It's the sacral hiatus, a normally occurring gap at the lower end of the sacrum, exposing the vertebral canal, due to failure of the laminae of the last sacral segment to coalesce.
It corresponds to the posterior caudal opening at the end of the sacral canal, which usually occurs at the fifth sacral vertebra (S5), at the posterior surface of the sacrum.
Sacral hiatus is flanked bilaterally by the articular process called sacral cornua. Anteriorly there is the sacral canal, which, at this level, has no dural sac and contains only extradural fat, vertebral venous plexus, lower sacral nerve roots and the filum terminale. Posteriorly, covering the hiatus, is the sacrococcygeal ligament.
It provides cannular access to the sacral epidural space for administration of anesthetics (caudal nerve blocks).
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