The spine is a column composed of bones (vertebra) interspersed with intervertebral discs that act as shock absorbers. The spine transmits the spinal cord in a bony canal and nerve roots exit the spinal cord at each vertebral level. Nerve roots supply power and sensation to different parts of the body.
The cervical spine (neck) and lumbar spine (lower back) are mobile structures, allowing for neck movements and movements of the low back. These parts of the spine are most at risk of wear and tear or age-related degenerative changes. The thoracic spine is a more rigid structure as it is connected by the ribs to the breastbone (sternum). The thoracic spine is less likely to accumulate wear and tear.
This is a cross-section through the level of a lumbar intervertebral disc in a young, healthy spine. The disc structure is maintained and the disc is well hydrated. There is no degeneration or overgrowth of the facet joints. There is plenty of space for the nerves in the central canal and also for the nerve roots as they exit the canal.
Acute disc prolapse
Spinal degeneration usually begins with wearing out of the discs or shock absorbers. This can lead to acute disc herniation and irritation of a nerve root.
Note the tear in the tough outer part of the disc, and the herniation or prolapse of the soft, gelatinous interior outside the confines of the disc space. There is irritation and compression of the nerve root as it leaves the canal, marked in red.
Mechanical compression and subsequent inflammation of the nerve root causes leg pain or sciatica.
In more chronic degenerative disease there can be overgrowth of bone and ligament and broad based disc bulge, leading to compression of the spinal cord or cauda equina (lumbosacral nerve roots).
Note the combination of disc herniation and facet joint overgrowth. There is less space for the nerves in the central canal and as they leave the canal, marked in red.
Stenosis in the neck causes cervical myelopathy. Stenosis in the back causes neurogenic claudication.
Spinal degeneration with age
Studies with MRI scans collectively show the likelihood of pain-free spinal degeneration with age.
37% of pain-free 20-year-olds have disc degeneration
96% of pain-free 80-year-olds have disc degeneration