The posterior sacroiliac ligaments (short and long dorsal) are the strongest ligaments in the body by cross-sectional area, providing the primary restraint against posterior pelvic ring disruption. The long dorsal SI ligament is palpable as the cord from the PSIS to the sacrum and is commonly tender in SI joint dysfunction and posterior pelvic pain of pregnancy. Their integrity is what makes the SI joint the load-transferring structure for all upper body weight.
| Origin | Posterior iliac crest and PSIS |
|---|---|
| Insertion | Lateral sacral crest (short — between S1-S2); to the sacrum and PSIS (long — attaches at S3-S4 level) |
| Actions | The strongest ligament system in the body — resists sacral nutation, posterior sacral shear, and iliac distraction |
|---|
Posterior pelvic pain from long dorsal SI ligament strain is the most common musculoskeletal pain of pregnancy — relaxin-mediated laxity allows excessive SI joint motion. The long dorsal SI ligament is assessed by palpation at the PSIS while the patient actively raises the straight leg (positive posterior pain provocation — P4 test). Posterior SIJ injection precisely targets the dorsal ligament complex.
Long dorsal SI ligament strain from relaxin-induced laxity producing PSIS and buttock pain in pregnancy managed with pelvic belt stabilisation and physiotherapy.
