Disc height loss is the common theme with back pain.
It is a radiological finding that is associated with disc herniation, disc bulging, spondylolisthesis, disc protrusion, degeneration, stenosis just to name a few. Jill Urban in Arthritis and Research Therapy writes : new treatments are in development that are aimed at restoring disc height and biomechanical function.
In the 1970’s, Kirkaldy-Willis described three phases of degenerative disc disease.
- First, there is significant dysfunction caused by the acute back pain of the injury.
- Next, there is a long phase of relative instability at that particular vertebral segment and the patient will be prone to intermittent bouts of back pain.
- Finally, the body re-stabilizes the segment and the patient experiences fewer episodes of back pain.
What has never been explained clearly is how the discs become unstable after an acute injury and lose disc height due to annular fissures/tearing. Here, I will explain in a simple format.
Each disc is centred by a hydraulic cylinder and surrounded by numerous alternating fibrous sheets.
The angle fibres are oriented 30 degrees from horizontal. As you move inwards, each layer has fibres oriented in a mirrored fashion measuring -30 degrees. Therefore, when the upper vertebrae moves in an opposite direction to the lower one, only half the fibres come to tension. The other half of the fibres go into a slackened state.
Now, as the disc height reduces, so does the angle of the fibres. This leads to a greater displacement of the vertebrae and constitutes to the instability phase that Kirkaldy-Willis eluded to.
As you can see in the bulged image above, the angle of the annulus fibrosus becomes more acute.