The lumbar spine is the third region of the vertebral column, located in the lower back between the thoracic and sacral vertebral segments.
It is made up of five distinct vertebrae, which are the largest of the vertebral column. This supports the lumbar spine in its main function as a weight bearing structure.
This article will look at the osteology of the lumbar vertebrae, examining their characteristic features, joints and their clinical correlations.
Although the lumbar vertebrae lack some of the more distinctive features of other vertebrae, there are several characteristics that help to distinguish them.
The vertebral bodies are large and kidney-shaped. They are deeper anteriorly than posteriorly, producing the lumbosacral angle (the angle between the long axis of the lumbar region and that of the sacrum). The vertebral foramen is triangular in shape.
Other features of a typical lumbar vertebrae:
- Transverse processes are long and slender.
- Articular processes have nearly vertical facets.
- Spinous processes are short and broad.
- Accessory processes can be found on the posterior aspect of the base of each transverse process. They act as sites of attachment for deep back muscles.
- Mammillary processes can be found on the posterior surface of each superior articular process. They act as sites of attachment for deep back muscles.
The fifth lumbar vertebrae, L5, has some distinctive characteristics of its own. It has a notably large vertebral body and transverse processes as it carries the weight of the entire upper body.
There are two types of joint in the lumbar spine. Both of these articulations are not unique to the lumbar vertebrae, and are present throughout the vertebral column.
- Between vertebral bodies – adjacent vertebral bodies are joined by intervertebral discs, made of fibrocartilage. This is a type of cartilaginous joint, known as a symphysis.
- Between vertebral arches – formed by the articulation of superior and inferior articular processes from adjacent vertebrae. It is a synovial type joint.
The joints of the lumbar vertebrae are supported by several ligaments. They can be divided into two groups; those present throughout the vertebral column, and those unique to the lumbar spine.
Present throughout Vertebral Column
- Anterior and posterior longitudinal ligaments: Long ligaments that run the length of the vertebral column, covering the vertebral bodies and intervertebral discs.
- Ligamentum flavum: Connects the laminae of adjacent vertebrae.
- Interspinous ligament: Connects the spinous processes of adjacent vertebrae.
- Supraspinous ligament: Connects the tips of adjacent spinous processes.
(Note: In the cervical spine, the interspinous and supraspinous ligaments thicken and combine to form the nuchal ligament).
Unique to Lumbar Spine
The lumbosacral joint (between L5 and S1 vertebrae) is strengthened by the iliolumbar ligaments. These are fan-like ligaments radiating from the transverse processes of the L5 vertebra to the ilia of the pelvis
Throughout the vertebral column, the spinal cord travels through the vertebral canal (made up by the foramina of all vertebrae). At around the level of L1, the spinal cord terminates and the cauda equina begins. This is a bundle of lumbar, sacral and coccygeal nerve roots.
Spinal nerves exit the vertebral canal through the intervertebral foramina.
Clinical Relevance: Abnormalities of the Lumbar Spine
Lumbar Spinal Stenosis
Thought to be hereditary, lumbar spinal stenosis results in a stenotic (narrow) vertebral foramen in one or several lumbar vertebrae. This can cause compression of the spinal cord and exiting nerves.
This condition can be worsened by age-related degenerative changes, such as bulging of the intervertebral discs.
Lumbar spinal stenosis can sometimes be treated surgically with a decompressive laminectomy.
Excessive Lumbar Lordosis
This is an abnormal anterior curvature of the vertebral column in the lumbar region, characterised by anterior tilting of the pelvis.
Women develop temporary excessive lumbar lordosis in late pregnancy, but this resolves after childbirth. In either sex, obesity can similarly cause lordosis due to the increased weight of the abdomen. Both examples can cause back pain, and occur as a result of an altered line of gravity.