Spine surgeons and radiologists (who both may read and interpret MRI scans) sometimes differ in their method of labeling a particular spinal segment. This creates confusion - among both patients and insurance companies - triggering the question: "Where is the problem?"
A Sixth Lumbar Vertebra
One of the reasons for the confusion is that some people, approximately 10% of adults, have a congenital anomaly in their lower back. One of the most common anomalies is the presence of a sixth lumbar vertebra. Having one extra lumbar vertebra provides no advantage or disadvantage to the individual and is rarely a cause of back problems, but it can create some confusion. For example:
- Radiologists commonly count down from the last rib when numbering the lumbar vertebral bodies.
- Surgeons on the other hand, count up from the sacrum when numbering the lumbar vertebrae.
Neither method of labeling lumbar vertebrae is incorrect, but obviously it can create confusion.
For an individual with 5 vertebral bodies, they would be in agreement when labeling the L4-L5 level. If the individual has 6 lumbar vertebrae, however, the radiologist would typically refer to the lowest level as L6-S1 and the level above that L5-L6, which in the surgeon's mind, would be correctly labeled L4-L5.
It is obviously very important to clearly identify the location of lumbosacral anomalies in order to avoid injection or surgical exposure of the incorrect level. This becomes particularly important for minimally invasive procedures. A physical exam and complementary imaging studies, such as an MRI scan and X-ray, can help improve the accuracy and the communication of the diagnosis.
When Spinal Anomalies Cause Back Problems
Patients are sometimes told that a spinal anomaly - such as a sixth lumbar vertebrae or an extra sacral bone - has been found on an X-ray and is causing their back problems. However, anomalies such as these in the lumbar spine and sacral spine are simply variants of normal bony architecture and are typically of no consequence. In other words, it would be very rare for an abnormality such as a sixth lumbar vertebra or extra bone in the sacrum to cause back problems.
The one exception to this general guideline is in cases where the last transverse process (a bony protrusion near the vertebra) is partially attached to the sacrum, or "sacralized". If this bony protrusion is attached to the sacrum, it can create a rudimentary joint (pseudoarticulation) where there shouldn't be one. The resulting motion in this section of the spine can sometimes be a cause of localized low back pain. This condition can usually be successfully treated without surgery. For instance, an injection of steroid medication at the pseudoarticulation of the transverse process and the sacrum can often be both diagnostic and therapeutic.