SI joint pain originates in the sacroiliac joints, the two joints where your lower spine meets your pelvis. It accounts for a surprisingly large share of chronic low back pain. A systematic review in The Lancet found that among people investigated for low back pain, the SI joint was the source in roughly half of cases, making it at least as common as disc-related or facet joint pain. The causes range from everyday biomechanical stress to inflammatory disease, pregnancy, and prior surgery.
How the SI Joint Works
You have two sacroiliac joints, one on each side of the triangular bone (the sacrum) at the base of your spine. These joints don’t move much. In a healthy person, they shift less than one millimeter during normal activity. Their job is to transfer the weight of your upper body into your pelvis and legs while absorbing shock from walking and running.
A thick web of ligaments holds each SI joint in place. The iliolumbar ligaments, which run from the lowest lumbar vertebra to the pelvis, are especially important for limiting front-to-back motion. When these ligaments or the joint surfaces themselves are damaged, loosened, or inflamed, pain follows.
Too Much or Too Little Joint Movement
SI joint pain generally traces back to one of two mechanical problems: the joint moves too much or too little.
When the joint is hypermobile (too loose), surrounding ligaments can’t restrain normal motion. Pain typically settles in the lower back or hip and sometimes radiates into the groin. This pattern is common in younger women, athletes, and anyone who has experienced ligament damage from injury or hormonal changes.
When the joint is hypomobile (too stiff or “locked”), the restricted movement irritates the joint and surrounding tissue. This version of SI pain tends to show up on one side of the lower back or deep in the buttock and can send pain down the back of the thigh. It’s more common in older adults whose joints have stiffened with age or wear.
Where SI Joint Pain Shows Up
SI joint pain has a distinctive pattern that helps distinguish it from other causes of back pain. It usually centers over the buttock, close to the bony bump you can feel at the back of your pelvis (the posterior superior iliac spine). Many people describe it as a sharp, stabbing, or shooting sensation that travels down the back of the thigh but typically stops before reaching the knee. That cutoff point is one way clinicians differentiate SI pain from sciatica, which often extends all the way to the foot.
Pregnancy and Hormonal Changes
Pregnancy is one of the most common triggers for SI joint pain. During pregnancy, the body releases a hormone called relaxin that loosens the muscles, joints, and ligaments around the pelvis. This helps the birth canal widen for delivery, but it also destabilizes the SI joints. The combination of loosened ligaments, weight gain, and a shifting center of gravity puts substantial stress on these joints, especially in the third trimester.
Recovery isn’t instant. Relaxin levels drop after delivery but remain elevated for months. Some sources estimate it can take up to 12 months for relaxin to return to pre-pregnancy levels, which is why many new mothers continue to experience pelvic and SI joint pain well after giving birth.
Inflammatory Conditions
Inflammation of the SI joint, called sacroiliitis, is a hallmark of a group of conditions known as spondyloarthritis. The most well-known form is ankylosing spondylitis, a chronic inflammatory disease that primarily affects the spine and SI joints. But sacroiliitis also appears in psoriatic arthritis, reactive arthritis, and arthritis linked to inflammatory bowel diseases like Crohn’s disease and ulcerative colitis.
Inflammatory SI joint pain behaves differently from mechanical pain. It tends to be worst in the morning or after long periods of rest, improves with movement, and often affects both sides. In early stages, standard X-rays may look normal. MRI can reveal active inflammation, fatty lesions, and erosions in the joint before visible structural damage appears on plain films. This distinction matters because early detection allows treatment that can slow disease progression.
Leg Length Differences and Gait Problems
When one leg is shorter than the other, even slightly, the pelvis tilts with every step. Over time, this asymmetry can strain the SI joint on one or both sides. The question of how large the difference needs to be before it causes symptoms doesn’t have a clean answer. Some research places the threshold at 5 millimeters, others at 11 or even 30 millimeters. In practice, the likelihood of developing pain depends less on the exact measurement and more on how much time you spend on your feet and how vigorous your activity is. A 5-millimeter discrepancy in a distance runner may cause more trouble than a 20-millimeter difference in someone with a sedentary lifestyle.
Other gait-related problems can produce similar effects. Altered walking patterns from hip or knee arthritis, foot injuries, or muscle weakness can shift load unevenly through the pelvis and stress the SI joints over time.
Spinal Fusion and Prior Surgery
People who have had lumbar spinal fusion surgery face an elevated risk of developing SI joint pain afterward. The reason is mechanical: fusing vertebrae together eliminates motion at that level of the spine, which forces the joints above and below the fusion to compensate. The SI joints sit directly below the lumbar spine and absorb much of this transferred force.
According to Mayo Clinic spine specialists, SI joints that normally move less than a millimeter can begin moving a couple of millimeters after fusion surgery. That small increase in motion, sustained over months and years, can lead to premature degenerative changes and chronic pain. This is sometimes grouped under “adjacent segment disease,” a broader term for problems that develop near a surgical fusion site.
Other Common Triggers
Falls and direct trauma to the pelvis can damage the SI joint’s cartilage or ligaments. A hard landing on one side, a car accident, or a sports collision can all initiate pain that persists long after the initial injury heals. Osteoarthritis also affects the SI joint, particularly in older adults. The cartilage that cushions the joint wears down over time, leading to stiffness, inflammation, and bone-on-bone contact.
Repetitive stress from activities that involve asymmetric loading (running, golf, or jobs that require standing on one leg or twisting) can gradually irritate the joint. Obesity increases load on the SI joints with every step, accelerating wear.
How SI Joint Pain Is Diagnosed
Diagnosing SI joint pain is tricky because it mimics other causes of low back pain. Clinicians typically use a combination of provocative physical tests that stress the SI joint in specific ways to reproduce your pain. Research shows these tests have positive predictive values above 80 percent, meaning that when a test is positive, it’s quite likely the SI joint is the source. The thigh thrust test is the most sensitive of the common provocative tests (correctly identifying about 74 percent of true cases), while the FABER test, which involves flexing and rotating the hip, is the most specific (best at ruling out other causes).
No single test is definitive, so most clinicians require at least three positive provocative tests before attributing pain to the SI joint. In uncertain cases, a diagnostic injection of numbing medication directly into the joint can confirm the diagnosis. If the injection temporarily eliminates the pain, the SI joint is the source.
Treatment Timelines and What to Expect
Conservative treatment is always the starting point. This typically includes physical therapy focused on stabilizing the pelvis, therapeutic exercise to strengthen the muscles around the joint, and over-the-counter pain relief. Current guidelines require at least four weeks of conservative care before any procedures are considered, and most clinicians recommend sticking with this approach for 12 weeks or more.
If conservative treatment fails, the next step is usually a corticosteroid injection into the joint to reduce inflammation. For people whose pain persists beyond six months despite physical therapy, exercise, and injections, surgical fusion of the SI joint becomes an option. The threshold for surgery is deliberately high: guidelines from the International Society for the Advancement of Spine Surgery specify at least six months of pain that affects quality of life and failure of all nonsurgical treatments before fusion is appropriate.