(/ˈsækrəm/ or /ˈseɪkrəm/; plural: sacrums or sacra) is a large, triangular bone at the base of the spine and at the upper, back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and its lower part with the coccyx (tailbone). Usually, it begins as five unfused vertebrae which begin to fuse between the ages of 16–18 years and have usually completely fused into a single bone by the age of 34 years.
The sacrum has three different surfaces which are shaped to accommodate various structures. It articulates with four other bones. It is curved upon itself and placed obliquely (tilted forward). It is concave, facing forward. The base projects forward as the sacral promontory internally, which is the superiormost portion of the sacrum. The central part is curved outward toward the posterior, allowing greater room for the pelvic cavity. The two lateral projections of the sacrum are called the alae (wings), and articulate with the ilium at the L-shaped sacroiliac joints.
The sacral vertebrae develop by the end of the first month of embryonic development, the higher vertebrae having developed first. There are congenital disorders that develop in the early stages of the fetus.
See sacral lesions