The intervertebral disc binds two adjacent vertebral bodies. Disc degeneration is the progressive failure of the structure of the disc, leading to several changes such as tears, fissures, release of inflammatory substances, loss of water content, and reduction of disc height up to collapse, among others.
The picture above shows the MRI of a lumbar spine in a patient with low back pain. The difference between the three normal discs and the degenerated disc is striking, and it seems straightforward to attribute the cause of the low back pain to the degenerated disc. However, this may not be true.
Disc degeneration is common in individuals with no back pain
For instance, consider this systematic review by my colleagues at the University of Washington CLEAR Center. The review included 33 articles reporting imaging findings for 3,110 individuals without back pain: disc degeneration was present in a high proportion, increasing with age (see table below).
Age (people without pain) | 20 | 30 | 40 | 50 | 60 | 70 | 80 |
Disc degeneration | 37% | 52% | 68% | 80% | 88% | 93% | 96% |
Given this finding, how do we know that our patient' pain is caused by the degenerated disc? Can the pain have another cause that is not detected by the image, the degeneration being irrelevant to the pain? The question is of crucial importance, as disc degeneration is one major reason for undergoing back surgery. If the pain does not stem from the disc, surgery will fail to improve the patient's condition, and may even cause worsening.
And yet, disc degeneration is associated with low back pain
This statement may confuse the reader, but does not contradict the above data. In a population-based study of Finnish individuals born in 1966, an MRI was performed in 1,505 participants who reached the age of 47 years: the more the disc degeneration, the higher the chance of having low back pain. This tells us that disc degeneration is potentially important, at least in some patients with low back pain. The challenge is to know for which patients disc degeneration causes pain, and for which does not.
What are we missing?
We still do not know why disc degeneration is associated with back pain in certain individuals, and not in other ones. Furthermore, an association does not necessarily imply a causal relationship: the association could be coincidental, and the degeneration may not be the cause of the back pain. It has also been discussed that disc degeneration and other changes of the spine are unimportant, and psychosocial factors are the main determinants of low back pain.
The confusion arises from a common misconception. Under clinical conditions, disc degeneration remains an image finding. Image findings are not equal to pain. Therefore, a weak correlation between image findings of disc degeneration and pain does not mean that degeneration is unimportant. It merely means that diagnostic imaging is unable to reliably identify the source of pain.
What triggers a pain signal from any tissue, such as the disc, is the activation of specialized nerve endings called nociceptors. The nociceptors translate events occurring in the disc (such as a tear or inflammation) into a signal that runs along the nerve. After elaboration in the spinal cord and the brain, the signal reaches the brain centers where pain perception occurs. Obviously, these processes are not visible with diagnostic imaging of the disc.
It is never "just about the disc"
For those patients with pain from the disc, a lot happens between the trigger of the signal from the disc and the perception of pain. The pain experience is always the result of complex processes at different sites of the pathways that are involved in the generation, transmission, elaboration and perception of pain. These processes can amplify or attenuate the signal, resulting in excessive or minimal pain, respectively (read more here). A known contributing factor is psychosocial distress. In fact, the above mentioned study found that disc degeneration is associated with low back pain in individuals without mental distress, but not at all in those with high mental distress.
Perspectives
We still do not know what molecular mechanisms activate the pain-specific nerve endings of the disc, and how the nerves translate the activation into a signal that is transmitted to the spinal cord and ultimately to the brain. As mentioned above, these processes are not detectable with diagnostic imaging, and this is one of the reasons why we do not understand why degeneration is associated with pain in certain individuals and not in others.
However, recent research developments offer some hope. Previous studies had to rely on animal research, in which disc degeneration is induced by different means, such as chemical agents. Limitations of animal studies are obvious when findings have to be applied to human pain. Recently, there have been enormous progresses in our ability to study molecular mechanisms of human pain. More information about a large project on this topic can be found here. Importantly, the project is not focused exclusively on molecular mechanisms, but involves a comprehensive evaluation of different dimensions of the patients' condition, including physical and psychological domains. If successful, this research would allows us not only to better understand pain from the disc in a clinical context, but also to develop effective treatments.
What should you do if you have disc degeneration?
If you do not have pain, or you pain is mild
You should ignore the finding of disc degeneration.
There is a good chance that your pain does not stem from the degenerated disc. Your disc degeneration can be a normal aging process that occurs in most individuals and has no meaning. Consider it equal to wrinkles on your face, they do not hurt.
If the pain impacts your life
Consider that there are many other different potential causes. Joints, muscles, ligaments, and fasciae, among others, can be a source of low back pain. If you see different health professionals, you are likely to receive different explanations. This reflects the limited agreement on the diagnostic criteria for low back pain.
Psychosocial distress, such as depression or negative thoughts, can greatly contribute to low back pain and disability, also when pain primarily originates from the disc. The treatment of psychological distress is a priority, and together with other conservative measures can be sufficient to improve your quality of life.
Surgery should be considered very restrictively because of the diagnostic uncertainty and the potential for failure.