The sacrum articulates with four bones: the last lumbar vertebra above via a disc space and facet joint complex, the coccyx below with a ligamentous attachment and occasional bone union, and on either side with the ilium via the sacroiliac joint. The long auricular articulation of the ilium with the sacrum is broad above and narrows below, it extends from the first to the second sacral segment. In adults, the sacroiliac joint is essentially immobile acting primarily as a shock absorber for the axial column. The sacrum is stabilized by several ligaments. The sacrotuberous and sacrospinous ligaments hold the inferior part of the sacrum; these ligaments are connected to the ischium. The coccyx is attached to sacrum by the sacrococcygeal ligaments. The interosseous and dorsal sacroiliac ligaments, the strongest ligament of the pelvis, bind the upper sacrum to the superior aspect of the ilium posteriorly.