Neuraxial Anatomy (Anatomy Relevant to Neuraxial Anesthesia)
The vertebral column forms part of the axis of the human body, extending in the midline from the base of the skull to the pelvis. Its four primary functions are protection of the spinal cord, support of the head, provision of an attachment point for the upper extremities, and transmission of weight from the trunk to the lower extremities. Pertinent to regional anesthesia, the vertebral column serves as the landmark for a wide variety of regional anesthesia techniques. It is important, therefore, that the anesthesiologist be able to develop a three-dimensional mental image of the structures comprising the vertebral column.
The vertebral column consists of 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal segments) (Figure 22–1). In the embryonic period, the spine curves into a C shape, forming two primary curvatures with their convex aspect directed posteriorly. These curvatures persist through adulthood as the thoracic and sacral curves. The cervical and lumbar lordoses are secondary curvatures that develop after birth as a result of extension of the head and lower limbs when standing erect. The secondary curvatures are convex anteriorly and augment the flexibility of the spine.
The vertebral column and the curvatures of the adult spine, lateral view.
A typical vertebra consists of a vertebral arch posteriorly and a body anteriorly. This holds true for all vertebrae except C1. Two pedicles arise on the posterolateral aspects of each vertebra and fuse with the two laminae to encircle the vertebral foramen1 (Figures 22–2A, 22–2B). These structures form the vertebral canal, which contains the spinal cord, spinal nerves, and epidural space. Fibrocartilaginous disks containing the nucleus pulposus, an avascular gelatinous body surrounded by the collagenous lamellae of the annular ligament, join the vertebral bodies. The transverse processes arise from the laminae and project laterally, whereas the spinous process projects posteriorly from the midline union of the laminae (Figures 22–2A, 22–2B). The spinous process is frequently bifid at the cervical level and serves as an attachment for muscles and ligaments.
A typical vertebra. A: Superior view of the L5 vertebra. B: Posterior view of the L5 vertebra.
C1 (atlas), C2 (axis), and C7 (vertebra prominens) are described as atypical cervical vertebrae due to their unique features. C1 is a ringlike bone that has no body or spinous process. It is formed by two lateral masses with facets that connect anteriorly to a ...