Lumbar Degenerative Disc Disease as a Cause of
Lower Back Pain
Chronic back pain and the reasons why lower back pain exists, such as lumbar degenerative disc disease (DDD), can often be explained by looking at the overall anatomy of the spine. Mechanically, everything associated with the anatomy of the lumbar spine revolves around the disc. To understand this better, it helps to gain an understanding of the function of the lumbar spine. The disc functions like a shock absorber. This structure absorbs impact and still allows for motion of the spine. It also has to have restraints to prevent damage to itself and the other spinal structures such as the nerves and facets. These requirements create some major demands. We will discuss those demands and offer causes of low back pain and reasons to why lower back problems exist.
In appearance, a disc of the lumbar spine looks like a jelly donut. The jelly (called the nucleus) is made of sugars attached to a protein backbone called a proteoglycan. This structure allows it to act like a giant sponge. The jelly pulls in water from the body of the vertebra to create a high-pressure interior matrix (think of the jelly as the air pressure in a tire).
The outside of the donut is made up of about thirty rings of collagen, called the annulus, just like the plies of a tire. These rings are normally quite tough. Each layer of these rings alternates in angulation in their attachment to the bone of the vertebra. The endplates of the disc separate the bone of the vertebral body from the interior of the disc. They are made of hyaline cartilage-the same cartilage that lines the hip and knee joints. This material creates a barrier to nutrients and oxygen entering and exiting the disc.
Common Causes of Degenerative Disc Disease
Blood Supply and Imbibition
Spine doctors are often asked to explain what is a degenerative disc and why does lower back pain exist. When someone experiences lower back pain that does not go away, they may assume they have a slipped disc, a pinched nerve in the lower back, or a compressed nerve in the spine. The problem, in reality, goes much further than this. Problems exist with the design of the disc that causes the “disease” we know today as lumbar degenerative disc disease or DDD. The first problem is that the blood supply, for all intensive purposes, disappears from the disc by about the age of eight. This means that collectively, the discs are the largest structures in the body that have no blood supply.
Without a blood supply, there is very poor oxygen penetration into the interior of the disc. The only fluids that can be exchanged are under hydrostatic and osmotic pressure. This means that motion of the disc exchanges fluids similarly to a squeezing a sponge under water and releasing it. The water and nutrients that this material absorbs and releases, transfers through the endplate of the vertebra under a process called imbibition.
The fluids that are transferred into the disc are poor in oxygen and limited in nutrients. This creates a problem for the living cells inside the disc. These cells produce the glycoproteins, which make up the nucleus, (the gel or sponge) and they need oxygen to function well. Without oxygen, these cells become much less effective keeping up with maintaining the jelly. Without the production and maintenance of these gel proteins, the pressure in the disc drops with age.
When the pressure drops inside the disc, this is similar to letting some air out of a car tire. As the pressure in the car tire drops, the sidewalls bulge outward from the weight of the car. When you then try to drive on a partially filled tire, there is less stability to hold the road, especially with unpredicted maneuvers such as a quick turn to avoid an obstacle or driving over an unexpected speed bump. Just as the car has trouble holding the road, a degenerative disc of the spine is much less resistant to abnormal movements and these motions can tear the disc wall. An injury or unexpected lift or fall may injure the disc. That is why once an individual has been diagnosed with degenerative disc disease or degenerative facet disease, proper lifting techniques become very important. Proper lifting is one of the key areas regarding how to prevent back injuries.
Annular Tears and Nociceptors
If a tear occurs, without a blood supply, the disc can’t heal if injured. One of the most common causes of low back pain is a tear. All injuries to the disc therefore are cumulative or add up. To say it another way, any injury to the disc is essentially, a permanent injury. A tear of the collagen in the donut won’t repair itself. The cells lining the outside wall of the annulus attempt to repair the outer defect but are unsuccessful and actually get in the way.
There is a tenuous blood supply in the outer 1/3 of the disc. Tears that go out to here will attempt to heal but the scar tissue laid down is not nearly as strong as the collagen fibers it attempts to replace. To make matters worse, in regards to lumbar degenerative disc disease, any blood vessels that grow into the torn fibers of the disc carry along with them new pain fibers. These fibers are highly sensitive and another major cause of pain sensitization of the disc.
The back portion of the disc (posterior annulus) is full of pain sensors called nocicepters. When a tear occurs in the annulus of the disc, these pain receptors come in contact with the nucleus of the disc. These pain receptors themselves then become inflamed and much more easily transmit pain signals.
Your Mom and Dad
The second problem with the disc (and whether you will have the symptoms associated with lumbar degenerative disc disease) is how you picked your mom and dad— genetics. This has to do with the type of collagen that makes up the plies of the tire (the annulus.) The collagen in the annulus is not uniform. Some fibers are pliable and resilient and can undergo multiple loading cycles without any wear just like thick bungee cords. Other types of collagen are unfortunately brittle, like wire coat hangers. You can bend these only a limited amount of times before they break from fatigue failure.
Good examples of this genetic relationship are the variety of patients I am asked to see. Some are injured on the ski slopes. Eighty, and occasionally, ninety years olds will sporadically develop pain in their backs and have an MRI performed through the emergency room (it helps to review another section on this website: understanding an MRI of the lumbar spine). There are times these octo and nanogenarians have perfect appearing discs. On the other end of the spectrum are the unfortunate twelve year olds in my practice with severe degenerative disc disease; while back pain in children is not too uncommon, finding severe DDD in a child does cause some alarm.
There is a big caveat here. Remember, just because the patient may have degenerative disc disease does not mean they are going to have symptoms of lower back pain. A study out of Emory University noted that more than 60% of individuals without back or leg pain had degenerative changes in their discs. These people had no symptoms. I am always reminded of this when I see older NFL players with horrible looking spines and minimal to no pain. This leads into the theory of wiring.
There are 2 groups of patients with the same problems found on x-rays and MRI. One group has no symptoms and the other may be incapacitated with the same type of degenerative spine. It appears that some individuals are “wired” to have low back pain and others are not. This effect has to do with the sensitivity of the central nervous system and the number of pain receptors present and active in the area of injury. For example, I personally have a degenerative disc at L5-S1 and have virtually no low back pain. Others with the same type of disc on MRI are incapacitated. I have lousy wiring and am so grateful that my spine is so uneducated.