Posterior sacro-iliac ligament

At a glance

English

Posterior sacro-iliac ligament

Latin (TA2)

Ligamentum sacroiliacum posterius

System

Ligament

Region

Abdomen

Side

Paired, left and right

About the posterior sacro-iliac ligament

Behind the sacro-iliac joint the sacrum sits deep between the two iliac bones, leaving a hollow on each side above the buttock. This hollow is filled by the posterior sacro-iliac ligament, which together with the interosseous ligament beneath it forms the main bond between the bones. It lies superficial to the interosseous ligament and is itself covered by the deep back muscles and the thoracolumbar fascia.

The ligament is made of many bundles running in different directions, but two parts are usually described. The upper, short posterior sacro-iliac ligament has almost horizontal fibres passing from the upper two tubercles of the lateral sacral crest (those of S1 and S2) to the iliac tuberosity and nearby ilium. The lower, long posterior sacro-iliac ligament runs obliquely from the S3 tubercle of the lateral sacral crest, and sometimes S4, up to the posterior superior iliac spine. Its lateral fibres blend with the upper part of the sacrotuberous ligament, and its medial fibres with the thoracolumbar fascia and erector spinae aponeurosis.

The short fibres help hold the sacrum wedged between the hip bones. The long part tightens when the sacral base tilts backwards (counternutation), whereas the sacrotuberous ligament tightens with the opposite movement, so the two balance each other. Lateral branches of the posterior rami of the sacral nerves pass across and between its bundles to reach the skin of the buttock.

The long ligament can be felt just below the posterior superior iliac spine, the dimple above the buttock, and tenderness here is a common finding in sacro-iliac and low back pain. Injections for sacro-iliac pain are often directed at this region.

Attachments and function

Attaches from

The transverse tubercles of the lateral sacral crest: the first and second for the short part, the third (and sometimes the fourth) for the long part.

Attaches to

The iliac tuberosity for the short part and the posterior superior iliac spine for the long part.

Function

Forms the chief bond behind the sacro-iliac joint, holds the sacrum between the hip bones and resists backward tilting of the sacral base.

Nerve supply

Lateral branches of the posterior rami of the lower lumbar and upper sacral nerves.

Blood supply

Branches of the lateral sacral and superior gluteal arteries.