Last revised by Bruno Di Muzio on 5 Apr 2024

Sacroiliitis (rare plural: sacroiliitides), is an inflammation of one or both sacroiliac (SI) joints, and a common cause of buttocks or lower back pain. Sacroiliitis can be a manifestation of a wide range of disease processes.

Symptoms of sacroiliitis can vary. People with sacroiliitis commonly present with ipsilateral or bilateral buttock and/or midline lower lumbar area pain. Up to 50% may have pain radiating to the lower extremity.

The causes of sacroiliitis can be divided into unilateral or bilateral. See article: sacroiliitis (differential)

Conventional radiography remains the first line of imaging despite its poor sensitivity and specificity in early disease. Specific sacroiliac joint views are helpful in the evaluation and comparing both sides of sacroiliac joints.

Radiograph findings include:

  • sclerosis of the endplates particularly on the iliac side

  • irregular joint end plates

  • widening of joint spaces

  • erosions

  • ankylosis (end stage)

CT examinations offer greater sensitivity, accuracy and detailed information compared to plain radiography. However, due to higher radiation exposure, it is not advisable to use CT for diagnosis or follow-up purposes.

Bone scans demonstrate increased radioisotope activity of the joints and helpful in localizing the source of the pain. It is also valuable in excluding stress fractures and other bone pathologies.

Though not routinely used for evaluating the sacroiliac joints, MRI is capable of identifying early inflammatory changes of joints when other imaging is negative and excludes other differential causes such as disc prolapse which may resemble clinical symptoms of sacroiliitis.

MRI features of sacroiliitis can be divided into inflammatory and structural lesions 6,7:

  1. inflammatory lesions

    • marrow edema (first to appear): high signal on water sensitive sequences

    • synovitis and capsulitis: thickening and contrast enhancement of the synovium and joint capsule

    • enthesitis: thickening and contrast enhancement of ligaments and tendons at their attachments to bone

  2. structural lesions

    • subchondral sclerosis: bands of low signal (on all sequences) paralleling the joint margins, at least 5 mm from the joint space

    • erosions: marginal foci of articular bone loss

      • low T1 signal

      • high T2/STIR signal if active

      • more prominent anteroinferiorly and on the iliac side of the SIJ

      • when confluent may appear as joint space widening

    • backfill: intra-articular high T1 signal filling up excavated bone erosions

    • fat metaplasia: periarticular fat deposition

    • ankylosis

Treatment depends on the underlying cause of the sacroiliitis. Physiotherapy may also be helpful in strengthening the pelvic muscle and increase the mobilization of the SI joint. Analgesics such as NSAIDs may be useful in symptomatic management. Corticosteroid injection to the affected sacroiliac joint can be performed to reduce inflammation and pain.

Surgical fusion of the SI joint is only considered as a last resort when conservative management is ineffective. 

The following conditions may mimic sacroiliitis:

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