Degenerative Disc Disease: the Natural Degenerative Process

Fig. 1: Degenerative Disc Disease
(larger view)

Fig. 2: MRI of severely degenerated
L₅-S₁
(larger view)

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 (see Figure 1).

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 (see Figure 2).

Magnetic Resonance Imaging (MRI scan) has contributed a great deal to our understanding of lumbar 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 back 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.

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 is not synonymous with a diagnosis of degenerative disc disease and low back pain. The MRI findings need to be corroborated by the findings of the patient’s history and physical exam.

Pain from degenerative disc disease

It is not exactly clear why some degenerated discs are painful and some are not. As with many common causes of back pain, 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 which results in 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 pain generator.

MRI findings without significant degenerative disc disease

The two findings most correlated with a pathological disc—a degenerating disc that is painful—are:

1. Disc space collapse
2. Cartilagenous end plate corrosion

The cartilagenous end plate is the source of disc nutrition. If this becomes eroded, the disc is likely to go through a degenerative cascade leading to the inflammation and micromotion instability, which in turn causes pain. As it goes through the process, the disc space will collapse.

MRI findings of disc dehydration (often referred to as a dark disc, because a disc with less water in it looks dark on an MRI scan), annular tears, or disc bulges are not specific causes of low back pain. These findings may or may not be the cause of the patient’s low back pain. It is well known that the results of surgically fusing a spine with these findings will be much more unreliable than fusing a disc space that has disc space collapse and cartilagenous endplate erosion.

• Article written By:
• Peter F. Ullrich, Jr, MD

November 29, 2005

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