Anatomy, Back, Lumbar Vertebrae

Excerpt

Vertebrae, along with intervertebral discs, compose the vertebral column or spine. The vertebral column extends from the skull to the coccyx and includes the cervical, thoracic, lumbar, and sacral regions. The spine has several significant roles in the body, including protection of the spinal cord and branching spinal nerves, support for the thorax and abdomen, and enabling flexibility and mobility. The intervertebral discs are responsible for this mobility without sacrificing the supportive strength of the vertebral column. The lumbar region contains five vertebrae, denoted L1-L5. The intervertebral discs, along with the laminae, pedicles, and articular processes of adjacent vertebrae, create a space through which spinal nerves exit. The lumbar vertebrae, as a group, produce a lordotic curve.

Typical vertebrae consist of a vertebral body, a vertebral arch, and seven processes. The vertebral body bears the majority of the force placed on the vertebrae. Vertebral bodies increase in size as the column descends. The vertebral body consists of trabecular bone, which contains the red marrow, surrounded by a thin external layer of compact bone. The arch, along with the posterior aspect of the body, forms the vertebral (spinal) canal, which houses the spinal cord. The arch consists of bilateral pedicles, pieces of bone that connect the arch to the body, and bilateral lamina, bone segments that form most of the arch, connecting the transverse and spinous processes. A typical vertebra also contains four articular processes, two superior and two inferior, which contact the inferior and superior articular processes of the two adjacent vertebrae, one superior and one inferior.

The point at which superior and articular facets meet is known as a facet, or zygapophyseal, joint. These maintain vertebral alignment, control the range of motion, and are weight-bearing in certain positions. The spinous process projects posteriorly and inferiorly from the vertebral arch and overlaps the inferior vertebrae to various degrees, depending on the region of the spine. Lastly, the two transverse processes project laterally from the vertebral arch in a symmetric fashion.

Typical lumbar vertebrae have several features distinct from those typical of cervical or thoracic vertebrae. The most notable distinction is the presence of a large vertebral body. The spinous process is short and thick, relative to the size of the vertebra, and projects perpendicularly from the body. The articular facets are markedly vertical, with the superior facets directed posteromedially and medially. The facets also have the unique feature of a curved articular surface. This is one feature that differentiates lumbar vertebrae from thoracic. There is also the mammillary process on the posterior aspect of the superior articular process. Disc thickness generally increases from rostral to caudal, with the lumbar intervertebral disc height greater than cervical and thoracic intervertebral discs.

There is only one lumbar vertebra that may be considered atypical. L5 has the largest body and transverse processes of all vertebrae. The anterior aspect of the body has a greater height compared to the posterior. This creates the lumbosacral angle between the lumbar region of the vertebrae and the sacrum.