The lumbar vertebrae are located at the bottom section of the vertebral column, inferior to the rib cage and superior to the pelvis and sacrum. Since these vertebrae are most largely responsible for bearing the weight of the upper body (and permitting movement), they are logically also the largest segments of the vertebral column. These vertebrae are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body.
There are 5 lumbar vertebrae (denoted as L1-L5) found in adult humans, and they are situated beneath the thoracic vertebrae, They are the largest, in terms of size, out of all the vertebrae because the lumbar vertebrae must be able to support the weight of the body when a person is standing due to the effects of gravity.
To summarize, the main anatomical components of a lumbar vertebra are:
- Vertebral body
- Vertebral arch
- Spinous process
- Pedicles and laminae
- Vertebral foramen
- Superior and inferior articular processes/facets
- Transverse processes
Distinguishing features of the lumbar vertebrae include a thick and stout vertebral body, a blunt, quadrilateral spinous process for the attachment of strong lumbar muscles, and articular processes that are oriented differently than those found on the other vertebrae. The vertebral body is large, wider laterally compared to longitudinally, and thicker in the front than in the back. It is also flattened or slightly concave superiorly and inferiorly, concave behind, and deeply restricted at the front and laterally.
Each lumbar vertebra has a vertebral body and a vertebral arch. The vertebral arch consists of a pair of pedicles and a pair of laminae, which encircles the vertebral foramen that ultimately supports 7 processes. The pedicles are very strong and are directed backwards from the upper part of the vertebral body. As a result, the inferior vertebral notches have considerable depth. The pedicles also change in morphology from L1 to L5, increasing in width from 9 mm up to 18 mm at L5. The angle in the axial plane also increases from 10 to 20 degrees from L1 to L5.
The laminae are strong, broad, and short in morphology, and form the posterior portion of the vertebral arch. The upper lumbar laminae regions are taller instead of wider, whereas the lower lumbar laminae regions are wider instead of tall. Laminae connect the spinous process to the pedicles.
The vertebral foramen is triangular-shaped within the arch, and larger in size than in the thoracic vertebrae, but smaller than in the cervical vertebrae. The superior and inferior articular processes are well-defined, and project upward and downward from the junctions of pedicles and laminae, respectively. The superior processes are concave and face medially (like when the palms of the hands are facing each other when about to clap), whereas the inferior processes are convex and face lateralward towards the superior processes of the next vertebra. This anatomical conformation allows for resistance against the twisting of the lower spine.
Transverse processes are long and slender, with changing morphology from L1 to L5. These processes are horizontal in L1-L3, and incline a little upward in L4-L5. In L1-L3, the transverse processes arise from the junctions of the pedicles and laminae, but in L4-L5, they arise from the pedicles and posterior portions of the vertebral bodies since they are set farther forward. The transverse processes are positioned in front of the articular processes instead of behind them as in the thoracic vertebrae, and are homologous with the ribs.
Specific Lumbar Vertebrae
- First lumbar vertebrae (L1) - L1 is roughly inline with the anterior end of the 9th rib at a level called the transpyloric plane (since the pylorus of the stomach is found at this level).
- Fifth lumbar vertebrae (L5) - L5 is significantly different in morphology, with its body being much deeper in front than behind, which allows for articulation with the sacrovertebral prominences. The spinous process is smaller, there is a wider interval between the inferior articular processes, and the transverse processes are thicker and spring from the body as well as the pedicles.
The spinal cord extends down to the L2 vertebra. Below the L2 level, the spinal canal surrounds a bundle of nerves known as the cauda equina (“horse’s tail”), which reaches down into the lower limbs and pelvic organs.
Muscles Affecting Lumbar Vertebrae Function
- Longissimus muscle - This is a long muscle with a lumbar vertebral origin at the transverse processes and spinous processes,. The longissmus muscle can extend the spine upon bilateral contraction, and unilateral contraction can bend the spine laterally to the same side.
- Spinalis muscle - This is a long muscle that is part of the erector spinae bundle of muscles and tendons, which helps the spine with movement and also helps in maintaining posture and staying erect when standing.
- Multifidus muscle - This is a long muscle that traverses the length of the back and functions in stabilizing and rotating the lumbar spine.
- Intertransversarii muscle (mediales lumborum and laterales lumborum) - This muscle is specifically found from L1-L5. It bilaterally stabilizes and extends the lumbar spine and unilaterally bends the lumbar spine laterally to the same side.
- Psoas major - This muscle helps bend the trunk laterally, and raises the trunk from the supine position bilaterally.
Spondylolysis & Spondylolisthesis
The pedicles are sometimes used as an entryway into the vertebral body for fixation with pedicle screws, or for placement of bone cement during kyphoplasty or vertebroplasty. The L5 vertebra is the most common site of spondylolysis and spondylolisthesis.
Also, while most people have 5 lumbar vertebrae, there are individuals who have 4 or 6 lumbar vertebrae. In these individuals, the lumbar disorders that normally affect L5 will now affect L4 or L6.
Similar to in the thoracic spine, disk herniation can also occur in the lumbar spine. Since the stress resistance of the annulus fibrosus declines with age, under great enough strain or weight loading, the tissue of the nucleus pulposus can bulge through weak spots. The herniated material can end up compressing the contents of the intervertebral foramen, which include the nerve roots and blood vessels, which can result in lower back pain. Muscles affected in this region can also be weakened when the motor part of a spinal nerve is affected.
Abnormal Spinal Curvatures
Scoliosis - This refers to an abnormal lateral deviation or curvature, most often in the thoracic region, and common among adolescent girls. It can result from a developmental abnormality where the body and arch will fail to develop on one side of a vertebra, therefore leading to an imbalance and irregular curvature.
Kyphosis - An exaggerated thoracic curvature is known as kyphosis. The upper back where the thoracic vertebrae are will be abnormally curved, thus leading to the more commonly used term “hunchback.” This condition is most often the result of osteoporosis, a progressive bone disease characterized by decreased bone mass and density, but can also occur in people with osteomalacia (defective bone mineralization) or spinal tuberculosis. The predisposing risks for osteoporosis include: inadequate calcium or vitamin D, menopause, lack of exercise, weight, old age, and/or race. However, adolescent boys who often and actively engage in spine-loading activities or sports (weight lifting, wrestling, etc.) may also get kyphosis.
Lordosis - This condition is due to an exaggerated lumbar curvature. In common jargon, this is known as “swayback,” where the lower back will be abnormally curved instead of the upper back (as in kyphosis). The causes of lordosis are the same or similar to those for kyphosis. Lordosis is especially common in pregnancy or obesity due to the added abdominal weight that will force the lumbar vertebrae into an abnormal curvature.
Lumbarization - This refers to a spinal anomaly defined by the nonfusion of the first and second segments of the sacrum. Upon imaging, the lumbar spine will appear to have 6 vertebrae or segments instead of 5, and the sacrum will conversely appear to have only 4 segments instead of 5.
Sacralization of L5 - Sacralization is a congenital anomaly where the transverse process of L5 fuses to the sacrum on one or both sides, to the ilium, or to both sacrum and ilium. This anomaly usually presents bilaterally. While sacralization can cause lower back pain, it is often asymptomatic, especially in bilateral presentations since the biomechanics will not be as destabilized as a non-bilateral presentation. Finally, the L5-S1 intervertebral disc also may be thin and narrow.