Korsbenet

Korsbenet, known in anatomy as the sacrum, is a large triangular bone at the base of the spine. It is formed by the fusion of five sacral vertebrae (S1–S5) in most adults and sits between the two hip bones, completing the posterior part of the pelvic ring. Its superior end articulates with the fifth lumbar vertebra (L5), and its inferior end articulates with the coccyx.

Structure and surfaces: The anterior (pelvic) surface is concave and features the sacral promontory at its superior border. The lateral aspects have auricular surfaces that articulate with the ilia at the sacroiliac joints. The dorsal surface displays the median sacral crest, along with the intermediate and lateral crests, and four pairs of posterior sacral foramina for the dorsal rami of sacral nerves. The anterior sacral foramina transmit the ventral rami. The sacrum contains the sacral canal, the continuation of the vertebral canal, which ends at the sacral hiatus near the coccyx.

Function: The sacrum transmits body weight from the axial skeleton to the pelvic girdle, helps form the posterior wall of the pelvis and pelvic floor, and protects pelvic organs. It provides attachment points for ligaments and muscles and serves as a conduit for sacral nerves and blood vessels.

Clinical notes: Variations can occur, such as sacralization of L5 or lumbarization of S1. The promontory is an important obstetric landmark, and sacroiliac joint dysfunction can affect pelvic stability and lower back or hip regions.