The sacrum and coccyx are formed by the fusion of eight vertebrae (five sacral and three coccygeal vertebrae). There is a natural defect resulting from incomplete fusion of the lower portion of S4 and entire S5 in the posterior midline. This defect is termed the sacral hiatus and is covered by the sacrococcygeal ligament. The hiatus is bounded laterally by the sacral cornua, and the floor is comprised of the posterior aspect of the sacrum. The epidural space extends from the base of the skull to the level of the sacral hiatus. It is the space confined between the dura mater and the ligamentum flavum and surrounds the dural sac. It is divided into anterior and posterior compartments and bounded anteriorly by the-posterior longitudinal ligaments, laterally by the pedicles and neural foramina, and -posteriorly by the ligamentum flavum. The epidural space contains the spinal nerve roots and the spinal artery that pass through the neural foramina and the epidural venous plexus. Below the level of S2, where the dura terminates, the epidural space continues as the caudal epidural space that can be accessed via the sacral hiatus which is covered by the sacrococcygeal membrane. The sacral epidural canal contains the sacral and coccygeal roots, spinal vessels, and the filum terminale. The epidural venous plexus is concentrated in the anterior space in the caudal epidural canal.
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