How a Disc Becomes Painful

Each spinal disc is a unique and well-designed structure in the spine. It is strong enough to resist terrific forces in multiple different planes of motion, yet it is still highly mobile and permits motion in multiple directions.

The disc has several functions, including acting as a shock absorber between the boney vertebral bodies.

Disc Anatomy and Function

The intervertebral disc has been likened to a jelly donut. It is comprised of a series of bands that form a tough outer layer, and soft, jelly-like material contained within.

Annulus Fibrosus - the disc's firm, tough outer layer

Nerves to the disc space only penetrate into the very outer portion of the annulus fibrosus. Even though there is little innervation to the disc, it can become a significant source of back pain if a tear in the annulus reaches the outer portion and the nerves become sensitized.

With continued degeneration, the nerves on the periphery of the disc will actually grow further into the disc space and become a source of pain.

Nucleus Pulposus - the jelly-like inner disc material

The inner material contained in the disc, the nucleus pulposus, contains a great deal of inflammatory proteins. If this inner disc material leaks out of the disc and comes in contact with a nerve root, it will inflame the nerve root and create pain down the leg (sciatica or lumbar radiculopathy) or down the arm (cervical radiculopathy).

In the same manner, if any of the inflammatory proteins within the disc space leak out to the outer annulus and touch the pain fibers in this area, it can create a lot of low back pain or neck pain.


When we are born, the disc is comprised of about 80% water, which gives it its spongy quality and allows it to function as a shock absorber. As we age, the water content decreases and the disc becomes less capable of acting as a shock absorber.

The proteins within the disc space also change composition, and most of us will develop tears into the annulus fibrosus (the outer hard core of the disc).

Most people will have some level of disc degeneration by their sixth decade, yet most do not have back pain.

Degenerated Disc on an MRI Scan

Magnetic Resonance Imaging, called an MRI scan, has contributed a great deal to our understanding of degenerative disc disease and the natural degenerative process. With the advent of MRI technology, good anatomic detail of the disc can be imaged and correlated with the individual's pain.

Through studies with MRI scans, it was found that:

  • A large number of young patients with chronic low back pain had evidence of disc degeneration on their MRI scans, and;
  • Up to 30% of young healthy adults with no back pain had disc degeneration on their MRI scans.

Variability in Degenerative Disc Disease

It is not exactly clear why some degenerated discs are painful and some are not. There is probably a variety of reasons that discs can become painful.

Some theories about pain from degenerative disc disease are:

  • If a disc is injured or degenerated, it may become painful because of the resultant instability from the disc injury, which in turn can lead to an inflammatory reaction and causes low back pain.
  • Some people seem to have nerve endings that penetrate more deeply into the outer annulus than others, and this is thought to make the degenerated disc more susceptible to becoming a source of pain.

While the exact causes are not known, there is a generally agreed upon theory of how a disc degenerates over time, as explained on the next page.