The posterior sacroiliac ligaments are the thickest and strongest ligaments of the body, comprising the short and long posterior sacroiliac ligaments (the interosseous ligament is the deepest component) that run between the posterior iliac crest and the sacral tuberosity and transverse tubercles. They are the primary restraint to posterior sacral rotation (nutation) and to vertical displacement of the hemi-pelvis in posterior pelvic ring fractures.
The primary stabilisers of the posterior pelvic ring, preventing vertical displacement of the sacrum relative to the ilium, resisting posterior sacral rotation, and maintaining the sacroiliac joint as the structural keystone of the pelvic ring under axial load.
The posterior SI ligaments determine pelvic stability: Tile type C injuries involve complete disruption of both the anterior and posterior SI ligamentous complex, producing unstable vertical shear fractures requiring posterior fixation with iliosacral screws or spinopelvic fixation. The posterior SI ligament complex is palpated in the posterior pelvis between the PSIS and the sacral posterior surface and its disruption produces significant posterior pelvic tenderness. Iliosacral screw fixation targets the interosseous ligament zone to compress the posterior pelvic ring back into stability.
Complete posterior SI ligament disruption in vertical shear pelvic injury produces Tile C instability with superior migration of the hemi-pelvis; CT confirms posterior ring disruption and iliosacral screws placed along the S1 and S2 body corridors compress and stabilise the posterior ring, with anterior fixation addressing associated symphysis or anterior column fractures.
