Sacrococcygeal symphysis

At a glance

English

Sacrococcygeal symphysis

Latin (TA2)

Symphysis sacrococcygea

System

Cartilage

Region

Abdomen

Side

Single, on the midline

About the sacrococcygeal symphysis

The sacrococcygeal symphysis, or sacrococcygeal joint, joins the bottom of the sacrum to the top of the coccyx. Like the joints between vertebral bodies higher up, it is a secondary cartilaginous joint: the oval surface at the apex of the sacrum and the base of the first coccygeal segment are each covered by hyaline cartilage and joined by a thin disc of fibrocartilage.

Several ligaments hold the joint together. The anterior sacrococcygeal ligament runs down the pelvic surface and continues the anterior longitudinal ligament. Behind lie a deep posterior ligament, continuing the posterior longitudinal ligament on the back of the bodies, and a superficial posterior ligament, which closes the lower opening of the sacral canal, the sacral hiatus. Lateral sacrococcygeal ligaments run from the lower lateral angles of the sacrum to the transverse processes of the coccyx, and intercornual ligaments join the sacral and coccygeal cornua.

Only a little movement occurs, mostly a slight bending forwards and backwards of the coccyx on the sacrum. This happens passively, as when a person sits down and the coccyx is pressed backwards, and with the pull of the pelvic floor muscles and gluteus maximus, which attach to the coccyx and the adjacent ligaments. In many people the joint becomes partly or wholly fused with age.

Falls onto the buttocks can sprain, sublux or fracture the region, and pain at the tip of the spine, called coccydynia, often comes from this joint. The superficial posterior ligament over the sacral hiatus is the membrane pierced by the needle in a caudal epidural injection.

Attachments and function

Attaches from

The oval articular surface at the apex of the sacrum.

Attaches to

The base of the first coccygeal segment, joined to the sacrum by a fibrocartilaginous disc and the sacrococcygeal ligaments.

Function

Allows slight flexion and extension of the coccyx on the sacrum, mainly passive, as in sitting and in the action of the pelvic floor muscles.

Nerve supply

Not well described in standard texts; probably small twigs of the coccygeal plexus, formed from the fourth and fifth sacral and coccygeal ventral rami.

Blood supply

Small branches of the median sacral artery and the lateral sacral arteries.