and can be opened to fill with fluid during disease processes or therapeutic intervention. Superficial to the dura, between the dura and the inner aspect of the vertebral canal, is the epidural space, filled with fat and the epidural venous plexus (Figs. 16 and 17). Loss of resistance to air or saline is commonly used to indicate successful placement of a needle into the epidural space. The ligamenta flava plays an important role in this loss of resistance, and the anatomic variability of this ligament may contribute to false-positive assessments of needle placement. It has been suggested that epidurography can improve the accuracy of needle placement and medication delivery to targeted areas of spinal pathology (15) (Fig. 18).
Deep to the dura, between it and the arachnoid is a potential space, the subdural space. During administration of spinal anesthesia, the needle may push the arachnoid membrane away from the dura, instead of piercing the arachnoid. In this case, the injectate may be delivered to the subdural space and may contribute to the variable success of spinal anesthesia (16). Deep to the meningeal layer of arachnoid, between the arachnoid and the pia, is the subarachnoid space filled with cerebrospinal fluid. Together, the arachnoid and the pia mater are referred to as the leptomeninges.
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