The role of Schmorl's nodes in disc degeneration is the topic of this study from Southern China. To understand Schmorl's nodes, picture two vertebral (spinal) bones with a disc between them. Now imagine a circular layer of cartilage between the disc and each vertebra. That bit of cartilage is called the end plate. Schmorl's nodes occur when the disc pushes through the endplate and into the next vertebra. On imaging studies these look like small hollowed areas.
Schmorl's nodes can be seen on X-rays and are often present in patients diagnosed with disc herniation. Is this a coincidence or does it have some particular meaning? Do Schmorl's nodes occur without disc degeneration? If a person does have Schmorl's nodes, does that mean the disc is going to degenerate? And final question, just what does the presence of these nodes signify?
There are some theories about why Schmorl's nodes develop. In many cases, we just don't know why they show up. They seem to develop without cause. That's referred to as idiopathic. Sometimes, there is a clear cause such as a tumor, decreased bone mineral density, or trauma. In order to understand the etiology (cause) of Schmorl's nodes better, the authors of this report examined the lumbar spine of 2,449 (adult) volunteers using MRIs.
The participants in the study were everyday people from the general population. Some had low back pain but the majority had no history of back pain or problems. Anyone with a history of back surgery, spinal tumors or infection, or any diseases of the spine was not allowed to join the study.
After all the MRIs were read and interpreted, participants were divided into two groups: 1) those who had no evidence of Schmorl's nodes and 2) anyone with clear evidence of one or more Schmorl's nodes. As with most research, the patients' ages, activity level and sports participation, height and weight (also known as body mass index or BMI), and use of tobacco was recorded.
For both groups, the radiologists also looked at the condition of the disc, disc height, and any other signs of disc degeneration (tears, fissures, disc narrowing). Then they compared the two groups to look for any clues that might explain the role or function of Schmorl's nodes. They found that in the general population, slightly more than 80 per cent did not have any Schmorl's nodes. For the nearly 20 per cent who did have evidence of this anatomic feature, more than half had multiple levels affected.
The upper lumbar spine (especially L23) was the most common site for Schmorl's nodes to be seen. Age wasn't a significant factor except that older adults were more likely to develop disc degeneration. And it was those participants with disc degeneration who had the most Schmorl's nodes. In fact, the more severe the degeneration, the more likely they were to have Schmorl's nodes. Body mass index was also much higher in the group with Schmorl's nodes. This seems to suggest that being overweight or obese puts older adults at increased risk for Schmorl's nodes as a complication of disc degeneration.
Perhaps the most surprising finding was that disc bulging wasn't linked with Schmorl's nodes as clearly as disc narrowing. In fact, the people with disc bulging were less likely to have node development. One possible explanation for this is the fact that disc narrowing puts more pressure on the disc, forcing the disc material into the endplate. The most severe cases of disc degeneration also had the greatest disc narrowing.
Because this study was done in Southern China, the authors compared their results with other studies reporting on the prevalence of Schmorl's nodes in other ethnic groups (e.g., European-Americans, African Americans. The percentage of Chinese affected was much less than these two other groups. What factors might account for the low rate of Schmorl's nodes in this group of Southern Chinese adults? The results of this study don't answer that question directly. The authors review other research results and suggest factors such as genetic influences, differences in posture and subesquent load on the spine, and perhaps ethnic differences in the size and strength of the endplates. In all groups, men with a higher body weight (and therefore greater upper body load placed on the endplates) had a greater risk of endplate failure and the development of Schmorl's nodes.
The authors conclude that taking all the information gathered from human studies (cadavers), animal studies, and comparison studies like this one with live humans, it seems reasonable to think that damage to the endplate can lead to disc degeneration. It's likely that there are multiple factors involved and not just one reason why Schmorl's nodes develop. Genetics, nutrition, body mass index, and severity of disc degeneration may all work together to result in end-plate changes leading to Schmorl's nodes.
Reference: Florence P. S. Mok, MSc, et al. ISSLS Prize Winner: Prevalence, Determinants, and Association of Schmorl Nodes of the Lumbar Spine with Disc Degeneration. In Spine. October 1, 2010. Vol. 35. No. 21. Pp. 1944-1952.