The sacrum sits at the base of the vertebral column, inferior to the lumbar vertebrae and to the coccyx superiorly. It is composed of five (occasionally four or six) sacral vertebrae fused together to form one functional unit.
Dorsally, the spinous processes of the sacral vertebrae diminish in prominence inferiorly. The spinous processes are variably fused together, forming the median crest of the sacrum. The most inferior portion of the sacrum is termed the apex . This apex articulates with the coccyx. The superior and lateral sides of the sacrum bear the large, shallow auricular surfaces, which articulate with the corresponding surfaces on the os coxae.
In life, the sacral promontory articulates with L5 at an angle such that the apex of the sacrum is significantly more dorsally placed than the promontory. However, especially in males, the sacrum is ventrally concave, and the inferior portion of the sacrum projects into the bony pelvic canal. Females, adapted to handle the demands of giving birth, have a shorter and less curved sacrum which is less intrusive into the pelvic canal. This sexual difference facilitates the passage of an infant through the female’s birth canal.
The 3D model seen here is comprised of segments 2 and 3 fused together.