Lumbar Vertebrae and Intervertebral Disk

The pedicles are short and strong and arise from the upper and posterolateral aspects of the bodies; the superior vertebral notches are therefore less deep than the inferior notches. The laminae are short, broad plates that meet in the midline to form the quadrangular and almost horizontal spinous processes. The intervals between adjacent laminae and spinous processes are relatively wide.

The articular processes project vertically upward and downward from the junctional areas between the pedicles and the laminae. The superior facets are gently concave and face posteromedially to embrace the inferior facets of the vertebra above, which are curved and disposed in a reciprocal fashion. This arrangement permits some flexion and extension but very little rotation. The transverse processes of the upper three lumbar vertebrae are long and slender, whereas those of the fourth, and especially of the fifth, are more pyramidal.

Near the roots of each transverse process are small accessory processes; other small, rounded mammillary processes protrude from the posterior margins of the superior articular processes. The former may represent the true transverse processes (or their tips) because many of the so-called transverse processes are really costal elements. In the first lumbar vertebra, these elements occasionally develop into lumbar ribs.

The fifth lumbar vertebra is atypical. It is the largest, its body is deeper anteriorly, its inferior articular facets face almost forward and are set more widely apart, and the roots of its stumpy transverse processes are continuous with the posterolateral parts of the body and with the entire lateral surfaces of the pedicles.

The intervertebral disks are interposed between the adjacent vertebral bodies from the axis to the sacrum and are immensely strong fibrocartilaginous structures that provide powerful bonds and elastic buffers. They consist of outer concentric layers of fibrous tissue—the annulus fibrosus (the fibers in adjacent layers are arranged obliquely but in opposite directions, to assist in resisting torsion)—and a central springy, pulpy zone, the nucleus pulposus. The blood and nerve supplies to the disks are inconspicuous. If the annular fibers give way as a result of injury or disease, the enclosed turgid nucleus pulposus may prolapse and press on related nervous and vascular structures.

In health and maturity, the intervertebral disks account for almost 25% of the length of the vertebral column; they are thinnest in the upper thoracic region and thickest in the lumbar region. In vertical section, the lumbar disks are rather wedge shaped, with the thicker edge anteriorly. The forward convexity of the lumbar spine is due more to the shape of the disks than to disparities between the anterior and posterior depths of the lumbar vertebrae. The more defined wedge shape of the lumbosacral disk helps to minimize the effects of the marked lumbosacral angulation.

As age advances, the nucleus pulposus undergoes changes: its water content decreases, its mucoid matrix is gradually replaced by fibrocartilage, and it ultimately comes to resemble the annulus fibrosus. The resultant loss of depth in each disk is small, but overall, it may amount to a decrease of 2 to 3 cm in the height of the spinal column.

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