A degenerating spinal disc does not always lead to pain or other symptoms. Because the disc itself has very little innervation, pain usually occurs when the degenerating disc affects other structures in the spine (such as muscles, joint, or nerve roots).
Pain associated with degenerative disc disease generally stems from two main factors:
- Inflammation. Inflammatory proteins from the disc space interior can leak out as the disc degenerates, causing swelling in the surrounding spinal structures. This inflammation can produce muscle tension, muscle spasms, and local tenderness in the back or neck. If a nerve root becomes inflamed, pain and numbness may radiate into the arm and shoulder (called a cervical radiculopathy in cases of cervical disc degeneration), or into the hips or leg (called a lumbar radiculopathy, in cases of lumbar disc degeneration).
- Abnormal micro-motion instability. The cushioning and support a disc typically provides decreases as the disc’s outer layer (the annulus fibrosis) degenerates, leading to small, unnatural motions between vertebrae. These micro-motions can cause tension and irritation in the surrounding muscles, joints, and/or nerve roots as the spinal segment becomes progressively more unstable, causing intermittent episodes of more intense pain.
Both inflammation and micro-motion instability can cause lower back or neck muscle spasms. The muscle spasm is the body's attempt to stabilize the spine. Muscle tension and spasms can be quite painful, and are thought to cause the flare-ups of intense pain associated with degenerative disc disease.
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What Happens in The Spine During Disc Degeneration?
Degenerative disc disease primarily concerns a spinal disc, but will most likely impact other parts of the spine as well. The two findings most correlated with a painful disc are:
- Cartilaginous Endplate Erosion. Like other joints in the body, each vertebral segment is a joint that has cartilage in it. In between a spinal disc and each vertebral body is a layer of cartilage known as the endplate. The endplate sandwiches the spinal disc and acts as a gatekeeper for oxygen and nutrients entering and leaving the disc. As the disc wears down and the endplate begins to erode, this flow of nutrition is compromised, which can hasten disc degeneration. As the disc goes through this process, the disc space will collapse.
- Disc space collapse. As a disc degenerates the disc space will collapse, placing undue strain on the surrounding muscles as they support the spine and shortening the space between vertebrae, leading to additional micro-motion and spinal instability.
These processes typically progress gradually rather than all at once. Endplate erosion and disc space collapse can add to spinal instability, tension in the surrounding muscles, and both local and nerve root pain.