Osteology of the Vertebral Column: Sacrum & Coccyx

The first and largest of the fused sacral element (S1) on the sacrum is the most cranial portion, which is called the sacral promontory. Sacral vertebrae decrease in size inferiorly along the series. The superior articular facets of S1 are termed the articular processes of the sacrum.  Adjacent vertebrae are usually fused, but in a ventral view transverse lines are visible, delineating the ontogetically separate sacral vertebral bodies.  Lateral to the sacral bodies, the sacrum is perforated by intervertebral foramina.  The transverse processes of the sacral vertebrae are fused together to form the ala, or wing, on each side of the sacrum.

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 coccyx is comprised of three to five (usually four) coccygeal vertebrae.  Usually the vertebrae are all fused together, but frequently the first coccygeal vertebra is separate from the rest.  Occasionally, the first coccygeal vertebral segment is fused to the last sacral vertebra. There are few useful morphological features on the coccyx.

The 3D model seen here is comprised of segments 2 and 3 fused together.