the anulus, and loss of this support mechanism allows focal decompression within fissures. This in turn may allow focal swelling and PG loss. Results support the hypothesis: annulus fissures are conducive to nerve and blood vessel in-growth.
