Sacroiliac (SI) joint fusion is a surgical procedure designed to treat chronic low back pain caused by dysfunction or instability of the sacroiliac joint—the joint where the sacrum (the triangular bone at the base of the spine) meets the ilium (the pelvic bone). The SI joint normally allows very limited movement, but when it becomes inflamed, unstable, or degenerative, even small amounts of motion can generate significant pain that radiates into the lower back, buttock, groin, or thigh. SI joint fusion is typically considered only after conservative treatments such as physical therapy, medications, activity modification, bracing, and image-guided injections have failed to provide lasting relief. Before surgery, patients undergo a detailed diagnostic process to confirm the SI joint as the primary pain source. This often includes physical exam maneuvers that stress the joint, imaging studies (X-rays, CT, or MRI), and diagnostic SI joint injections. A strong response to anesthetic injections is an important indicator that fusion may be beneficial. Proper patient selection is critical, as SI joint pain can mimic other spine or hip conditions. Most modern SI joint fusions are performed using a minimally invasive approach. The procedure is typically done under general anesthesia and often on an outpatient basis. The patient is positioned face down, and the surgeon uses real-time imaging guidance (fluoroscopy or navigation) to precisely access the SI joint through a small incision, usually only a few centimeters long. Damaged cartilage within the joint is removed, and specialized implants—commonly titanium screws, rods, or triangular devices—are placed across the joint from the ilium into the sacrum. These implants stabilize the joint immediately and serve as a scaffold for bone growth, allowing the two bones to gradually fuse into a single solid structure over time. Bone graft material may also be used to encourage fusion. The primary goal of SI joint fusion is to eliminate painful motion by permanently stabilizing the joint. Unlike spine fusion, which can significantly limit motion, SI joint fusion generally does not noticeably reduce functional mobility because the joint normally moves very little. Most patients begin walking shortly after surgery, often the same day or within a few days, sometimes with the assistance of a walker or cane. Early recovery focuses on protecting the fusion site while maintaining gentle mobility. Patients are typically advised to avoid heavy lifting, twisting, prolonged sitting, and high-impact activities during the early healing phase. Recovery timelines vary, but many patients return to light daily activities within 2 to 6 weeks. Physical therapy may be recommended to restore strength, improve posture, and reduce strain on surrounding structures. Full fusion and maximal pain improvement may take several months, with higher-impact activities often delayed for 3 to 6 months. While many patients experience substantial and lasting pain relief, outcomes depend on accurate diagnosis, surgical technique, and adherence to post-operative guidelines. As with any surgery, SI joint fusion carries risks, including infection, bleeding, nerve irritation, implant-related issues, or failure of the bones to fuse (nonunion). Some patients may continue to experience residual pain or develop stress on nearby spinal segments over time. However, for appropriately selected patients with confirmed SI joint–mediated pain, minimally invasive SI joint fusion has demonstrated high success rates and meaningful improvements in function and quality of life. Overall, SI joint fusion is considered a definitive treatment for severe, persistent SI joint dysfunction when conservative care has been exhausted, offering long-term stabilization and pain relief for many patients struggling with chronic low back pain.