The epidural space lies circumferentially around the dural sac and extends from the foramen magnum to the sacrococcygeal membrane at the sacral hiatus (Fig. 1). Its anterior boundary is the posterior longitudinal ligament. Posteriorly, it is limited by the ligamentum flavum, the laminae, and pedicles. It is connected to the paravertebral space through the intervertebral foraminae, which contains the exiting nerve roots. This communication is utilized in the transforaminal epidural approach. The content of the space includes nerve roots as they exit from their intradural course to the neural foramen, lymphatic tissue,

Anatomy Exiting Nerve Roots
Fig. 1. The transverse anatomy of the lumbar spine.

fatty tissue, loose connective tissue, and arteries and veins. It varies from a potential space up to 5-6 mm, measured in the midlumbar spine, and varies in depth (23). The epidural space is narrowest at the rostrale lamina and widest at the caudal lamina and the adjacent interlaminar space. Reynolds (23) described a sawtooth appearance along its posterior margin that accounts for this variation (Fig. 2). The epidural space is circumferential when looked at in cross section. There is a posterior raphe in the midline formed by the dorsal median fold of the dura mater, which may occasionally divide the posterior compartment. This can be seen as a midline lucency during epidurography. In the majority of cases, the space is contiguous, surrounding the spinal cord and cauda equina and dural membranes. The epidural space does increase slightly in depth in the prone position (24). The epidural space has a pressure, which is slightly less than the ambient pressure, explaining the so-called "loss of resistance" technique used for epidural punctures.

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