Sacrum

The sacrum is a large, triangular bone at the base of the vertebral column, formed by the fusion of five sacral vertebrae. It forms the posterior wall of the pelvis and articulates with the hip bones to form the sacroiliac joints.

The sacrum is situated between the two hip bones at the base of the vertebral column, forming the posterior portion of the pelvic girdle. The sacral promontory projects anteriorly at the lumbosacral junction.

The sacrum transfers the weight of the upper body to the lower limbs via the sacroiliac joints and forms the posterior wall of the pelvis. The sacral canal, as the inferior continuation of the vertebral canal, transmits sacral nerve roots.

The sacrum may be involved in sacral fractures, sacroiliac joint dysfunction, and sacral stress fractures particularly in runners and those with osteoporosis. Sacral nerve compression may be associated with lower limb radiculopathy or bladder and bowel dysfunction.

The sacrum is a wedge-shaped, curved bone with a smooth anterior surface and a rough posterior surface bearing the median sacral crest. Four pairs of sacral foramina are visible on its anterior and posterior surfaces, and the sacral promontory forms the prominent anterior projecting edge at its superior end.