The interosseous sacroiliac ligament is the massive, multi-layered ligamentous complex filling the posterior iliac fossa behind the articular surface of the SIJ — the strongest ligament in the body by unit cross-sectional area. It is entirely intra-articular, filling the potential space behind the articular cartilage.
The primary stabiliser of the SIJ — resists all directions of sacroiliac displacement. Its multi-layered interdigitating fibre bundles resist both shear and tension forces, making it the reason the SIJ rarely dislocates without associated bone fracture.
The interosseous SIJ ligament is not directly palpable but its volume is assessed indirectly by the SIJ gap on CT — disruption allows the joint to widen on AP pelvis X-ray. Sacroiliac screw fixation (the standard for posterior ring instability) engages the interosseous ligament zone for mechanical purchase.
Disruption of the interosseous sacroiliac ligament producing the most severe form of pelvic instability — vertical shear injury — requiring posterior sacroiliac screw fixation for stability.
Posterior iliosacral screws placed into S1 body pass through the interosseous SIJ ligament zone, using this dense fibrous region for screw purchase in pelvic ring instability fixation.
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