Sacrum

Function

Five fused vertebrae forming the keystone of the pelvic ring. It transmits the entire body weight from the spine into the ilia through the sacroiliac joints, and its canal carries the sacral nerve roots that exit to the bladder, bowel and perineum.

Landmarks

• Promontory
• Sacral canal
• Anterior and posterior foramina
• Median sacral crest
• Auricular surface
• Sacral hiatus

Articulations (joints)

• L5 above
• Coccyx below
• Left and right hip bones (sacroiliac joints)

Clinical relevance

Caudal epidural anaesthesia (most used in children) is injected through the sacral hiatus. Sacral insufficiency fractures afflict osteoporotic elders; chordoma is the classic sacral tumour.