Anatomy

Dorn Spinal Therapy

Spine Healing Therapy

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The epidural space extends from the foramen magnum down through the sacral hiatus. It consists of the space or potential space between the external surface of the spinal dura mater and the inner margin of the spinal canal (5,6). Thus, the outer limit of the epidural space is confined by the peridural membrane, which is demarcated by the periosteum of the osseous canal, the posterior margins of the intervertebral discs, the inner border of the ligamenta flava, and, sometimes, the anterior border of the interspinous ligament (7,8). Noteworthily, some patients have a midline posterior dural reflection, the plica mediana dorsalis, which may divide the epidural space and limit the spread of injectate to one side (5,6,9,10). This variant appears to be more common in the sacral levels.

In the lumbar spine, the epidural space is typically largest in the posterior mid-line, where the triangular posterior recess is usually filled with fat and readily visible on magnetic resonance imaging (MRI) and computed tomography (CT) scans. This recess is the target for epidural injection if the "interlaminar" approach is used (Fig. 1). Postsurgical scarring often obliterates it. Also, the posterior epidural space is commonly only a potential space at L5/S1 or in patients with spinal stenosis. In the cervical spine, the posterior epidural space is often only a potential space, with no focal fat to target. Therefore, reaching the epidural space here relies on feel as much as it does on image guidance.

The sacrum is formed by the fusion of five embryonic vertebrae and is convex dorsally. The coccyx consists of three to five rudimentary vertebrae attached to the base of the sacrum. The sacral hiatus, the portal used for accessing the epidural space, is a natural defect resulting from an incomplete midline fusion of the posterior elements of the lower portion of S4 and the entire S5 vertebra. The hiatus, which is covered by the sacrococcygeal ligament, is bordered laterally by the sacral cornua and its floor is made up of the posterior aspect of S5 (Fig. 2). The thecal sac has a variable termination depending on age: it ends at the lower border of the S1

FIGURE 1 Posterior epidural space. (A) Axial computed tomography demonstrating fat in the posterior epidural space {arrow). (B, C) Axial proton density and sagittal Tl-weighted magnetic resonance images of a different patient demonstrating fat in the posterior epidural space at L3/4, providing a suitable target for epidural steroid injection. (Note that the posterior recess at L5/S1 is characteristically negligible.)

FIGURE 1 Posterior epidural space. (A) Axial computed tomography demonstrating fat in the posterior epidural space {arrow). (B, C) Axial proton density and sagittal Tl-weighted magnetic resonance images of a different patient demonstrating fat in the posterior epidural space at L3/4, providing a suitable target for epidural steroid injection. (Note that the posterior recess at L5/S1 is characteristically negligible.)

foramen in adults and at the S3 foramen in children, but studies of cadavers suggest that the position of the thecal sac tip is at the middle-third of the S2 body in the average patient (11,12).

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