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

Disc Degeneration: Animal Models

Animal models provide a reliable guide to biologic processes within degenerating discs because they preserve the complex mechanical and biochemical environment of disc cells. However, they are less useful for investigating how degenerative changes are initiated in human beings because the interventions (or genetic defects) may not represent common occurrences in living people. Small animal models of disc degeneration have some additional limitations relating to their increased cell density, improved metabolite transport, and the presence of notochordal cells in the mature nucleus.[80] These differences can result in an increased propensity for disc repair.

Surgical disruption of the endplate or anulus leads inexorably to degenerative changes throughout the disc.[57,81] Perforation of the endplate from the side of the vertebral body causes nucleus decompression, proteoglycan loss, and internal disruption of the anulus.[57] The anulus disruption model, which simulates a peripheral rim tear, causes subsequent changes in the nucleus and endplate,[30,81,82] and shows that degenerative changes, unlike aging, need not start in the nucleus. Compressive loading of rodent tail discs can result in cell death, impaired matrix synthesis, and disruption of the anulus and vertebral body.[83,84,85] Compression without immobilization affects disc cell metabolism and matrix composition but does not lead to any architectural degenerative changes.[86] Injecting cement into the vertebral body to block nutrient transport through the endplate does not lead to disc degeneration within 1 year.[87]

The time span for detectable degenerative changes to occur ranges from 1 week for mice88 to many months for pigs and sheep.[57,81] For comparison, in human adolescents, it takes several years for disc degeneration to become apparent after endplate injury,[89] and narrowing in adult human discs progresses at approximately 3% per year.[74]

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