SACROILIAC JOINT INJECTION
Your MRI keeps aiming at the spine. Some of that pain lives an inch to the side of it, in a joint nobody tested.
The sacroiliac joint connects the spine to the pelvis, sits deep behind muscle and bone, and cannot be diagnosed by exam or imaging alone. An image-guided injection is how we find out whether it is the source.
Why it keeps getting blamed on the spine instead
Sacroiliac joint pain sits in the same neighborhood as lumbar pain and can refer into the buttock and the back of the thigh in a pattern that overlaps disc and facet referral almost exactly. An MRI ordered for “low back pain” is framed to look at the spine, not the joint just beside it, so a normal-looking disc gets treated first while the actual generator sits one structure over, untested. No physical exam maneuver reliably isolates this joint on its own either — it sits deep under the gluteal muscles and the posterior ligaments, which is exactly why imaging guidance is not optional here the way it might be for a more superficial target.
Who this actually shows up in
POSTPARTUM
Pregnancy hormones loosen the ligaments stabilizing the pelvis to allow delivery, and that laxity does not reverse itself on the day the baby is born. A joint that was asked to move more than it was built to move during pregnancy and delivery can stay irritated for months afterward, and it is routinely mistaken for ordinary postpartum back strain rather than tested as its own diagnosis.
POST-FUSION
A lumbar fusion locks the treated segment in place, and the joints above and below it — the sacroiliac joint included, when the fusion runs down to the sacrum or close to it — absorb motion they were not previously asked to carry. New pain after a fusion that “worked” on imaging is often this joint compensating, not a failure of the hardware.
Diagnosis and treatment from the same needle
Unlike a medial branch block, which tests a nerve without treating anything, this injection goes directly into the joint capsule with steroid and anesthetic together. Immediate relief that matches the anesthetic’s known duration confirms the joint as the source — the diagnostic half of the visit — and the steroid component can then provide weeks to months of relief on its own, doing therapeutic work in the same appointment. That dual role is part of why this is usually the first step rather than a separate diagnostic block followed by a separate treatment.
What comes next if it keeps confirming the joint
A joint that responds well but repeatedly needs re-treatment is a candidate for lateral branch radiofrequency ablation, the longer-lasting option aimed at the nerves supplying this joint rather than the joint itself. Cooled radiofrequency technique is generally preferred here because the lateral branches supplying the SI joint run a less predictable course than the lumbar medial branches, and a larger, more forgiving lesion covers that variability better.
A joint that will not settle is also worth reading in the context of what is trying to heal it. Ligament and cartilage repair depend on collagen synthesis, and both insulin resistance and systemic metabolic inflammation degrade collagen quality and slow that repair — two biological drags on the same joint that already has a mechanical problem. Add a caregiver’s schedule that does not allow rest — lifting a car seat, carrying a toddler on one hip, standing through a shift with no real break — and the joint never gets the loading holiday it needs to settle, which is as much of the reason it stays inflamed as the original ligamentous laxity.
The evidence, and where it is thinner than we would like
Systematic review of sacroiliac joint interventions finds this joint genuinely difficult to diagnose with certainty — prevalence estimates among patients with low back pain vary considerably across studies, in part because the reference standards used to confirm the joint as the source are themselves imperfect. That is not a reason to skip testing it; it is a reason to test it with image guidance and a real anesthetic-response protocol rather than treating on suspicion, and to expect that a meaningful share of patients tested will turn out negative — which is useful information, not a wasted visit. For lateral branch radiofrequency specifically, a placebo-controlled trial and a more recent multicenter comparative study both found meaningful, durable benefit over standard medical management in patients selected this way.
What happens on the day
- Pain map recorded, including the specific movements — standing from sitting, rolling in bed — that provoke it.
- Positioned face down, skin cleaned and numbed.
- Needle guided under fluoroscopy into the joint, with contrast confirming an intra-articular arthrogram pattern before anything else is injected.
- Medication injected into the joint, then the needle comes out.
- A short recovery period, and someone else drives you home. Over the rest of that day, the numbing effect tells you whether the joint was the right target; over the following two weeks, watch for the steroid to take hold.
Common questions
How do you know it is the SI joint and not my spine?
The exam pattern — pain over the joint itself, reproduced with specific pelvic maneuvers — raises suspicion, but the injection is what confirms it. A concordant response to anesthetic placed in the joint is stronger evidence than any single exam finding, including the pattern seen in hip pain that can mimic this closely.
Can this be done without imaging guidance?
Not reliably. The joint, discussed generally on the sacroiliac joint pain page, is irregularly shaped and sits deep enough that a blind injection frequently misses the capsule entirely, which means an unguided attempt risks both a false negative and no real treatment at all.
How long does relief last?
Highly variable — weeks to several months. A joint that responds but keeps needing repeat injections is the profile that moves toward lateral branch ablation for something more durable.
Is this common after a lumbar fusion?
Common enough that it should be on the differential whenever new pain appears after a fusion that otherwise looks solid on imaging — particularly relevant reading if you are working through failed back surgery syndrome.
I just had a baby and my back pain will not resolve. Could this be it?
It is common enough postpartum that it deserves a direct test rather than being assumed away as normal recovery, especially when the caregiving schedule described on the sleep, movement and behavior page never allows the joint any real rest. Bring the specific location and the movements that trigger it to the visit.
Related reading
- Sacroiliac joint pain
- Radiofrequency ablation
- Failed back surgery syndrome
- Hip pain
- When injections stop working
We will test the joint next to your spine, not just the one on the film
If your low back and pelvic pain has never been evaluated for this joint specifically, that is worth a direct conversation before another round of spine-focused imaging.
4479 Woodson Rd, Suite 401
St. Louis, MO 63134
Next to St. Louis Lambert International Airport, off I-70 at Woodson Road.
Sources
- Simopoulos TT et al. A systematic evaluation of prevalence and diagnostic accuracy of sacroiliac joint interventions. Pain Physician, 2012. PubMed 22622915
- Cohen SP et al. Randomized placebo-controlled study evaluating lateral branch radiofrequency denervation for sacroiliac joint pain. Anesthesiology, 2008. PubMed 18648237
- Cohen SP et al. Cooled radiofrequency ablation versus standard medical management for chronic sacroiliac joint pain: a multicenter, randomized comparative effectiveness study. Regional Anesthesia & Pain Medicine, 2024. PubMed 37407279