The true incidence of an additional lumbar vertebra (L6) is not well-studied but is also not insignificant, affecting up to 5.5% of the general population.
There is a theoretical link between the presence of L6 and low back pain, which requires further investigation.
Anatomical variations of the spine are often missed and can lead to wrong-level surgery, making the identification of L6 of utmost importance in spinal patients.
Chronic back pain encapsulates multiple possible underlying causes and can be quite disabling. Most recently in the news, singer Adele’s collapse backstage at a concert was attributed to her struggles with sciatica and chronic back pain, and some physicians deduce this is related to the presence of an L6 vertebra, which Adele also disclosed to the media.
But what, if any, is the link between the presence of an L6 vertebra and her pain? Should we be screening our low back pain patients for the presence of an L6?
Facts and figures
The incidence of supernumerary lumbar vertebrae, specifically an L6 vertebra, is not well studied, but a range in incidence from 0.8-5.5% of the general population has been reported. Physicians don’t often know if a patient has 6 lumbar vertebrae until they present with spinal problems. Even then, L6 can be missed due to how the vertebrae are counted and the presence of other anatomical differences, such as lumbosacral transitional vertebrae (LSTV).
The question becomes, then, is having an L6 vertebra simply just an anatomical variation? Or is it a congenital anomaly that should be a cause for concern?
And what is the relationship, if any, to lower back pain? The global point prevalence of low back pain in 2017 was estimated to be a significant 7.5%. In fact, low back pain is the number-one global cause of disability. So is the finding of an L6 really the problem? Or is it just incidental?
Theoretically, the presence of an extra vertebra can change the angle of the spine. From an evolutionary standpoint, the number of vertebrae in a primate is thought to be related to the type of movement needed and used. Therefore, it’s easy to assume that an extra vertebra would change this. A study done on cadavers, with results published in the Global Spine Journal, assessed the relationship between the number of lumbar vertebrae and pelvic incidence, or, the balance of the pelvis.
Although having fewer vertebrae (4 lumbar vertebrae) was found to affect the balance of the pelvis, the effect of 5 or 6 lumbar vertebrae was not statistically significant. However, the literature can’t seem to agree on this, as some smaller studies have in fact shown a statistically significant difference.
LSTV is diagnosed when there is lumbarization of S1 or sacralization of the lower lumbar vertebrae. The issue with this is simply that, unless we are imaging the entire spine, labeling LSTV vs a true L6 is difficult and often missed. As well, LSTV can occur to varying degrees including fusion, and can therefore alter the mobility of the lower spine.
Although there is still some controversy as to whether LSTV is associated with a higher incidence of low back pain, the evidence leans towards a correlation.
The authors of a study published in Asian Spine Journal did find a statistically significant correlation between LSTV and low back pain, especially among females. Sacralization or fusion of L6 is also a possibility and a potential concern with regards to low back pain.
The importance of recognizing a patient with L6
We need to recognize the presence of L6 vertebrae for fairly obvious reasons; with addition of an extra vertebra, the patient’s anatomy, including both spinal and neuroanatomical structures, changes.
Wrong-level surgery has been performed by as many as 50% of spinal surgeons. In up to 40% of those cases, variations in spinal anatomy seems to be the cause.
Due to the location and pressures in the spine, lumbar vertebrae are prone to disc herniation and osteoarthritic changes; adding an extra vertebra simply adds another location where problems may arise.
What this means for you
The presence of an L6 vertebra is not an insignificant finding in the general population. Although studies are not clear on the implication of L6 on low back pain, recognition of its presence is still imperative, especially in patients with related complaints. A thorough approach of assessing for supernumerary vertebrae may improve patient outcomes by reducing wrong-level surgery. Further research needs to be done on best practices to evaluate for spinal variations and the association between L6 and lower back issues.
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