Sacroiliitis
Sacroiliitis is inflammation of one or both sacroiliac joints, the strong joints that connect the lower spine (sacrum) to the pelvis. These joints play a critical role in transferring weight between the upper body and the legs. Inflammation of the sacroiliac joints can cause lower back, buttock, hip, or thigh pain and may be associated with inflammatory arthritis, injury, pregnancy-related changes, infection, or age-related degeneration. MRI is particularly valuable because it can detect sacroiliac joint inflammation before permanent damage becomes visible on X-rays.
What is it?
The sacroiliac joints are two large joints located where the triangular bone at the base of the spine (the sacrum) connects with the iliac bones of the pelvis. Although these joints move only slightly, they play an important role in distributing forces between the spine, pelvis, and legs during standing, walking, and lifting.
Sacroiliitis occurs when one or both of these joints become inflamed. The inflammation may be temporary or chronic depending on the underlying cause.
One of the most important causes of sacroiliitis is inflammatory arthritis, particularly the group of conditions known as spondyloarthritis. These include ankylosing spondylitis, psoriatic arthritis, reactive arthritis, and inflammatory bowel disease–associated arthritis.
Sacroiliac joint inflammation can also result from mechanical stress, degenerative arthritis, prior injury, leg-length differences, pregnancy-related ligament changes, or previous spinal surgery.
Less commonly, the joint may become infected, a condition known as septic sacroiliitis, which requires prompt medical treatment.
Symptoms typically include pain in the lower back, buttocks, hips, groin, or upper thighs. The pain may affect one side, alternate sides, or involve both joints.
Inflammatory sacroiliitis often produces morning stiffness lasting more than 30 minutes and pain that improves with activity while worsening during rest. Some patients notice pain that wakes them during the second half of the night.
Mechanical or degenerative sacroiliac pain is usually more activity-related and may worsen with prolonged standing, walking, climbing stairs, or transitions between sitting and standing.
Pain can sometimes mimic sciatica, although sacroiliac joint pain typically remains above the knee and follows a different pattern than true nerve-root compression.
MRI of the sacroiliac joints is the preferred imaging test for detecting inflammatory sacroiliitis because it can identify active inflammation, including bone marrow edema, before structural damage develops.
Pelvic X-rays are useful for identifying more advanced changes such as joint erosion, sclerosis, narrowing, or fusion.
CT scanning may be used in selected cases when detailed evaluation of the joint surfaces and bony anatomy is needed.
When inflammatory disease is suspected, blood tests such as HLA-B27 and inflammatory markers may be performed alongside imaging and rheumatologic evaluation.
Important to Know
Treatment depends on the underlying cause of sacroiliitis.
Mechanical and degenerative sacroiliac joint pain is often managed with physical therapy, strengthening exercises for the core and hips, activity modification, ergonomic improvements, and anti-inflammatory medications.
Image-guided sacroiliac joint injections may help both confirm the diagnosis and provide significant pain relief.
When sacroiliitis is part of an inflammatory condition such as ankylosing spondylitis, treatment is typically coordinated by a rheumatologist and may include disease-modifying medications and biologic therapies designed to control inflammation and prevent long-term joint damage.
Septic sacroiliitis is uncommon but requires urgent treatment with antibiotics and sometimes drainage procedures.
Sacroiliac joint fusion surgery is rarely necessary and is generally reserved for carefully selected patients with persistent severe symptoms that do not respond to conservative treatment.
Because sacroiliitis may be the earliest sign of a broader inflammatory disorder, early detection can lead to earlier diagnosis and treatment, helping prevent long-term complications and preserve mobility.
Red flag symptoms include severe pain accompanied by fever, rapidly worsening symptoms, severe pain following trauma, progressive weakness or numbness, difficulty walking, loss of bowel or bladder control, or unexplained weight loss. These symptoms require prompt medical evaluation.