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Posterior sacroiliac ligament

The posterior sacroiliac ligament is a strong fibrous structure connecting the sacrum and ilium at the sacroiliac joint. It reinforces the joint posteriorly and is essential for pelvic stability, transmitting weight and forces between the axial skeleton and lower limbs. It consists of short (interosseous) fibers and long fibers, contributing to the integrity of the sacroiliac articulation.

This ligament is clinically important in low back pain, pelvic instability, sacroiliitis, and trauma. It is readily evaluated with MRI and CT when sacroiliac pathology is suspected.

Synonyms

  • Dorsal sacroiliac ligament

  • Posterior interosseous sacroiliac ligament

  • Long dorsal sacroiliac ligament

Attachments and Parts

  • Short (interosseous) posterior sacroiliac ligament:

    • Runs between the sacral tuberosity and iliac tuberosity

    • Very strong, lies deep, filling the gap posterior to the joint

  • Long posterior sacroiliac ligament:

    • Extends from the posterior superior iliac spine (PSIS) and adjacent iliac crest

    • Attaches to the third and fourth transverse tubercles of the sacrum

    • Lies superficial to the interosseous ligament, spanning downward and medially

Course

  • The fibers pass obliquely from the ilium to the sacrum, oriented both vertically (long part) and horizontally (short part)

  • The ligament blends with fibers of the sacrotuberous ligament and thoracolumbar fascia, forming a strong stabilizing complex

Relations

  • Anteriorly: Sacroiliac joint and interosseous sacroiliac ligament

  • Posteriorly: Erector spinae muscles, multifidus, and gluteus maximus muscle

  • Superiorly: Posterior superior iliac spine (PSIS)

  • Inferiorly: Sacrotuberous ligament and posterior sacrum

Function

  • Provides strong posterior support to the sacroiliac joint

  • Prevents posterior and inferior displacement of the sacrum

  • Assists in stabilizing the pelvis during weight-bearing and locomotion

  • Works in concert with sacrotuberous and sacrospinous ligaments for pelvic stability

Clinical Significance

  • Involved in sacroiliac joint dysfunction and chronic low back pain

  • Can be strained in trauma or pregnancy due to increased joint laxity

  • Tenderness at ligament insertion is a diagnostic clue in posterior pelvic pain

  • May show enthesopathy or calcification on imaging

  • Surgical landmark in posterior pelvic approaches

MRI Appearance

T1-weighted images:

  • Ligament appears as a thin, dark (low-signal) band connecting ilium to sacrum

  • Surrounded by bright fat, providing contrast

T2-weighted images:

  • Ligament remains low signal

  • Injury or inflammation appears as focal bright signal within or adjacent to ligament

STIR (Short Tau Inversion Recovery):

  • Normal ligament is dark

  • Pathology (strain, enthesitis, sacroiliitis) appears bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal ligament shows no significant enhancement

  • Inflamed ligament shows linear or focal enhancement

  • Enthesitis or sacroiliitis shows enhancement at sacral and iliac attachments

3D T2 SPACE / CISS:

  • Ligament shows low signal band

  • Surrounded by bright fat or joint fluid, providing excellent delineation

  • Helpful in detecting subtle tears or inflammatory changes

CT Appearance

Non-Contrast CT:

  • Appears as a dense fibrous band between ilium and sacrum

  • Enthesopathy or chronic stress may show calcification or ossification

  • Helpful for detecting associated sacral/iliac fractures

Post-Contrast CT:

  • Ligament itself does not enhance

  • Adjacent soft tissue inflammation or abscess may show enhancement

  • In sacroiliitis, enhancement may be seen along adjacent bone and soft tissue

MRI image

Posterior sacroiliac ligament mri coronal anatomy image-img-00000-00000

MRI image

Posterior sacroiliac ligament mri axial image

CT image

Posterior sacroiliac ligament ct axial image