What are spinal discs?
Most of your back is made up of structures we are familiar with, such as bone and muscle. The intervertebral disc, however, is a unique structure that doesn’t appear anywhere else in the body, so it’s not as widely understood. Unlike other tissues, there is no blood supply to the disc, and it metabolises very slowly. The disc absorbs fluid at night while you are lying down and releases this fluid during the day while you are standing up. The process of sucking in the nutrients and squeezing out the waste means that cell turnover is very slow, and it can take a considerable amount of time for a disc to heal after an injury.
Common misconceptions about spinal discs
Here are three commonly held misconceptions about spinal discs.
1. Your disc is full of gel
Similar to grey hair and wrinkles, the discs undergo an ageing process. This is known as disc degeneration and is a natural process that occurs in everyone. As we age, part of the process is a change in the molecules that hold water (called proteoglycans). It is similar to how a new sponge can hold water much better than an old one.
By the time you reach adulthood, the nucleus of the disc dries out, changing from almost liquid to solid. As we become older, the nucleus continues to dry out and begins to fragment. Similarly, the annulus also changes as you age. The once tough and rubbery structure becomes brittle and over time begins to split.
2. Your disc wears out from overuse
The second thing people often believe is that our discs wear out because of something we did (like heavy work or sport) or something that was done to them (such as an injury). We now understand that genetics plays a larger role in determining the rate at which a disc degenerates than previously thought.
In the last decade, there have been multiple studies investigating the factors affecting disc degeneration. A study of similarities in degenerative findings of the lumbar spines of identical twins showed that the main factor influencing disc degeneration was genetics rather than occupational or environmental exposure. In other words, even though the twins had very different lifestyles, their degree of disc degeneration was similar.
Prior to these studies, heavy physical loading was the main suspected risk factor for disc degeneration however the findings suggest that factors associated with occupation and sport have a minor role in the degeneration of discs.
3. Disc degeneration equals pain
The third commonly held belief is that disc degeneration always results in pain. Many studies have shown that signs of disc degeneration on scans are very common in people with no back pain. A large study showed that 37% of 20-year-olds without back pain had disc degeneration on MRI and by age 50, 80% had degeneration on their scans! (See Table 1).
That doesn’t mean that degenerative changes don’t cause pain. However, the exact role of disc degeneration in pain production is unclear.
It can be hard to distinguish between physiology (the normal ageing process) and pathology (serious problems). It is estimated that in 75% of cases, a definite cause for back pain cannot be found.
Table 1: Age-specific prevalence estimates of degenerative spine imaging findings in asymptomatic patients a
|Disc signal loss||17%||33%||54%||73%||86%||94%||97%|
|Disc height loss||24%||34%||45%||56%||67%||76%||84%|
a Prevalence rates estimated with a generalised linear mixed-effects model for the age-specific prevalence estimate (binomial outcome) clustering on the study and adjusting for the midpoint of each reported age interval of the study.
J. P. G. Urban, S. Roberts, J. R. Ralphs, The Nucleus of the Intervertebral Disc from Development to Degeneration, American Zoologist, Volume 40, Issue 1, February 2000, Pages 53–061, https://doi.org/10.1093/icb/40.1.53