The human spine has 5 lumbar vertebrae with interspaced intervertebral disks. They are the largest vertebrae in the body and connect the thorax to the pelvis. The lumbar vertebrae are designated L1–L5 starting with the segment below the last thoracic vertebra (T12) (Fig. 29.1). However, there can be variations in lumbar anatomy. When the first sacral segment fails to fuse to the second segment during development, the so-called “transitional anatomy” will be present and the lumbar spine will appear to have six segments and the sacrum appears to have four segments. This is called lumbarization of the sacral spine (Fig. 29.2). Conversely, the transverse processes of L5 can become fused to the sacrum. This is called sacralization of the lumbar spine and the lumbar spine in these cases appears to have four lumbar vertebrae.

The lumbar vertebrae are characterized by vertebral bodies that are wider and larger than the cervical and thoracic vertebrae. In the upper lumbar region the laminae are taller than wide but in the lower lumbar vertebra the laminae are wider than tall. The lamina connects the spinous process to the pedicles. The morphometry of the pedicles of the lumbar spines varies considerably from level to level, as well as from patient to patient. At L1, the angle inclines medially about 10 degrees and gradually increases 5 degrees at each subsequent vertebra to approximately 30 degrees at L5 (Fig. 29.3). An understanding of these dimensions and angles is important when considering the use of pedicle screws.

The superior and inferior articular processes are well defined, projecting upward and downward from the junctions of pedicles and laminae, respectively. The superior articular processes face backward and medial; the inferior articular processes face forward and lateral. The superior articular processes are wider apart than the inferior articular processes and the inferior articular processes sit in the bowl-like surface formed by the superior articulating processes (Fig. 29.4). This can be seen in ballet dancers and football linemen. The transverse processes are long and slender. They are horizontal in the upper three lumbar vertebrae and incline a little upward in the lower two. In the upper three vertebrae they arise from the junctions of the pedicles and laminae, but in the lower two they are set farther forward and spring from the pedicles and posterior parts of the vertebral bodies. Finally, the mammillary process is connected in the lumbar region with the back part of the superior articular process and is unique to the lumbar spine.

In the lumbar spine, the average lordosis is 50 degrees, with a range of 32 to 84 degrees. In the lumbar spine, the disks have an increased height anteriorly, which helps create lordosis. Also, the anterior walls of the lumbar vertebrae are taller than the posterior walls. This also helps to create regional lordosis. Eighty percent of lordosis in the lumbar spine comes from the disks while 20% comes from the shape of the vertebrae. Two-thirds of the total lordosis in the lumbar spine is from L4 to S1 while the remaining one-third is from L1 to L3. As the human body ages, lumbar disk degeneration and lumbar compression fractures can lead to loss of lumbar lordosis due to reductions in height of the anterior column. This can be a cause of low back pain that may be independent from the pain caused by disk degeneration and compression fractures. Also, if an instrumented fusion is being performed, it is important to recreate the natural lordosis in the lumbar spine as failure to do so may result in a “flatback deformity.”

The conus medullaris usually begins at T11 and, in most males, ends at the L1–L2 disk space. The conus in females frequently stops slightly more proximally. The conus medullaris can occasionally extend much lower into the lumbar spine and is often associated with a hypertrophic filum terminale. The neural elements of the lumbar spine below the L1–L2 disks are usually purely spinal nerve rootlets (cauda equina).

One of the important components of the lumbar spinal anatomy is the soft tissue that connects the bony elements. This complex interaction of ligaments, disk, and musculature allows for both controlled motion and stability of the spine.

The anterior longitudinal ligament is a strong, broad-based ligament that runs on the anterior aspect of the

vertebral body from the atlas to the sacrum. It is firmly attached to both the ventral aspect of the disk and periosteum of the vertebral body. It is a major contributor to spinal stability and limits hyperextension of the vertebral column. The posterior longitudinal ligament also runs the length of the spinal column, but it is narrower and weaker than its anterior counterpart. Its primary function is to limit hyperflexion. The intervertebral disk is composed of the annulus fibrosus and the nucleus pulposus. The annulus is formed by concentric bands of fibrocartilage that run obliquely from one vertebral body to another. This arrangement allows for some motion, yet is one of the strongest connections between vertebral segments. The nucleus, which is encased in the annulus, acts as a shock absorber for axial forces.

Posteriorly, the laminae are joined by the ligamentum flavum, a broad band of elastic fiber. The spinous processes are joined by a weak interspinous ligament and a strong supraspinous ligament. The intrinsic muscles of the back include the erector spinae group of muscles (spinalis, longissimus, iliocostalis) and the transversospinalis group (rotators, multifidus, and semispinalis). The intrinsic muscles maintain posture and provide movement of the vertebral column. Any deformity resulting from trauma can alter the function of these muscles. In addition, it is important to have an understanding of these muscle groups when considering the various anatomic approaches to the spine described later in this chapter.