Ristiluu

Ristiluu, known in English as the sacrum, is a large triangular bone at the base of the spine and a key part of the pelvic girdle. It is formed by the fusion of five sacral vertebrae (S1–S5) during adulthood. The base of the sacrum articulates with the fifth lumbar vertebra (L5) at the lumbosacral joint, while its lateral parts, the ala (wings), join the ilia at the sacroiliac joints. The apex of the sacrum meets the coccyx.

The sacrum has both anterior and posterior surfaces with openings that transmit nerves and vessels. The anterior surface is concave and contributes to the pelvic wall, while the posterior surface features the median sacral crest and four pairs of posterior sacral foramina for the dorsal rami of sacral nerves. The anterior sacral foramina accommodate the ventral rami. The sacral canal runs within the bone as a continuation of the vertebral canal, and features such as the sacral promontory, sacral hiatus, and sacral cornua are notable landmarks.

Functionally, the sacrum transmits body weight from the spine to the pelvis, stabilizes the pelvis, and helps protect sacral nerves within the sacral canal and foramina. It provides attachment points for ligaments and muscles of the pelvic floor and lower back.

Development and variation: typically, the sacral vertebrae fuse by adulthood. Variation can include sacralization of L5 (partial fusion with the sacrum) or lumbarization of S1 (less common separation from the sacrum).

Clinical relevance: sacral fractures, usually from high-energy trauma, can affect the sacral nerves and pelvic stability. Other concerns include sacroiliac joint disorders and congenital variations affecting anatomy and function.