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Spine

Sacroiliitis

Inflammation of the joint between the sacrum and pelvis, producing one-sided low back and buttock pain.

What sacroiliitis is

The sacroiliac (SI) joints transfer load between the spine and the pelvis. When one becomes inflamed or moves abnormally, it produces pain that patients usually point to just below the belt line, off to one side.

Sacroiliitis is a common cause of pain after pregnancy, after lumbar fusion surgery, and following falls or motor vehicle collisions. It is also one of the most frequently missed sources of low back pain, because it is often treated as a disc problem.

A carefully performed diagnostic injection into the joint under fluoroscopy remains the most reliable way to confirm the SI joint as the source.

Common symptoms

  • One-sided pain below the beltline, over the dimple of the pelvis
  • Pain with rising from a chair, climbing stairs or standing on one leg
  • Referred pain into the buttock, groin or back of the thigh
  • Discomfort sitting on the affected side

Common causes

  • Pregnancy-related ligament loosening that alters joint mechanics
  • Lumbar fusion surgery, which shifts extra motion onto the SI joints
  • A fall onto the buttocks or a twisting injury to the pelvis
  • Leg length differences or altered gait that load one joint more than the other
  • Inflammatory arthritis conditions affecting the joint on both sides

How it is diagnosed

Because the SI joint sits deep in the pelvis, no single physical exam maneuver is perfect on its own. Your provider typically performs a cluster of provocative tests, such as pressing on the joint and stressing the pelvis in specific directions, looking for a consistent pattern that reproduces your usual pain.

Imaging with X-ray or MRI can identify inflammatory changes or confirm the joint is structurally normal, but many patients with clear SI joint pain have unremarkable imaging.

As with the facet joints, a fluoroscopically guided diagnostic injection directly into the joint is the most dependable way to confirm it as the pain source before moving to a longer-lasting treatment.

When to see a specialist

  • One-sided low back or buttock pain that has lasted more than a few weeks
  • Pain clearly worse with standing on one leg, climbing stairs, or getting out of a car
  • Pain that began after childbirth, a fall, or spine surgery
  • Difficulty finding a comfortable sitting position
  • Fever, unexplained weight loss or pain that wakes you at night, which warrant urgent evaluation rather than routine follow-up

Common questions

How is sacroiliitis different from a herniated disc?
Disc herniations usually cause pain that radiates in a line down the leg with numbness or tingling in specific toes. Sacroiliitis produces a duller, more localized ache just below the beltline on one side without that same nerve pattern.
Can the SI joint be seen clearly on MRI?
MRI can show inflammation or structural change in some cases, but a normal MRI does not rule out SI joint pain, which is why a diagnostic injection is often used to confirm the diagnosis.
Is sacroiliitis common after a fusion surgery?
Yes, it is a well-recognized cause of new pain after lumbar fusion, because the fused segment transfers more motion and stress to the SI joints below it.
Does sacroiliitis affect both sides at once?
It is usually one-sided when related to mechanical causes such as injury or prior surgery, though inflammatory arthritis conditions can involve both joints.

Related conditions

Ready to get a real diagnosis?

Most patients are seen within two weeks. Call our office or send a request and our staff will help you choose the most convenient location.

Call 540-444-5670