Sixth lumbar vertebra

How it works

The lower back usually has five vertebrae, the lumbar vertebrae. Below them sits the sacrum, a single bone made of fused vertebrae². Some people have six lumbar vertebrae.

In lumbarization, the top segment of the sacrum does not fuse with the rest. The lower back then seems to have six vertebrae, and the sacrum four segments instead of five². In other people the spine has one extra vertebra, or the split between chest and lower-back vertebrae is different³⁵. The reverse also happens. In sacralization, the lowest lumbar vertebra fuses to the sacrum on one side or both⁷.

A bone that is half lumbar and half sacral is called a lumbosacral transitional vertebra¹. These are common. In CT scans of 450 young adults with no history of spine disease, 26.4% had one⁵. People with a cervical rib at the top of the spine were more likely to have one at the bottom⁵.

During surgery, doctors often call the last box-shaped vertebra on the X-ray L5 and count up from it. With a transitional vertebra, that bone may really be L4 or L6¹. This can lead to an injection or surgery at the wrong level⁵. In a review of surgical studies covering 885 patients with a transitional vertebra, 1.4% had surgery on the wrong level⁶. Only whole-spine imaging counts the vertebrae reliably³.

How common it is

No review gives a pooled figure for six lumbar vertebrae. “Roughly 1 in 20” is a summary of single studies. In 211 abdominal X-rays in the US, 6.6% had six⁴. In 418 whole-spine MRI scans at a UK orthopaedic hospital, 4.3% had one extra movable vertebra³.

Rates of transitional vertebrae range from 4.5% to 35.6% between studies¹. A 2025 review gives a plain total of 15.3%, not a pooled estimate¹. Many of its studies scanned people with back pain or scoliosis, where rates are higher¹. Reviews disagree on whether men have them more often¹⁸.

Is it a problem?

Usually not. It is a normal variation, and many people with one have no spine problems⁵. One X-ray study suggests the rate may be similar in the general population and in patients with back pain. The authors call its role in back pain controversial⁴.

In some people a transitional vertebra can play a part in low back pain. Doctors call this Bertolotti syndrome only after they show that the vertebra causes the pain⁷. If a report says you have one, mention it to any doctor who plans a spine injection or surgery.

History and culture

The Italian doctor Mario Bertolotti first described back pain linked to a transitional vertebra in 1917⁷⁸.