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Interosseous sacroiliac ligament

The interosseous sacroiliac ligament is one of the strongest ligaments in the human body. It is a thick, dense bundle of short fibers located deep between the sacrum and ilium, binding their rough articular surfaces. It is the primary stabilizing structure of the sacroiliac joint (SIJ), transmitting the forces of the spine to the pelvis and lower limbs. Because of its strength and location, it is difficult to visualize directly, but its integrity is crucial for pelvic stability.

Synonyms

  • Interosseous ligament of the sacroiliac joint

  • Posterior interosseous SI ligament

  • Sacroiliac interosseous ligament

Attachments

  • Extends from the tubercles of the sacrum (sacral fossae, posterior to the auricular surface)

  • To the tuberosity of the ilium (just posterior and superior to the iliac auricular surface)

Relations

  • Anteriorly: Synovial sacroiliac joint cavity

  • Posteriorly: Posterior sacroiliac ligaments and gluteal muscles

  • Superiorly: Continuous with fibers blending into the posterior superior iliac spine region

  • Inferiorly: Related to sacrotuberous and sacrospinous ligaments forming pelvic floor stability

Function

  • Acts as the chief stabilizer of the sacroiliac joint

  • Transfers axial load from the spine to the pelvis and lower limbs

  • Prevents anterior and inferior displacement of the sacrum relative to the ilium

  • Provides strong fibrous support between the articular bones, resisting shear and rotation

Clinical Significance

  • SI joint dysfunction: Damage or laxity can lead to pelvic instability and low back pain

  • Trauma: Disruption occurs in high-energy pelvic fractures

  • Degeneration: May calcify or weaken with age, contributing to sacroiliitis

  • Inflammation: Target of autoimmune diseases such as ankylosing spondylitis, leading to fusion of the SI joint

  • Imaging relevance: Difficult to visualize directly but assessed indirectly through SI joint alignment, signal changes, or adjacent inflammation

MRI Appearance

T1-weighted images:

  • Appears as a dark band of low signal intensity between sacrum and ilium

  • Surrounded by bright signal of adjacent fat planes

T2-weighted images:

  • Normally dark, low signal intensity

  • Pathology (sprain, inflammation, enthesitis) shows bright signal areas within or around the ligament

STIR (Short Tau Inversion Recovery):

  • Normal ligament: dark low signal

  • Abnormal ligament: bright hyperintensity, especially in inflammatory sacroiliitis

T1 Fat-Sat Post-Contrast:

  • Normal ligament: minimal or no enhancement

  • Pathology: enhancement of ligament fibers and adjacent bone marrow/periarticular tissues in infection or inflammation

CT Appearance

Non-Contrast CT:

  • Ligament not separately visualized; appears as a dense fibrous band between sacrum and ilium

  • Chronic changes may show calcification or ossification within the ligament

  • Adjacent SI joint irregularities may be present in degeneration or sacroiliitis

Post-Contrast CT:

  • Normal ligament: no significant enhancement

  • Pathology: adjacent soft tissue enhancement or periarticular changes in infection, inflammation, or trauma

MRI image

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

MRI image

Interosseous sacroiliac ligament MRI AXIAL image

CT image

Interosseous sacroiliac ligament ct axial image