SI joint fusion is a surgical procedure that permanently connects the sacrum (the triangular bone at the base of your spine) to the ilium (the upper part of your pelvis). The goal is to eliminate painful movement at the sacroiliac joint by encouraging the two bones to grow together into one solid structure. It’s typically considered after months of conservative treatment have failed to relieve chronic lower back or buttock pain traced to this joint.
What the SI Joint Does
You have two sacroiliac joints, one on each side of your lower spine, and they act as shock absorbers between your upper body and your legs. Unlike your knee or shoulder, these joints allow only a few millimeters of movement. They’re reinforced by some of the strongest ligaments in the body. But when the joint becomes inflamed, unstable, or degenerative, even that small amount of motion can produce significant pain in the lower back, buttocks, groin, or legs.
SI joint dysfunction can result from degenerative sacroiliitis (wear-and-tear inflammation), trauma like a fall or car accident, pregnancy-related ligament loosening, or prior lumbar spine surgery that shifted extra stress onto the joint. It accounts for an estimated 15 to 30 percent of chronic lower back pain cases, though it’s frequently misdiagnosed as a lumbar disc problem because the symptoms overlap.
How Doctors Confirm the SI Joint Is the Problem
Diagnosing SI joint pain is tricky because imaging alone rarely tells the full story. The gold standard is a diagnostic injection: a doctor uses fluoroscopy or ultrasound to guide a numbing agent directly into the joint. If that injection produces at least 75 percent pain relief for the duration of the anesthetic, the SI joint is confirmed as the pain source. If the first injection doesn’t clear that 75 percent threshold, a second diagnostic block isn’t considered useful, and surgeons will look elsewhere for the cause.
This strict cutoff matters because fusion is irreversible. Fusing a joint that isn’t actually causing the pain won’t help and may create new problems. Most insurance coverage and surgical guidelines require this level of diagnostic confirmation, along with documented failure of conservative treatments like physical therapy, anti-inflammatory medications, and joint injections over a period of at least six months.
What Happens During the Procedure
Most SI joint fusions today are performed as minimally invasive surgery. The surgeon makes a small incision (typically 3 to 5 centimeters) on the side of the buttock and, using real-time X-ray or navigation guidance, places implants that bridge the sacrum and ilium. These implants are usually made of titanium with a textured or porous coating designed to encourage bone to grow into and around them. Current FDA-cleared systems include threaded implants roughly 10 to 11.5 millimeters in diameter, and newer robotic navigation platforms allow surgeons to place them with greater precision.
The procedure generally takes about an hour and is performed under general anesthesia. Many patients go home the same day, though some stay overnight. As healing progresses over the following months, new bone growth fuses the sacrum and ilium together permanently, eliminating the motion that was causing pain.
Recovery After Fusion
The first few weeks involve limited weight-bearing. Most surgeons allow you to walk with a walker or crutches right away, but ask you to avoid bending, twisting, or lifting anything heavy. You’ll likely need to limit activities for four to six weeks, with a gradual return to normal movement after that.
Physical therapy typically begins within the first month, focusing on gentle range of motion for the hip and core stabilization. Full bone fusion takes roughly three to six months, and most people notice progressive pain improvement during that window. Desk workers often return to work within two to four weeks, while those with physically demanding jobs may need three months or more before resuming full duties.
How Well It Works
Compared to ongoing conservative management (physical therapy, injections, medication), fusion produces substantially better results for patients with confirmed SI joint dysfunction. In one study published in the Journal of Spine Surgery, patients who underwent fusion saw a 41.6-point improvement in pain scores, compared to a 14-point improvement in those managed conservatively. Disability scores followed the same pattern: a 25-point improvement with surgery versus 8.7 points without it.
A double-blind, placebo-controlled trial published in The Lancet’s eClinicalMedicine compared minimally invasive fusion to a sham (fake) surgery. At six months, the surgical group had a 2.6-point reduction on a 10-point pain scale, versus 1.7 points in the sham group. That difference is meaningful but also shows that some of the improvement people feel after any procedure comes from the placebo effect. Still, the surgical group had a clear and statistically significant advantage, and many patients continue to improve beyond six months as the fusion solidifies.
Risks and Complications
SI joint fusion is considered low-risk as surgeries go, but it’s not without complications. In a study of minimally invasive fusion patients, the overall complication rate was about 13 percent at 90 days and 16 percent at six months. These complications range from temporary surgical pain and minor infections to more significant issues.
One concern specific to fusion surgery is the development of new lumbar spine problems. When the SI joint is locked in place, adjacent structures sometimes take on extra mechanical stress. In the same study, about 3.6 percent of patients developed new lumbar pathology within 90 days, rising to 5.3 percent at six months. Nerve irritation near the implant site, wound infection, and failure of the bones to fully fuse (non-union) are other possible outcomes, though individually each of these is relatively uncommon.
Who It’s Best Suited For
The ideal candidate has a clear diagnosis confirmed by at least one positive diagnostic injection, has tried and failed conservative treatment for six months or longer, and has pain that meaningfully limits daily activities. The procedure is indicated for sacroiliac joint disruption, degenerative sacroiliitis, and certain pelvic fractures. It’s also used to stabilize the SI joint in patients who’ve already undergone lumbar or thoracolumbar spinal fusion, since those longer fusions can destabilize the pelvis below.
Fusion is not a first-line treatment. For many people, physical therapy focused on core and pelvic stability, combined with periodic joint injections, provides enough relief to avoid surgery entirely. But for the subset of patients whose pain persists despite those efforts, fusion offers a durable solution with a strong track record of reducing both pain and disability.