Caudal epidural block involves injection of medication through the sacral hiatus, which is an arch shaped opening in the dorsal sacral surface. The sacral canal is continuous with the lumbar spinal canal, and contains the cauda equina nerve roots, spinal meninges, epidural fat and epidural venous plexus. The subarachnoid space terminates typically at S2.
The sacral hiatus is located below the fourth (or third) sacral spinous tubercles. An equilateral triangle is described with the two posterior superior iliac spines forming the base and the sacral hiatus at the apex. The two sacral cornua can be palpated flanking the rostral margin of the sacral hiatus. The sacral canal is roofed by the sacrococcygeal ligament, a continuation of the ligamentum flavum.
- Senoglu et al. Landmarks of the sacral hiatus for caudal epidural block: an anatomical study. Br. J. Anaesth. 2005;95:692-695.