Transitional vertebrae are abnormally formed vertebral bones that display the characteristics of two different types of vertebrae. The human spine is made up of 33 bones called vertebrae, with the spinal cord running through the middle. In adults, five of these bones fuse to form the sacrum, which is the bony back part of the pelvis, and another four fuse to form the coccyx or tailbone. The other vertebrae of the spine are individual bones. Although they are not normal, transitional vertebrae are not usually a problem for most people who have them.
The vertebrae in different sections of the spine have different structures and characteristics. The cervical spine forms the neck, the thoracic spine is in the region of the chest, and the lumbar spine supports the lower back. The locations at which the segments join are called the cervicothoracic, thoracolumbar, and lumbosacral regions of the spine. Vertebrae that occur at these junctions may develop abnormalities; for example, a lumbosacral transitional vertebra would have characteristics of both a normal lumbar vertebra and a normal sacral vertebra.
The structural changes that are seen in these vertebrae are usually related to either the vertebral arch, the transverse processes, or both. The vertebral arch is the posterior part of the vertebra and forms the back of the vertebral foramen, the hole at the center of the vertebra through which the spinal cord passes. The transverse processes are two bony protrusions that arise from each vertebra, which form the attachment points for muscles and ligaments.
A transitional vertebra found at the junction of the neck and the chest region of the spine is called a cervicothoracic transitional vertebra. An example of this type would be one that shows a change in structure in the lowest cervical vertebra so that the transverse processes, instead of being typical of a neck vertebra, take on the appearance of the processes of a thoracic vertebra. The thoracic vertebrae have special transverse processes that articulate with the ribs, and a vertebra that forms the base of the neck may have transverse processes that look as though they would be suitable for articulation with the ribs.
This type of abnormalities are quite common, found in about one in five adults, and usually do not cause any problems. Lumbosacral transitional vertebrae in the lower back are sometimes associated with back pain, however. Some types of arthritis, spinal disk disorders, and painful compression of the spinal cord may also be correlated with abnormal vertebrae at the lumbosacral junction.