SI joint pain is one of the most common sources of lower back pain, responsible for an estimated 15% to 30% of all chronic low back pain cases. The sacroiliac joints sit on either side of your lower spine where the sacrum meets the pelvic bones, and their job is to transfer the weight of your upper body down through your hips and into your legs. When something disrupts how these joints move or bear load, the result is pain that typically settles in the low back, buttocks, or groin.
How the SI Joint Works
The sacroiliac joints are the largest joints along your spine’s central axis. Each one is only about 1 to 2 millimeters wide, held together by a fibrous capsule and reinforced by some of the strongest ligaments in the body. Despite their small range of motion, these joints handle enormous forces. Every time you walk, bend, or lift something, the SI joints absorb and redirect large compression loads and bending forces from your spine to your legs. Their relatively flat shape, combined with their dense ligament network, is what makes this possible. When any part of that system is compromised, pain follows.
Traumatic Injury
A direct blow or sudden force is one of the most straightforward causes of SI joint dysfunction. Falls that land on the buttocks are particularly common triggers because the impact drives force straight into the joint. Motor vehicle collisions can do the same thing, especially rear-end impacts that jolt the pelvis. Heavy lifting, particularly with poor form or a sudden twist, can also strain the joint or its surrounding ligaments. In studies comparing SI joint pain to disc-related sciatica, a history of falling on the buttocks was significantly more common among people whose pain originated from the SI joint.
Pregnancy and Hormonal Changes
Pregnancy is one of the most well-recognized risk factors for SI joint pain, and the reason is largely hormonal. During pregnancy, your body releases a hormone called relaxin, which loosens the ligaments around the spine and pelvis to make room for childbirth. That loosening is necessary, but it also reduces the stability of the SI joint. With less ligament support, the joint moves more than it normally would, and the surrounding muscles and tissues can become irritated or inflamed.
This doesn’t always resolve after delivery. Some women experience lingering SI joint laxity for months or even years postpartum, particularly after multiple pregnancies. The combination of loosened ligaments, altered posture from carrying a baby, and the physical demands of labor can leave the joint in a state of chronic dysfunction.
Inflammatory Conditions
Not all SI joint pain comes from mechanical stress. Inflammatory diseases can attack the joint directly, causing a condition called sacroiliitis. The most notable of these is ankylosing spondylitis, a chronic inflammatory disorder that primarily targets the spine and SI joints. In ankylosing spondylitis, the immune system triggers inflammation at the points where ligaments and tendons attach to bone, gradually causing stiffness and, in advanced cases, fusion of the joint.
Inflammatory SI joint pain has a distinct pattern that separates it from mechanical pain. It typically comes on gradually, is worst first thing in the morning or after sitting still for a while, lasts at least 30 minutes after waking, and actually improves with movement and exercise. People with inflammatory sacroiliitis often report waking in the second half of the night because of pain, and they may notice the discomfort alternates between the left and right sides of the buttocks. If your SI joint pain fits this pattern, it’s worth investigating further, since ankylosing spondylitis can overlap with related conditions like psoriatic arthritis, reactive arthritis, and arthritis linked to inflammatory bowel disease.
Degenerative Wear and Tear
Like any joint, the SI joint breaks down over time. Degenerative changes are common in middle-aged and older adults and include narrowing of the joint space, thickening of the bone just beneath the cartilage surface (subchondral sclerosis), and the growth of small bone spurs along the joint margins. These changes reduce the joint’s ability to absorb and distribute force smoothly, which can produce chronic, aching pain that worsens with prolonged standing or walking. On imaging, degenerative SI joint disease can actually look similar to ankylosing spondylitis, which is one reason blood tests and symptom patterns matter for distinguishing the two.
Prior Spinal Fusion Surgery
If you’ve had lumbar spinal fusion, your SI joints may be absorbing forces they weren’t designed to handle. A study of 317 patients who underwent spinal fusion found that 12% developed new SI joint pain afterward. The mechanism is straightforward: when a section of the spine is fused and can no longer move, the joints above and below it have to compensate. The SI joint, sitting just below the lumbar spine, picks up increased motion and stress. The longer the fused segment, the greater the forces transferred to the SI joint. This is one of the more overlooked causes of persistent low back pain after an otherwise successful spinal surgery.
Leg Length Differences and Gait Imbalances
When one leg is longer than the other, the pelvis tilts slightly with every step. Over time, this uneven loading can accelerate degenerative changes in the SI joint. Research on cadaveric specimens found that a leg length difference greater than 10 millimeters (roughly half an inch) was associated with 1.4 times the expected amount of SI joint degeneration. Differences under 10 millimeters did not show the same effect. The association was strongest in individuals over 40, suggesting that it takes years of asymmetric stress before the joint shows measurable damage. Gait abnormalities from other causes, such as hip arthritis or foot problems, can produce similar uneven loading.
How SI Joint Pain Differs From Sciatica
SI joint pain and sciatica can feel remarkably similar, which is why misdiagnosis is common. Both can cause pain that radiates into the buttock and down the leg. But there are distinguishing features. SI joint pain is more likely to radiate into the groin, while disc-related sciatica more commonly causes numbness, tingling, or measurable muscle weakness in the leg. People with sciatica from a herniated disc also tend to have a harder time bending forward and are more likely to have pain with coughing or sneezing that follows a specific nerve path.
SI joint dysfunction also tends to affect women more often and produces symptoms of shorter overall duration before diagnosis. If your pain started after a fall on your buttocks and includes groin pain without significant leg numbness, the SI joint is a strong suspect. If you have notable leg weakness, reduced sensation, or pain that shoots below the knee in a narrow band, a compressed nerve root higher in the spine is more likely the source.