SI joint surgery is a procedure that fuses the sacroiliac joint, the large joint connecting your lower spine to your pelvis, to eliminate pain caused by excessive or abnormal movement in that joint. The most common version today is a minimally invasive fusion performed through a small incision on the side of the buttock, using titanium implants to lock the joint in place. It’s typically reserved for people whose SI joint pain hasn’t improved after at least six months of nonsurgical treatment.
What the SI Joint Does
The sacroiliac joints are the largest joints along your spine’s axis. You have one on each side, sitting where the triangular bone at the base of your spine (the sacrum) meets the broad pelvic bones. Their primary job is transferring weight and force from your upper body down into your legs. Every time you walk, climb stairs, or lift something, these joints absorb and redirect large compression loads.
No muscles are specifically designed to move the SI joint. It’s surrounded by some of the most powerful muscles in the body, but the joint itself moves very little. When it does become a source of pain, the problem is often too much motion, too little motion, or damage to the ligaments and capsule holding the joint together. Common triggers include soft tissue injury, inflammation, pregnancy, leg length differences, and prior spinal fusion surgery. Lumbar fusion in particular has been shown to increase motion and stress across the SI joint, which is why SI joint problems sometimes develop after back surgery.
SI joint pain is one of the most overlooked causes of low back pain. It can mimic sciatica or other nerve-related conditions, making accurate diagnosis critical before any surgical decision.
How Doctors Confirm You Need Surgery
Getting to surgery involves a structured diagnostic process. Criteria used by major healthcare systems require all of the following before fusion is considered medically necessary:
- Localized pain over the SI joint with tenderness at the sacral sulcus (the dimple area near your lower back) and no similar tenderness elsewhere, like the hip or lumbar spine.
- At least three positive provocative tests during a physical exam. These are specific maneuvers where a clinician applies pressure or positions your leg in ways that stress the SI joint to reproduce your pain.
- Six months of failed nonsurgical treatment, including anti-inflammatory medications, physical therapy targeting the lower back, pelvis, and hip, activity modification, and consideration of a supportive brace.
- Imaging to rule out other causes, including X-rays, CT, or MRI of the SI joint, pelvis, and lumbar spine to exclude tumors, fractures, hip problems, or nerve compression that could explain the pain.
- Diagnostic injections confirming the SI joint as the pain source. Most guidelines require at least 75% pain relief from a numbing injection placed directly into the joint under imaging guidance. Some research suggests patients with as little as 50% relief may still benefit from fusion, but 75% remains the standard threshold.
- A therapeutic steroid injection and consideration of radiofrequency ablation (a procedure that uses heat to disrupt pain signals from the joint’s nerves) before moving to surgery.
This layered approach exists because SI joint pain overlaps with so many other conditions. An inaccurate diagnosis is one of the main reasons some patients don’t improve after surgery.
What Happens During the Procedure
The minimally invasive approach is now the standard technique. You’re placed face-down under general anesthesia, and the surgeon uses real-time imaging or 3D navigation to guide the procedure. A single incision of about 3 to 5 centimeters is made on the side of the buttock. The surgeon works through the muscle to reach the outer surface of the pelvic bone, then passes a guide pin across the SI joint and into the sacrum, confirmed by imaging at each step.
The path is then drilled and shaped, and triangular titanium implants are placed across the joint. Typically three implants are used, two positioned in the upper sacrum and one in the lower sacrum. The triangular shape is designed to grip bone and resist rotation. Once imaging confirms proper placement, the incision is closed. The entire procedure generally takes under an hour.
The older alternative is open surgery, which uses a larger incision and more tissue dissection to directly expose the joint. Open approaches involve more blood loss and longer recovery, so they’re now reserved for complex cases like traumatic instability or situations where minimally invasive placement isn’t feasible.
Recovery Timeline
Most people begin walking the same day as surgery or the next morning, often with the help of a walker or cane to limit pressure on the surgical site. The goal in the first few weeks is controlled, gradual movement rather than strict bed rest.
Physical therapy typically follows to rebuild strength in the muscles around the pelvis and lower back. Most people can return to all of their usual physical activities within three to four months. The bone itself takes longer to fully fuse. Studies tracking fusion on imaging show about 62% of joints are fused at six months, rising to 96% at one year and 100% by 18 months.
Success Rates and Patient Satisfaction
A multicenter study tracking patients one year after minimally invasive fusion found that pain scores improved by an average of 6.1 points on a 10-point scale. About 90% of patients achieved what researchers define as substantial clinical benefit, meaning their pain either dropped by more than 2.5 points or fell to 3.5 or below.
Satisfaction rates were high: roughly 91% of patients said they were very satisfied, and another 14% were somewhat satisfied. When asked whether they would have the same surgery again, 92% said yes.
Risks and Reoperation
Minimally invasive SI joint fusion carries a relatively low complication profile compared to open spinal procedures, but it’s not risk-free. The main concerns include implant loosening, malposition of the hardware, nerve irritation, and infection. Reoperation rates in the published literature range from 0% to 17% depending on the study. One study using navigation-guided placement reported a reoperation rate of about 5%, with both cases involving a loose screw that needed replacement or reinforcement.
Implant malposition is a preventable factor that correlates with poor outcomes. The use of 3D navigation during surgery has improved placement accuracy, which is one reason the technique continues to evolve. Patients who don’t improve after fusion tend to fall into a few categories: the implant was placed incorrectly, they had an unrecognized lumbar spine problem contributing to their pain, or the original diagnosis was inaccurate.
Nonsurgical Alternatives
Before fusion is on the table, a full course of conservative treatment is expected. Anti-inflammatory medications and targeted physical therapy focused on core stabilization form the foundation. SI joint belts can provide external compression to reduce abnormal motion. Steroid injections into the joint offer temporary relief lasting weeks to months. Radiofrequency ablation, which heats the small nerves supplying the joint to interrupt pain signals, can provide longer-lasting relief and is often the last step tried before surgery. For some patients, these approaches manage the pain well enough that surgery is never needed.