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Sacroiliac joint

The sacroiliac joint (SI joint) is a paired synovial and syndesmotic joint that connects the sacrum to the ilium of the pelvis. It is located posteriorly in the pelvis, just inferior to the posterior superior iliac spine (PSIS).

The SI joint has a unique structure: the anterior one-third is a synovial joint, while the posterior two-thirds is a fibrous syndesmosis. Its articular surfaces are irregular, with complementary ridges and depressions that interlock, providing strong stability while allowing limited movement.

The joint is reinforced by some of the strongest ligaments in the body: the interosseous sacroiliac ligament, anterior sacroiliac ligament, posterior sacroiliac ligament, sacrotuberous ligament, and sacrospinous ligament.

Functionally, the sacroiliac joint transmits weight and forces between the spine and the pelvis/lower limbs, absorbs shock during ambulation, and provides limited motion (nutation and counternutation of the sacrum). Clinically, it is an important source of low back and pelvic pain and is commonly involved in sacroiliitis, trauma, pregnancy-related laxity, and spondyloarthropathies.

Synonyms

  • SI joint

  • Sacroiliac articulation

  • Articulatio sacroiliaca

Function

  • Transmits weight of upper body from the axial skeleton to pelvis and lower limbs

  • Provides stability with minimal mobility, mainly nutation and counternutation

  • Absorbs shock and torsional stresses during gait

  • Serves as a common pain generator in low back pain syndromes

MRI Appearance

T1-weighted images:

  • Normal cortical bone of sacrum and ilium: low signal

  • Marrow: intermediate signal

  • Cartilage appears thin, intermediate signal

T2-weighted images:

  • Joint space appears as a hyperintense line if fluid is present

  • Inflammation or effusion increases T2 signal

  • Cartilage thinning or erosion seen as irregular low-signal margins

STIR:

  • Excellent for detecting bone marrow edema, seen as hyperintense signal adjacent to joint

  • Sensitive for early sacroiliitis and inflammatory changes

T1 Fat-Saturated (Pre-contrast):

  • Improves conspicuity of subtle marrow signal abnormalities against suppressed fat background

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Enhances active inflammation, synovitis, and pannus formation

  • Used in distinguishing acute vs chronic sacroiliitis

MRI 3D Reconstructions:

  • Depict joint morphology, erosions, sclerosis, and ankylosis

  • Valuable in pre-surgical planning and detailed assessment of chronic changes

CT Appearance

CT Pre-Contrast:

  • Excellent for bony detail: joint space narrowing, sclerosis, erosions, osteophytes, or ankylosis

  • Detects subtle fractures around SI joint in trauma

CT Post-Contrast:

  • Bone itself does not enhance

  • Enhances visualization of adjacent soft tissue inflammation, infection, or tumor involvement

  • Used in complex cases of osteomyelitis or neoplastic disease

MRI image

Sacroiliac joint MRI axial anatomy image -img-00000-00000

MRI image

Sacroiliac joint MRI coronal anatomy image -img-00000-00000

CT images

Sacroiliac joint CT axial image

CT VRT 3D image

Sacroiliac joint CT 3D image