The sacroiliac joint connects the auricular surfaces of the sacrum with auricular surfaces of the iliac bones on each side. The auricular surfaces are roughened surfaces that match and interlock to some extent, yet permit limited gliding and rotatory movement. The joint capsule is thin and reinforced by strong, extracapsular ligaments. The ventral and dorsal sacroiliac ligaments and the dorsal interosseous ligaments are particularly strong. The dorsal interosseous ligament occupies the posterior two thirds of the space between the sacropelvic surface of the ilium and the lateral mass of the sacrum. Around the age of 50, the joint cavity disappears and the articulating bones fuse. Downward displacement of the sacrum tends to move the two iliac bones apart and to rotate the inferior aspect of the sacrum and the coccyx posteriorly. The sacrotuberous and the sacrospinous ligaments, which maintain the forward tilt of the lower sacrum and coccyx, oppose this rotation (Fig. 13).
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