Sacrospinous ligament

At a glance

English

Sacrospinous ligament

Latin (TA2)

Ligamentum sacrospinale

System

Ligament

Region

Abdomen

Side

Paired, left and right

About the sacrospinous ligament

The sacrospinous ligament, known in older texts as the lesser sacrosciatic ligament, is one of two ligaments that tie the sacrum to the ischium on each side. It is shaped like a fan. Its broad base attaches to the lateral margin of the lower sacrum and upper coccyx, and its fibres converge to a narrow apex on the ischial spine, the pointed projection on the back of the hip bone.

The ligament lies in front of the larger sacrotuberous ligament, and the two cross each other, their fibres mingling where they meet near the sacrum. On its pelvic surface lies the coccygeus muscle, which has the same triangular shape and attachments and is so closely bound to it that the ligament is often regarded as the fibrous back part of the muscle.

By bridging the sciatic notch, the ligament divides it into two openings. Above it lies the greater sciatic foramen, through which piriformis, the sciatic nerve, the gluteal vessels and nerves and other structures leave the pelvis. Below it lies the lesser sciatic foramen, between it and the sacrotuberous ligament. The pudendal nerve and internal pudendal vessels hook around the back of the ligament close to the ischial spine, leaving through the greater and returning through the lesser foramen. The inferior gluteal artery and its coccygeal branch also pass behind it.

With the sacrotuberous ligament it stops the lower end of the sacrum tipping backwards and upwards when body weight presses down on the sacral base, so it resists rotation of the sacrum relative to the ilium. Bending forward and rising from a chair load it most. The ischial spine and ligament are the landmark for a pudendal nerve block, and the nerve can become trapped where it passes between the two sacro-ischial ligaments.

Attachments and function

Attaches from

The lateral margins of the lower sacrum and upper coccyx.

Attaches to

The ischial spine.

Function

Resists backward tilting of the lower sacrum and coccyx, stabilises the sacro-iliac joint, and converts the sciatic notches into the greater and lesser sciatic foramina.

Nerve supply

Small twigs from the sacral plexus and pudendal nerve.

Blood supply

Branches of the inferior gluteal and internal pudendal arteries.