Pathophysiology
In the 1970s, Kirkaldy-Willis first described the "degenerative cascade" of degenerative disc disease. His landmark findings outlined how the disc degenerates in three general stages. Because this degenerative process generally takes place over twenty to thirty years, the third phase is the one most likely to affect elderly patients.
The Dysfunction Phase
The initiating event in the degenerative cascade is typically the loss of internal disc integrity. Circumferential tears or fissures of the annulus characterize dysfunction. Tears can result in interrupted blood supply, endplate failure, and poor waste elimination. The tears can also add up to form radial tears. Annulus tears can also be accompanied by facet synovial reaction, cartilage degradation, or endplate separation.
The symptoms of the dysfunction stage are often localized. Contracted muscles, specific point tenderness, and hypomobility at the spinal segment can result in this stage.
The Unstable Phase
The second phase is often characterized as a period of instability within the vertebral segment. This can result in a loss of integrity of the tri-joint segment, leading to significant changes in biomechanical balance of the spinal segment. Additional complications may occur, including multiple annular tears, internal disc disruption, and resorption.
During this phase, the patient is most prone to syndromes of segmental instability and discogenic pain.
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