The L5 Vertebra

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The fifth lumbar vertebra (or the L5 vertebra) is the largest and most inferior of the lumbar vertebrae. As the last of the lumbar vertebrae, the L5 vertebra bears more body weight than any of the other 23 vertebrae that sit atop it in the vertebral column. Thus the L5 is the largest and strongest lumbar vertebra, but is also the most susceptible to stress-related injuries.

The L5 vertebra is located in the spinal column of the lumbar (lower back) region inferior to the L4 vertebra and superior to the sacrum. Like the other lumbar vertebrae, L5 has a large, roughly cylindrical region of bone known as the body that makes up most of its mass. The bodies of lumbar vertebrae are much wider than they are deep and are bean-shaped when viewed transversely. The body lies anterior to the hollow vertebral foramen and supports the entire weight of the tissues of the upper body. Intervertebral disks made of strong, rubbery fibrocartilage lie between the bodies of each of the lumbar vertebrae and the sacrum. The intervertebral disks provide support, shock absorption, and flexibility to the lower back.

Posterior to the body is a thin ring of bone known as the vertebral arch. The vertebral arches of the lumbar vertebrae are quite small compared to their large vertebral bodies, but are considerably thicker and stronger than the arches of the cervical and thoracic vertebrae. In the lumbar region the vertebral arch plays the vital role of surrounding and protecting the many spinal nerves that run through the hollow, triangular vertebral foramen. Several bony processes extend from the vertebral arch to provide important attachment points for bones and the muscles that move the lower back, hips, and pelvis.

The transverse processes, a pair of thin bony projections extending laterally and posteriorly from the arch, provide attachment points for muscles that stabilize the spine and flex the thigh at the hip. Unlike the transverse processes of the other lumbar vertebrae, those of the L5 are considerably thicker and more anterior, originating from the union of the arch and the body. Extending posteriorly from the arch is the spinous process, which is considerably shorter and thicker in the L5 than it is in the other vertebrae. The spinous process anchors many muscles that work together to stabilize, laterally flex, rotate, and extend the trunk. Finally, extending superiorly and inferiorly on both sides of the arch are the articular processes that help stabilize the spine and connect the L5 vertebra to the L4 vertebra and sacrum. Planar joints formed between the articular processes of these neighboring vertebrae allow the bones to move independently while maintaining the stability of the spinal column.