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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 below.

There is minimal blood supply to the disc, and blood is what brings healing nutrients and oxygen to damaged structures in the body. This means that the spinal disc lacks any significant reparative powers.

Unlike muscles, which have good blood supply, once a spinal disc is injured it cannot repair itself.

Stages of Degenerative Disc Disease

In the 1970s, Kirkaldy-Willis first described the "degenerative cascade" of degenerative disc disease. He postulated that after an individual suffers a torsional (twisting) injury to the disc, the disc would degenerate in three general stages.

1. First, there is significant dysfunction caused by the acute back pain of the injury.
2. 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.
3. Finally, the body re-stabilizes the segment and the patient experiences fewer episodes of back pain.

Based on the observation that demographic studies show less back pain from degenerative disc disease in elderly adults (over 60 years) than in younger adults (30 to 50 year-olds), the studies also concluded that this process happened over a period of 20 to 30 years.

Although patients over 60 may have pain from other degenerative conditions, such as degenerative osteoarthritis and/or spinal stenosis, it is uncommon for them to have pain from degenerative disc disease or other disc problems.

While this summary is a simplification of Kirkaldy-Willis's extensive work, it lays the framework for what is known today. We do know that lumbar disc degeneration is a very common and natural process, and only in limited cases does it become painful.

The natural history of lumbar degenerative disc disease is relatively benign. The pain tends to be intermittent, and although at times the pain may seem to be getting worse, the painful symptoms are generally not progressive.

While the disc degeneration will progress, the low back pain and other symptoms do not tend to get worse with the progression of the degeneration.

Understandably, many patients worry that if they are have a lot of low back pain when they are only 35 years old, the pain will become much worse and they may be in a wheelchair by the time they're in their sixties.

However, if patients can find a way to manage their back pain and maintain their function, the natural history is really quite favorable. With continued disc degeneration, all the inflammatory proteins within the disc space will eventually burn out, and the disc will usually become stiffer, thus decreasing micro-motion.

In fact, someone who is 65 years old is actually less likely to have discogenic back pain than someone who is 35 years old.

Therefore, degeneration on an MRI scan cannot be used as the sole diagnostic tool for lumbar degenerative disc disease.

Disc degeneration present on an MRI scan, often described as disc dessication or a black disc/dark disc, is not synonymous with a diagnosis of lower back pain that is caused by degenerative disc disease. The MRI findings need to be corroborated by the findings of the patient's history and physical exam.