Is It Possible to Have an Extra Back Bone?

 Q: My chiropractor told me that I have an extra bone in my back. Is this likely, and is it serious?

A:  Your chiropractor has given you a simple explanation of something that appeared on your X-ray, probably of your low back. If your doctor took ten separate sets of low back X-rays, it would not be unlikely to find an “extra bone” on one of them.

There are typically five vertebrae in the low back, but it seems that you have a sixth. This can happen, and is rarely a cause for concern. The human spine comprises five separate regions: generally, the neck, mid-back, low back, sacrum, and coccyx. The neck (cervical spine) is composed of 7 vertebrae, the mid-back (thoracic spine) 12, the low back (lumbar spine) 5, the sacrum 5, and the coccyx as many as 5. The sacrum begins as five individual bones in a child, then fuses as the spine matures. One way to develop an “extra” vertebra in the low back is for the upper segment of the sacrum to remain independent of the fused, adult bones. This would leave six low back vertebrae and a sacrum composed of four fused vertebrae. A thoracic, or mid-back, vertebra technically bears a rib. If the lowest of twelve thoracic vertebrae does not bear a rib, it is often considered the first lumbar vertebra; this would also leave a total of six bones in the low back.

This condition is rarely a cause for concern. Bear in mind that an extra lumbar vertebra formed by the first process described is often a somewhat strange-looking bone (something like a defect). Typically the last lumbar vertebra has two separate bony joints with the sacrum, but this atypical bone may form other “accessory” joints in addition to these. There is research to suggest that these accessory joints may cause pain in the low back. Beyond this situation, it is generally accepted that a sixth low back vertebra will not cause an individual any appreciable discomfort.