The posterior (dorsal) sacroiliac ligaments are a layered group of short and long bands behind the sacroiliac joint, lying superficial to the very strong interosseous sacroiliac ligament. The short posterior ligaments run almost horizontally to the upper sacrum, while the long posterior ligament runs from the PSIS to the lateral sacral crest at S3-S4 and blends with the sacrotuberous ligament. Together they form the posterior tension band of the pelvic ring and fail in vertical shear injuries, producing catastrophic posterior ring instability.
| Origin | Posterior superior iliac spine and adjacent posterior iliac crest |
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| Insertion | Intermediate and lateral sacral crests of S1-S4 (short ligaments to S1-S2, long ligament to S3-S4) |
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Bind the posterior ilium to the sacrum, resisting sacral counternutation and separation of the sacroiliac joint.
Posterior SI ligament disruption in vertical shear (Tile C) injuries produces superior hemipelvis displacement visible on AP pelvis radiograph and CT. Posterior ring stabilisation with percutaneous iliosacral screws into the sacral ala and body is the standard, supplemented by anterior ring fixation. The long dorsal ligament can be palpated just below the PSIS and is often tender in pregnancy-related posterior pelvic pain, when hormonal laxity increases SI joint loading. Pelvic belts and stabilising exercise usually relieve such pain.
Complete posterior SI ligament disruption producing superior hemipelvis displacement requiring urgent posterior iliosacral screw fixation.
Pain and tenderness at the long dorsal sacroiliac ligament below the PSIS during pregnancy, managed with a pelvic belt and physiotherapy.
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