The sacrum and caudal block

Abstract

Typically, the sacrum represents five fused sacral vertebrae. Variations are common: lumbarization of S1, sacralization of L5 and fusion of the coccyx. Frequently, a degree of spina bifida occulta is seen. The sacrum has a central mass, four anterior sacral foramina and a lateral mass. The foramina transmit the anterior primary rami of S1–S4. The wings of the sacrum (the alae) are crossed by the lumbosacral trunk, L4 and L5, which joins the sacral plexus. Posteriorly, a median crest ends below as the sacral hiatus, bearing the cornu on either side. The hiatus is covered posteriorly by the tough posterior sacrococcygeal ligament. The posterior sacral foramina transmit the posterior roots of S1–S4. The auricular surface lies laterally and forms the sacroiliac joint with the corresponding surface in the ilium. The dural sac ends at the level of the second piece of the sacrum. The sacral canal below this level contains extradural fat, vertebral venous plexus, lower sacral nerve roots and the filum terminale. A sacral block can be performed by introducing a catheter into the sacral canal via the sacral hiatus, which is found by palpation of the sacral cornua.