What is Intervertebral Disc Degeneration, and What Causes It?

Michael A. Adams, PhD; Peter J. Roughley, PhD

Disclosures

Spine. 2006;31(18):2151-2161. 

In This Article

Interpretation: What Causes Disc Degeneration?

The aforementioned definitions simplify the issue of causality. Plainly, excessive mechanical loading causes a disc to degenerate by disrupting its structure and precipitating a cascade of nonreversible cell-mediated responses leading to further disruption. As discussed previously, and in detail elsewhere,[4] cadaveric experiments and mathematical models show how various combinations of compression, bending, and torsion can cause all the major structural features of disc degeneration, including endplate defects, radial fissures, radial bulging, disc prolapse, and internal collapse of the anulus. Injury or wear-and-tear fatigue loading can create damage. Animal experiments confirm that structural disruption to disc or endplate always leads to cell-mediated degenerative changes.

Although we suggest that mechanical loading precipitates degeneration, the most important cause of degeneration could be the various processes that weaken a disc before disruption, or that impair its healing response. The combined effects of an unfavorable inheritance, middle age, inadequate metabolite transport, and loading history appear to weaken some discs to such an extent that physical disruption follows some minor incident. A common example is that of disc herniation following a cough or sneeze. It could be argued that such a weakened disc should be considered degenerated, even if it remains structurally sound. However, a disc is unlikely to become painful until it becomes disrupted, so there is little to be gained by anticipating future events and applying the term degeneration before this crucial nonreversible event actually occurs. As suggested previously, accelerated biochemical or cellular events in a structurally sound disc could be designated early degenerative changes to distinguish them from changes that are entirely typical of the disc's age. The multifactorial nature of disc weakening suggests that, from a medicolegal standpoint, all discs are vulnerable to a greater or lesser extent, and the vulnerability can only be gauged from the violence, or otherwise, required to disrupt the disc and initiate degeneration.

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