Posterior Sacral Hiatus

Description

Sacral hiatus occupies the inferior midline of the dorsal sacrum as an inverted U-shaped defect where the laminae of S5 (and often S4) fail to fuse, leaving the terminal portion of the sacral canal open posteriorly. Sacral cornua flank the hiatus as paired bony prominences, positioned inferomedial to the posterior sacral foramina and just superior to the base of the coccyx at the sacrococcygeal joint. Superior to the opening, the median sacral crest continues cranially along the posterior surface, while the lateral sacral crests run parallel on either side. Caudal epidural access relies on confident identification of this hiatus and the adjacent sacral cornua, since the needle is advanced through the sacrococcygeal ligament into the sacral canal to reach the epidural space. This illustration supports teaching and preprocedural planning by emphasizing the posterior bony landmarks that are palpated clinically, helping differentiate the true hiatus from nearby irregularities of the dorsal sacral surface. Congenital variation is common, and a small or absent hiatus, partial dorsal wall defects, or a wide opening can influence success rates and risk of subperiosteal injection. Anesthesia curricula, pain medicine texts, and procedural guides for caudal epidural steroid injection can pair this illustration with surface anatomy and ultrasound discussions to anchor the osseous target. It also suits gross anatomy courses covering the pelvis and vertebral column, where the relationship between the sacral canal, dorsal crests, and coccyx is often difficult to visualize from isolated bone specimens. Anatomical accuracy verified by SciePro's Medical Advisory Board.