The sacroiliac (SI) joint sits where the base of the spine meets the pelvis. When it becomes inflamed or unstable, it produces a distinctive one-sided ache deep in the buttock or low back that often worsens with prolonged standing, climbing stairs, or rolling over in bed. SI joint pain is frequently mistaken for sciatica because the discomfort can spread into the back of the thigh — but the source is the joint itself, not a pinched nerve.
An SI joint injection places a small dose of local anaesthetic mixed with a long-acting corticosteroid directly inside the joint capsule, under live X-ray (fluoroscopy) guidance. The fluoroscopy step is non-negotiable here — the SI joint is deep, narrow, and easy to miss without contrast confirmation that the needle is truly inside the joint space.
The injection serves two purposes at once. The anaesthetic acts as a diagnostic confirmation: if your usual buttock pain quiets within minutes of the injection, we know the SI joint is the pain generator. The steroid then takes a few days to start working and calms the inflammation that’s driving your symptoms — typically buying you 6 to 12 weeks of meaningful relief, sometimes longer.
Most patients combine the injection with targeted physiotherapy (core and hip-stabilizer strengthening). When the injection consistently helps but the relief is short-lived, radiofrequency ablation of the lateral branch nerves that supply the joint is the natural next step.
What this treats
- sacroiliac joint dysfunction
- sacroiliitis
- SI joint pain
- ankylosing-spondylitis-related SI pain
Conditions this treats
Common diagnoses we use this procedure for. Tap one to read more.
Before, during, and after — what to do
Before your appointment
Eat a normal meal before your appointment — there is no fasting required for most injections. Take your usual medications unless we have specifically asked you not to. If you take a blood thinner (aspirin, warfarin, clopidogrel, apixaban, rivaroxaban, etc.), tell us in advance — we may need to adjust the timing. Bring your OHIP card and any imaging reports we asked for.
On the day
Wear loose, comfortable clothing — easy to roll up sleeves or pant legs, easy to lie face-down on the table. Plan to be at the clinic about 45 minutes total. You can drive yourself unless we told you otherwise. If you are anxious about the procedure, tell us — we will walk you through every step.
After the procedure
You may feel some soreness at the injection site for 1–3 days. Ice helps; so does Tylenol or ibuprofen if you can take it. Avoid heavy lifting or vigorous exercise for 24 hours, then resume normal activity as comfort allows. The therapeutic effect of the injection usually starts within a few days and reaches full strength by 2 weeks.
When to call us: Call us right away — or go to the nearest emergency department — if you develop a high fever, severe new pain, redness or swelling at the injection site, weakness in a limb, or loss of bladder or bowel control. These are rare but worth knowing.
What happens on the day
We use live fluoroscopy to see exactly where the needle is going. The procedure itself takes about 25 minutes, plus a little time to get you positioned and a short rest afterwards. Most people feel relief lasting 6–12 weeks.