The sacroiliac joint is where the spine meets the pelvis. It causes a large share of low back pain, and it is often overlooked.
What causes sacroiliac joint pain?
It is a large, strong joint that barely moves. Thick ligaments hold it steady.
It is often hurt by a fall onto the buttock, by a car crash, in pregnancy, and after lumbar fusion (surgery joining low back bones).
PRP and the other orthobiologics we use for the low back and pelvis are explained on our PRP injections in St. Louis page.
What does SI joint pain feel like?
- Pain just below and to one side of the belt line. People often point to it with one finger.
- Pain into the buttock and back of the thigh, usually not past the knee.
- Worse when standing up from sitting, on stairs, and when lying on that side.
How is SI joint pain treated?
- An exam using a group of provocation tests (moves that stress the joint), not any single one.
- Image-guided sacroiliac joint injection. It both finds the problem and treats it.
- Physical therapy for pelvic and hip stability.
To be assessed, call us or request an appointment. If this began with a crash or a fall, see accident and injury care.
Why is SI joint pain so hard to diagnose?
No single exam test is reliable for the sacroiliac joint. That is why it is missed so often. One provocation test alone does barely better than a coin flip. Three or more positive tests together are meaningfully predictive. A single positive one does not.
The other common miss is after lumbar fusion. A fused segment does not move, so the load shifts to the joints below it. New one-sided low back pain following a fusion comes from the sacroiliac joint far more often than from a failure of the fusion itself.
Does SI joint pain go away?
Sacroiliac pain after an injury, such as a fall onto the buttock or a car crash, often improves over months with targeted injection and work to steady the pelvis.
Degenerative (breaking-down) and post-fusion sacroiliac pain acts more like facet pain. It lingers. It responds well to image-guided injection. And where injections give good but short-lived relief, radiofrequency ablation of the lateral branches (small nerves to the joint) becomes the durable option.
When this becomes urgent
Sacroiliac pain with fever needs urgent assessment, because infection in this joint is uncommon but serious. Inflammatory signs point toward inflammatory arthritis: morning stiffness lasting over an hour, pain that improves with exercise and worsens with rest, and onset before forty. Those call for a rheumatologist’s input.
Common questions
Why was this missed before?
It sends pain in a pattern that looks like disc or hip pain. And it cannot be diagnosed from an MRI. Without image-guided injection, it is easy to blame the pain on something else.
Sacroiliac Joint Injection explains the image-guided injection that identifies it.
Where is SI joint pain felt?
Just below the belt line and off to one side, often in a spot you can point to with one finger. It can spread into the buttock and the back of the thigh, usually not past the knee. It tends to be worse when you stand up from sitting, climb stairs, or lie on that side.
Can SI joint pain start after a lumbar fusion?
Yes, and it is one of the most common misses. A fused segment no longer moves, so the load shifts to the joints below it. New one-sided low back pain after a fusion comes from the sacroiliac joint far more often than from a failed fusion. It responds well to image-guided injection. Radiofrequency ablation is the durable option when injections help but wear off.
What does a sacroiliac joint injection do?
It does two jobs at once. Placed under image guidance, it confirms whether the sacroiliac joint is really the source of the pain. An MRI cannot do that. And it treats the joint at the same time. When the injection gives good but short-lived relief, radiofrequency ablation of the lateral branch nerves becomes the longer-lasting option.
When does SI joint pain need urgent attention?
Sacroiliac pain with a fever needs a prompt check, because infection in this joint is uncommon but serious. Some signs point toward inflammatory arthritis: morning stiffness that lasts more than an hour, pain that improves with exercise and gets worse with rest, and pain that began before age forty. Those call for a rheumatologist to weigh in.
Sources
- McCormick ZL et al. Consensus practice guidelines on sacroiliac joint complex pain from a multispecialty, international working group. Pain medicine (Malden, Mass.), 2025. PubMed 41318933
- Cohen SP et al. Sacroiliac joint pain: a comprehensive review of epidemiology, diagnosis and treatment. Expert review of neurotherapeutics, 2013. PubMed 23253394
- Szadek K et al. 5. Sacroiliac joint pain. Pain practice : the official journal of World Institute of Pain, 2024. PubMed 38155419
- Han CS et al. Low back pain of disc, sacroiliac joint, or facet joint origin: a diagnostic accuracy systematic review. EClinicalMedicine, 2023. PubMed 37096189
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