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Contributors: Michelle Qiu, Stephanie E. Siegrist MD
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Causes / typical injury mechanism: Sacroiliitis is inflammation of the sacroiliac (SI) joint(s). Chronic inflammation of the SI joint can lead to fibrosis and ossification. This chronic inflammation can arise from rheumatic diseases, pregnancy, infection, trauma, or overuse.

Bilateral sacroiliitis is usually associated with ankylosing spondylitis, prevalent in 90% of cases. Unilateral cases are more associated with reactive arthritis, psoriatic arthrosis, and infection.

Classic history and presentation: A classic history involves a young man experiencing atraumatic lower back pain (below L5), pain in the buttocks, and/or pain radiating down the thighs. Pain may be associated with numbness, tingling, or weakness and may be worse with sitting, lying down, or climbing stairs.

Risk factors:
  • HLA-B27 positivity (ankylosing spondylitis)
  • Reactive arthritis
  • Pregnancy
  • Usually seen in adolescent to middle-aged individuals
  • More predominant in male individuals
Pathophysiology: The SI joints connect the iliac crests to the sacrum and help absorb shock from the upper and lower body. Because the joint capsule is relatively thin, fluid such as a joint effusion or pus can leak into surrounding areas to cause pain. The distribution of pain can vary if nerve roots are affected, from L2-S3.


M46.1 – Sacroiliitis, not elsewhere classified

55146009 – Inflammation of sacroiliac joint

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Differential Diagnosis & Pitfalls

Bilateral, symmetrical:
  • Ankylosing spondylitis
  • Inflammatory bowel disease (see Crohn disease and Ulcerative colitis)
  • Rheumatoid arthritis
Bilateral, asymmetrical:
  • Gout
  • Psoriatic arthritis
  • Reactive arthritis
  • Osteoarthritis
  • Infection due to Osteomyelitis or Cutaneous tuberculosis
  • Neoplasm
  • Primary hyperparathyroidism is not a true sacroiliitis but can mimic its pain presentation, often presenting as chronic, months-long low back pain. A combination of laboratory tests and CT imaging can help differentiate the diagnosis.
  • Vertebral or Sacral fracture – Caused by trauma.
  • Spondylolisthesis – Seen on lateral x-ray. The patient may have radicular pain.
  • Spondylolysis – More common in children and adolescents. Activity related and more painful with hyperextension. Radiolucent gap in the pars interarticularis is seen on x-ray with surrounding sclerosis.
  • Paget disease of bone – Increased serum alkaline phosphatase, normal calcium.
Nonorthopedic causes of low back pain:
  • Myofascial pain
  • Fibromyalgia
  • Complex regional pain syndrome
  • Pelvic organ disease (Ovarian cysts, Ovarian torsion, Endometritis, Acute prostatitis, Cutaneous endometriosis, Pelvic inflammatory disease)
  • Renal / genitourinary disease
  • Gastrointestinal disease
  • Intraabdominal abscess
  • Vascular claudication

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Last Reviewed:08/23/2023
Last Updated:11/28/2023
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