An extra vertebra is a very rare birth abnormality occurring in less than 4% of the population. In this defect, a human is born with six sacrum vertebrae in the spine instead of the normal five. While humans today generally have five lumbar vertebrae, Homo erectus, the first early humans, typically had six. At times, the extra vertebra is also referred to as a transitional vertebra, because it tends to develop at points where the spine transitions from one section to the next. For example, the vertebra may form at the point where the lumbar section of the spine connects with the sacrum section.

Traditionally, the human spine consists of 33 vertebrae and the spinal cord running down the middle. As humans become adults, five of the bones fuse together to form the sacrum or lumbar. The rest of the vertebrae are separate and individual bones. While an extra vertebra is not normal, it will not cause any pain in a patient unless it pushes on other bones.

Many times, this vertebra is not fully developed in the spine and may not even be detected until well into adulthood or after an accident puts added pressure on the spine. In some instances, the extra vertebra can cause chronic pain in the back, hips and other areas of the body if nerve groups are affected. Crowding caused by the vertebra can move the spine out of alignment and push other bones, such as the hip and pelvic bones, out of place. The extra vertebra may also cause pain from rubbing or pressing against the surrounding bones and nerve tissue.

Professional health care providers are usually able to identify an extra vertebra through magnetic resonance imaging (MRI) scans or x-rays of the spine. In the event an extra vertebra is discovered to be causing a patient back pain, then medication, physical treatment, spinal injections and sometimes even surgery may be suggested. If the vertebra does not cause discomfort or pain, however, there may be no treatment necessary.

In some cases, the first lumbar vertebra is not properly fused with the second in the spine, and this may cause a misdiagnosis of an extra vertebra. It is typically recommended that individuals seek a second opinion to ensure proper diagnosis before proceeding with treatment. On the other hand, it is also possible for the extra vertebra to go undetected even after x-rays have been taken. Depending on the location and stage of development the vertebra is in, it may be hard to notice during testing.