The sacroiliac joint, commonly called the SI joint, is the largest axial joint in your body. It sits on each side of your lower spine, connecting the bottom of the spine (the sacrum) to the large wing-shaped bones of the pelvis (the iliac bones). You have two of them, one on each side, and their primary job is transferring the weight of your upper body down through your pelvis and into your legs. SI joint problems account for an estimated 15% to 30% of all chronic lower back pain cases.
Where the SI Joint Sits
The SI joint forms where the sacrum, a triangular bone at the base of your spine, meets the ilium on each side. The joint surface typically spans the first three segments of the sacrum. If you place your hands on the bony bumps at the back of your pelvis (just below your belt line), you’re touching the posterior superior iliac spine, which is right next to where each SI joint sits beneath the surface.
Unlike your knee or shoulder, the SI joint isn’t built for big, sweeping movements. It allows only a few degrees of rotation and a small amount of gliding. What it lacks in mobility, it makes up for in stability. A dense web of ligaments surrounds the joint on all sides: short and long ligaments along the back, ligaments along the front, and a thick interosseous ligament packed between the two bone surfaces. Additional accessory ligaments, including the sacrotuberous and sacrospinous ligaments, reinforce the joint from below. Together, these structures lock the joint tightly so it can handle enormous compressive forces without shifting.
What the SI Joint Does
Every time you stand, walk, run, or climb stairs, your SI joints are working. They transfer large bending moments and compression loads from your lumbar spine down into your lower extremities through the hip joints. Think of them as the bridge between your trunk and your legs. Without functional SI joints, the forces generated by your upper body would have no stable pathway into the ground.
The joint also acts as a shock absorber. When your foot strikes the ground during walking or running, impact forces travel upward through your leg. The SI joint helps dissipate that energy before it reaches your spine. In women, the SI joint serves an additional role: its slight flexibility allows the pelvis to widen during childbirth.
Why the SI Joint Causes Pain
SI joint pain almost always centers on the buttock, directly over the joint. In pain-mapping studies, 100% of people with confirmed SI joint dysfunction reported pain in the buttock overlying the joint. The pain can also spread into the lower back, groin, hip, or upper thigh. This pattern often mimics sciatica or a herniated disc, which is one reason SI joint problems frequently go undiagnosed or get blamed on the lumbar spine.
Distinguishing SI joint pain from a lumbar disc problem matters because the treatments differ. Disc herniations tend to cause true nerve-related symptoms: sharp, shooting pain down the leg, numbness, tingling, and sometimes muscle weakness or loss of reflexes. SI joint pain produces what clinicians call “pseudoradicular” symptoms. The pain may travel into the leg, but it typically doesn’t follow a single nerve path, and numbness or weakness is uncommon. Interestingly, the two conditions often overlap. In one study, 84% of patients with lumbar disc herniations also had restricted SI joint movement, and patients with herniations at the lowest spinal level (L5-S1) were especially likely to have SI joint tenderness as well.
Common Causes of SI Joint Problems
Several things can destabilize or irritate the SI joint. Degenerative changes from aging gradually wear down the cartilage lining the joint surfaces, a condition called sacroiliitis. Trauma from a fall, car accident, or sudden impact can damage the ligaments or shift the joint alignment. Repetitive stress from activities like running or heavy lifting can inflame the joint over time. Inflammatory conditions like ankylosing spondylitis specifically target the SI joint and can eventually cause the bones to fuse together.
Pregnancy is a particularly common trigger. During pregnancy, the body releases a hormone called relaxin, which slows the production of collagen and speeds up its breakdown. Since collagen is the main structural protein in ligaments, this softening effect loosens the SI joint to allow the pelvis to expand for delivery. The tradeoff is reduced joint stability, which can cause significant pain during and sometimes after pregnancy. The SI joint in women is anatomically slightly different as well: it typically involves less of the lower sacral segment, which may contribute to the higher rates of SI joint dysfunction in women overall.
How SI Joint Problems Are Diagnosed
There’s no single test that confirms SI joint dysfunction. Diagnosis relies on a combination of physical examination maneuvers and, when needed, a diagnostic injection. During a physical exam, a clinician will typically perform a series of five specific hands-on tests that stress the SI joint in different ways. Having three or more of these tests reproduce your pain strongly suggests the SI joint is the source.
The most definitive diagnostic tool is a guided injection of numbing medication directly into the SI joint. If the injection eliminates at least 75% of your pain, that confirms the joint is the pain generator. Current best practice recommends two separate injections on different occasions (dual diagnostic blocks) to reduce false positives. Imaging like X-rays, CT scans, or MRIs can help rule out other problems, such as fractures, tumors, or disc herniations, but they often look normal even when the SI joint is the source of pain.
Treatment Options
Most people with SI joint pain start with physical therapy and exercise. The goal is to strengthen the muscles that provide secondary stability to the joint. The key muscle groups include the erector spinae (the long muscles running along your spine), the rectus abdominis (your front abdominal muscles), the gluteus maximus, and the biceps femoris (part of your hamstrings). Research shows that when these muscles are activated, SI joint stiffness increases, essentially compensating for loose or damaged ligaments. In one case study, a targeted strengthening program for these muscles reduced pain from severe to minimal in just three weeks.
When physical therapy alone isn’t enough, therapeutic SI joint injections combining a numbing agent with a corticosteroid can provide temporary relief lasting weeks to months. SI joint belts, which wrap around the pelvis and compress the joint, can also help by mechanically reinforcing stability.
For people whose pain persists despite conservative treatment, minimally invasive SI joint fusion is an option. The procedure uses small implants, typically made of titanium, to permanently stabilize the joint. Candidates for fusion generally need to have failed non-surgical treatment, have at least three positive physical exam findings, and have a confirmed positive response to diagnostic injections. When these criteria are met, success rates exceed 80%. Three-dimensional imaging before surgery helps the surgeon assess bone density and rule out other conditions that could be mimicking SI joint pain.
SI Joint Pain vs. Other Back Pain
Because the SI joint sits at the crossroads of the spine and pelvis, its pain patterns overlap with several other conditions. Lower lumbar disc herniations, hip arthritis, piriformis syndrome, and even problems with the pelvic ligaments can all produce similar buttock and lower back pain. The overlap is so significant that disc herniations should always be considered as a possible contributor to what looks like isolated SI joint pain.
A few patterns can help you and your provider sort it out. SI joint pain tends to be one-sided and centered below the belt line. It often gets worse with prolonged sitting, standing from a seated position, or climbing stairs. Bending forward usually doesn’t make it worse the way a disc problem would. Pain that shoots below the knee, comes with numbness in a specific patch of skin, or causes foot weakness points more toward a nerve root issue in the spine than to the SI joint itself.