This short contribution aims to explain how intervertebral disc ‘degeneration’ differs from normal ageing, and to suggest how mechanical loading and constitutional factors interact to cause disc degeneration and prolapse. We suggest that disagreement on these matters in medico-legal practice often arises from a misunderstanding of the nature of ‘soft-tissue injuries’.
The belief that an intervertebral disc must degenerate
before it can herniate has clinical and medicolegal significance,
but lacks scientific validity. We hypothesised that tissue changes
in herniated discs differ from those in discs that degenerate without
herniation. Tissues were obtained at surgery from 21 herniated discs
and 11 non-herniated discs of similar degeneration as assessed by
the Pfirrmann grade. Thin sections were graded histologically, and
certain features were quantified using immunofluorescence combined
with confocal microscopy and image analysis. Herniated and degenerated
tissues were compared separately for each tissue type: nucleus, inner
annulus and outer annulus. Herniated tissues showed significantly greater proteoglycan loss
(outer annulus), neovascularisation (annulus), innervation (annulus),
cellularity/inflammation (annulus) and expression of matrix-degrading
enzymes (inner annulus) than degenerated discs. No significant differences
were seen in the nucleus tissue from herniated and degenerated discs.
Degenerative changes start in the nucleus, so it seems unlikely
that advanced degeneration caused herniation in 21 of these 32 discs.
On the contrary, specific changes in the annulus can be interpreted
as the consequences of herniation, when disruption allows local
swelling, proteoglycan loss, and the ingrowth of blood vessels,
nerves and inflammatory cells. In conclusion, it should not be assumed that degenerative changes
always precede disc herniation. Cite this article:
We investigated the distribution of compressive ‘stress’ within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting ‘stress profiles’ were analysed in order to characterise the distribution of vertical and horizontal compressive stress within each disc. A total of 87 discs from subjects aged between 16 and 87 years was examined. Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the ‘functional nucleus’) by approximately 50%, and the pressure within this region fell by 30%. The width of the functional annulus increased by 80% and the height of compressive ‘stress peaks’ within it by 160%. The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions. We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High ‘stress’ concentrations within the annulus may cause pain, and lead to further disruption.
