Sacroiliac pain originates from the two joints where your spine meets your pelvis, and it accounts for an estimated 10 to 25% of all chronic low back pain. The causes range from everyday mechanical stress and pregnancy to inflammatory disease, trauma, and age-related wear. Understanding which category your pain falls into matters because treatments differ significantly depending on the underlying cause.
How the SI Joint Works
The sacroiliac (SI) joints sit on either side of your lower spine, connecting the triangular bone at the base of your spine (the sacrum) to the large wing-shaped bones of your pelvis (the ilia). Unlike your knee or shoulder, the SI joint barely moves. It allows only a few degrees of rotation and a small amount of sliding motion, just enough to absorb shock when you walk, run, or land from a jump.
Three layers of ligaments hold the joint together: one in front, one in back, and a dense set wedged between the two bone surfaces. Additional ligaments running from the sacrum down to the sit bones and from the lower spine to the pelvis add further reinforcement. This heavy ligamentous support is what makes the joint so stable. It can withstand a side-directed compressive load six times greater than the lumbar spine can. But it fails under far less axial (top-down) force, roughly one-twentieth of what the lumbar spine tolerates. That imbalance helps explain why certain movements and injuries are more likely to cause problems.
Mechanical Overload and Repetitive Stress
The most common cause of SI pain in otherwise healthy adults is mechanical: the joint is subjected to excessive compression or shear force that irritates the joint surfaces, ligaments, or surrounding tissues. Shear force is the key concept here. Instead of weight pressing evenly through the joint, one bone surface slides against the other. This happens with activities that load one side of the pelvis more than the other, such as running on a sloped surface, standing with weight shifted to one leg for long periods, or repeatedly lifting with a twist.
Athletes and military recruits are at particular risk because of repetitive high-impact loading. The stress can even produce small fractures in the sacrum (insufficiency fractures) that mimic or coexist with SI joint pain. For most people, though, the issue is subtler: slightly asymmetric movement patterns that accumulate over months or years until the joint becomes sensitized.
Leg Length Differences
When one leg is shorter than the other, even by a small amount, the pelvis tilts with every step. This creates uneven loading at the SI joint. A difference of less than 10 millimeters is usually well tolerated, but once the discrepancy reaches 10 mm or more, it creates enough asymmetry to contribute to lower back and hip pain. If you’ve had SI pain that doesn’t respond to typical treatment, a leg length check is worth pursuing. The fix can be as simple as a heel lift in one shoe.
Pregnancy and Hormonal Changes
Pregnancy is one of the most well-known triggers for SI pain, and the mechanism is hormonal. During pregnancy, the body produces a hormone called relaxin that loosens the muscles, ligaments, and joints around the pelvis, back, and abdomen. This loosening is necessary to allow the pelvis to expand during delivery, but it also destabilizes the SI joints. Many women experience this as a deep ache in the low back or buttock, sometimes with a feeling of the pelvis “giving way” during walking or climbing stairs.
Relaxin levels drop after birth but don’t return to pre-pregnancy levels immediately. Some sources estimate it can take up to 12 months for relaxin to fully normalize. That extended timeline explains why SI pain can linger well into the postpartum period, even after the physical demands of pregnancy are over. Women who have had multiple pregnancies or difficult deliveries are at higher risk for persistent SI dysfunction.
Age-Related Degeneration
Like any joint, the SI joint degenerates over time. The cartilage lining the joint surfaces thins, the joint space narrows, and bony spurs can form along the edges. A CT imaging study of symptom-free adults found that some form of degenerative change appeared markedly more often starting in the 40s, and every subject aged 50 or older showed at least some type of degeneration in their SI joints.
This is an important nuance: degeneration on imaging doesn’t automatically mean pain. Many people with visible SI joint changes have no symptoms at all. But when degeneration does become symptomatic, it typically produces a stiff, achy pain that’s worse with prolonged sitting or after getting up from a chair. The pain tends to come on gradually rather than with a single event.
Inflammatory Conditions
A distinct category of SI pain comes from inflammation driven by the immune system rather than by mechanical wear. The hallmark condition is ankylosing spondylitis, a type of inflammatory arthritis that primarily targets the spine and SI joints. This form of SI pain typically starts before age 45, improves with movement rather than rest, and is often accompanied by prolonged morning stiffness lasting 30 minutes or more.
Inflammatory SI joint disease (sacroiliitis) can also occur with psoriasis, inflammatory bowel disease like Crohn’s, and other conditions in the spondyloarthritis family. Diagnosis relies on a combination of clinical features and imaging. MRI is the most sensitive tool, with bone marrow swelling and erosions together providing 94% specificity for sacroiliitis. If your SI pain started gradually before age 45, gets worse with rest, and improves with anti-inflammatory medication, inflammatory disease is worth investigating. A blood test for the HLA-B27 gene and a family history of related conditions can strengthen or weaken the suspicion.
Trauma and Pelvic Injuries
A single high-energy event can damage the SI joint directly. Car accidents, falls from height, and lateral compression injuries to the pelvis are the most common culprits. These forces can tear the ligaments holding the joint together, fracture adjacent bone, or disrupt the joint alignment. SI joint problems are particularly common after lateral compression pelvic ring injuries, where force hits the side of the pelvis and drives one bone surface into the other at an abnormal angle.
Post-traumatic SI pain doesn’t always appear immediately. Some people develop symptoms weeks or months after the initial injury as scar tissue forms or the joint settles into a slightly altered position. A history of pelvic trauma, even years earlier, is a recognized risk factor for later SI dysfunction.
After Spinal Fusion Surgery
One cause that surprises many people is spinal fusion surgery. When segments of the lumbar spine are surgically fused together, they can no longer absorb and distribute movement. The joints above and below the fusion have to pick up the slack, and the SI joint is often the one that pays the price. A systematic review found that 32.9% of patients whose fusion extended down to the sacrum developed SI joint pain afterward. That’s roughly one in three patients, making it one of the most common complications of lumbosacral fusion.
The risk increases with the number of spinal levels fused and is highest when the fusion includes the lowest lumbar vertebra and sacrum. If you’ve had spinal fusion and develop new buttock or low back pain, SI joint involvement should be on the list of possibilities.
How SI Pain Is Identified
SI joint pain can be tricky to pin down because it overlaps with disc problems, hip conditions, and other causes of low back pain. The pain typically sits over the buttock on one side, sometimes radiating into the back of the thigh, but rarely below the knee. It often worsens with transitions: standing up from sitting, rolling over in bed, or climbing stairs.
Clinicians use a cluster of five physical provocation tests to identify the SI joint as the pain source. These tests apply specific stresses to the joint in different directions: compression, distraction, and shear. When three out of five tests reproduce the patient’s familiar pain, the cluster has a sensitivity of 85% and specificity of 79% for SI joint origin. No single test is reliable on its own, which is why the cluster approach matters. In uncertain cases, a diagnostic injection of numbing medication directly into the joint can confirm or rule out the diagnosis.
Multiple Causes Often Overlap
SI pain rarely has a single, clean explanation. A woman in her 40s might have mild degenerative changes that were never symptomatic until pregnancy loosened her ligaments. A runner with a subtle leg length difference might tolerate it fine until a fall jars the pelvis. Someone with well-managed ankylosing spondylitis might develop new SI pain after years because mechanical degeneration is now layered on top of their inflammatory condition. Identifying which factors are contributing, and in what proportion, is what guides effective treatment. The more precisely you and your clinician can trace the cause, the more targeted the approach can be.