Sacroiliac Posterior Long Ligament

ligamentum sacroiliacum posterius longum

The long posterior sacroiliac ligament connects the third and fourth sacral transverse tubercles and the sacral crest to the posterior superior iliac spine and the erector spinae aponeurosis, forming the most superficial posterior SIJ ligament.

Region: Lower Back

Biomechanics

Function & Actions

Resists sacral counter-nutation and excessive posterior sacral rotation, directly connects the sacrum to the thoracolumbar fascia system, and is the ligament palpated in posterior pelvic pain testing.

Clinical Relevance

Clinical Notes

The long posterior SIJ ligament is the structure responsible for the positive posterior pelvic pain provocation test (P4 test) — deep pressure over the PSIS-sacrum area reproduces posterior pelvic pain. It is a primary pain generator in pregnancy-related pelvic girdle pain and chronic SIJ dysfunction.

Pathology

Common Injuries & Conditions

Long Posterior SIJ Ligament Pain in Pregnancy

Ligamentous laxity from relaxin producing long posterior SIJ ligament pain in pregnancy-related pelvic girdle pain, managed by pelvic girdle support and physiotherapy.

P4 Test for Long Posterior SIJ Ligament

Posterior pelvic pain provocation test applying pressure over the ligament to reproduce SIJ pain, used for clinical SIJ pain diagnosis.