The posterior SIJ ligament complex (superficial posterior SI ligament plus the interosseous SI ligament) is the primary stabiliser of the posterior SIJ. The interosseous ligament fills the deep gap between the sacral and iliac surfaces posterosuperiorly — it is the strongest ligament in the body per unit area. Posterior SIJ pain from the long posterior SI ligament (LPSI) is reproduced by Gaenslen test and posterior palpation below the PSIS.
| Origin | Posterior iliac spine and posterior iliac crest |
|---|---|
| Insertion | Lateral sacral crest (S1-S4 levels) — the superficial posterior SI ligament; the deep interosseous ligament fills the posterior SIJ gap |
| Actions | Primary posterior SIJ stabiliser; resists sacral nutation and axial loading; the strongest SIJ ligament complex |
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The long posterior SI ligament (from the PSIS to the lateral sacral crest) is the primary target of the posterior SIJ palpation test — tenderness here distinguishes SIJ pain from piriformis pain (more lateral) and lumbar facet pain (more medial). The LPSI is also a potential source of the low back pain reported after prolonged sitting or carrying.
LPSI strain from pregnancy, trauma, or repetitive loading producing posterior pelvic pain reproduced by sitting and posterior palpation managed with SIJ belt and stabilisation.
