Posterior view of lower lumbar vertebrae and sacrum. The sacrum is an irregular triangular-shaped bone that is formed by the fusion of five sacral vertebrae. It unites with four bones: the fifth lumbar vertebra superiorly, the right and left hip bones laterally, and the coccyx inferiorly. The superior articular processes of the S1 vertebra articulates with the inferior articular processes of the L5 vertebra. The sacral canal is a continuation of the vertebral canal and conveys the sacral spinal nerves. It opens distally at the sacral hiatus, whose margins are formed by the two sacral cornua. The dural sac ends at the S2 level. Nerve roots of the fifth sacral nerve and the filum terminale externum exit the hiatus. The sacrum contains anterior and posterior sacral foramina where the anterior and posterior rami of sacral spinal nerves exit, respectively. The median sacral crest represents the remnants of the fused spinous processes and the lateral sacral crests form from fused transverse processes. The auricular surface is the site of articulation with the ilium (sacroiliac joint) and the sacral tuberosity located posterior to this is the attachment site of the posterior sacroiliac ligaments. (Image courtesy of AL Rhoton, Jr.)