When low back pain sits low and to one side, and imaging keeps coming back unremarkable, the sacroiliac joint is one of the structures worth testing directly. Up to 25% of low back pain may arise from the sacroiliac joint, yet it is easy to miss because the joint barely moves and its pain refers in patterns that imitate sciatica.1 The way it is confirmed is not a scan. It is an image-guided injection that anesthetizes the joint and shows whether the pain it is supposed to be producing actually goes away.
Why the sacroiliac joint is easy to overlook
The sacroiliac joints transfer load from the upper body to the lower limbs, and they are held by strong ligaments and muscles. Their motion is minimal, limited to roughly 2 mm to 4 mm in any direction.1 A structure that moves that little does not produce dramatic findings on a film, which is one reason a patient can have significant pain alongside imaging that shows no obvious target.
The lifetime prevalence of low back pain is about 85%, and in roughly 25% of those patients the sacroiliac joint may be the cause.1 That is a large group of people whose pain is attributed to the lumbar spine by default.
The referral patterns are the reason it gets called sciatica
Patients with isolated sacroiliac joint dysfunction often localize pain just inferior and medial to the posterior superior iliac spine — pointing one finger at that spot is the Fortin finger test.1 But localized pain is not reliable on its own. A 2000 study reported 18 different pain referral patterns from the sacroiliac joint, including pain down the posterior or lateral thigh in 50%, pain distal to the knee in 28%, and pain in the foot in 14%.1
Pain that travels below the knee is widely assumed to be a compressed nerve root. It is not always. That assumption is how a sacroiliac problem ends up being worked up as sciatica from a disc or the piriformis and treated at the wrong level.
What the examination can and cannot establish
A thorough history and physical examination comes first, and it has to consider alternative sources that refer pain to the same region — the thoracolumbar spine, the pelvic organs, and the hip.1 Range of motion, tenderness, and a neurological examination covering sensory, motor and reflex testing all narrow the field.
Provocative maneuvers are part of that examination. In the sacral thrust test, the patient lies prone and a downward force is applied with the heel of the examiner’s hand centrally over the sacrum; pain in the sacroiliac region suggests dysfunction. No single maneuver settles it — the presence of three or more positive provocative tests increases diagnostic accuracy.1
Imaging rules things out more than it rules things in
Imaging studies mainly help exclude other sources. Plain radiographs identify hip pathology and structural change at the sacroiliac joint. MRI has about 90% sensitivity for identifying spondyloarthritis, but it is not valuable for non-inflammatory conditions, and radionuclide imaging is not a useful tool for identifying sacroiliac joint dysfunction.1
That gap between what a scan shows and what is generating the pain is not unique to this joint. It is the same problem described in what imaging can and cannot show about pain and in deep low back pain with a normal MRI.
Why the injection is the diagnostic test
Image-guided injections are the gold standard for identifying sacroiliac joint dysfunction. The test documents two things: provocation of the patient’s familiar symptoms as the joint is inflated, and relief following infiltration of local anesthetic.1 Both halves matter. Relief alone, without reproduction of the concordant pain, is weaker information.
Image modalities in decreasing order of efficacy are CT-guided, fluoroscopy-guided, and ultrasound-guided.1 The needle is directed toward the inferior portion of the joint, and the joint is inflated with only 1 to 2 mL of injectate — this is a small, tight space, not a cavity that tolerates volume.1
What a positive result actually buys you
A positive diagnostic response does not mean the joint is the only thing wrong. It means one contributing pain generator has been identified and confirmed, so subsequent treatment can be aimed rather than guessed. In a patient who has already cycled through treatments aimed at the lumbar spine, that distinction is the whole point of the test.
Where a diagnostic block is used to confirm a target before a longer-acting treatment, the logic is the same one used for facet joint pain and medial branch blocks. You can review how these fit together across the full range of pain treatments.
Risks worth asking about
Injections carry a risk of introducing infection into the joint and of bleeding; strict asepsis and fine needles reduce those risks.1 During image guidance, targeting the inferior portion of the joint is essential. Radiofrequency denervation may cause numbness, and wrongly placed electrodes can damage sacral nerves, causing incontinence, further pain, or limb weakness.1
These are not reasons to avoid evaluation. They are the reasons image guidance and technique are not optional details, and they are fair questions to raise before consenting to anything.
Frequently asked questions
How do doctors confirm the sacroiliac joint is the problem when my MRI is normal?
By anesthetizing the joint under image guidance and observing whether the familiar pain is provoked on inflation and relieved by local anesthetic — a test with a result, rather than an inference from a picture. Why scans fall short here is covered in what imaging can and cannot show about pain.
My pain goes down my leg. Does that mean it is sciatica?
Not necessarily. Sacroiliac joint pain has been reported to refer down the posterior or lateral thigh in 50% of cases, below the knee in 28%, and into the foot in 14%, so leg pain alone does not identify a nerve root. The alternatives are compared in sciatica: disc, piriformis and cluneal nerve causes.
Is one positive provocative test enough to make the diagnosis?
No. Individual maneuvers are suggestive rather than conclusive, and diagnostic accuracy improves when three or more provocative tests are positive. That examination is part of a first interventional visit, described on the pain management doctors in St. Louis page.
What are the risks of a sacroiliac joint injection?
Principally infection and bleeding, which strict asepsis and fine needles reduce. If radiofrequency denervation is later considered, numbness is possible and incorrectly placed electrodes can injure sacral nerves. Ask how image guidance is used, and see the range of image-guided treatments.
Do you see patients from Illinois for this?
Yes. The practice serves the St. Louis region across Missouri and Illinois, and both offices are listed on the locations page.
To discuss whether the sacroiliac joint is contributing to your own pain, request an appointment, call (314) 481-5000, or text (314) 886-5902.
Sources
- Raj MA, Ampat G, Varacallo MA. Sacroiliac Joint Pain. StatPearls. StatPearls Publishing; last updated August 14, 2023. NCBI Bookshelf NBK470299
Dr. Gurpreet Singh Padda, MD, MBA, MHP
