SI joint instability happens when the ligaments holding your sacrum to your pelvis become too loose, too damaged, or too weak to keep the joint properly aligned under load. The sacroiliac joints sit at the base of your spine and transfer the weight of your upper body into your legs, so even small increases in movement at this joint can produce significant low back, hip, and buttock pain. The causes range from pregnancy hormones and traumatic injuries to connective tissue disorders and prior spinal surgery.
How the SI Joint Stays Stable
Understanding what keeps the joint stable helps explain what goes wrong. The SI joint is designed for minimal movement, typically just 1 to 3 degrees of rotation. Unlike your shoulder or knee, it relies almost entirely on ligaments and the interlocking shape of the bone surfaces rather than large muscles for its stability.
The interosseous sacroiliac ligament, a thick band filling the space between the sacrum and ilium, is the strongest of these stabilizers and provides resistance to movement in multiple directions. Additional ligaments on the front and back of the joint control the two primary motions: nutation (the sacrum tipping forward) and counternutation (the sacrum tipping backward). The sacrotuberous and sacrospinous ligaments, which connect the sacrum to bony landmarks on the pelvis, resist forward tipping. Muscles like the deep spinal stabilizers also play a supporting role, and when they weaken, more stress falls on the ligaments.
When any part of this system is compromised, whether through stretching, tearing, inflammation, or degeneration, the joint moves more than it should. That excess movement is what clinicians mean by “instability.”
Pregnancy and Hormonal Changes
Pregnancy is one of the most common triggers for SI joint instability. During gestation, the body produces the hormone relaxin, which remodels collagen in ligaments and connective tissue to allow the pelvis to expand for delivery. This loosening isn’t limited to the pubic symphysis at the front of the pelvis. It also affects the SI joints, increasing their mobility beyond what the surrounding muscles can easily control.
For most women, pelvic girdle pain that develops during pregnancy resolves within about four months after delivery. But it persists in roughly 20% of women, and 8% to 10% continue to have pain one to two years postpartum. In these cases, the ligaments may not fully recover their pre-pregnancy tension, leaving the SI joint chronically hypermobile. Repeated pregnancies can compound this effect, making each recovery period longer and the residual laxity greater.
Trauma and High-Impact Injuries
A hard fall onto the buttocks, a motor vehicle accident, or any impact that forces the pelvis into an asymmetric position can tear or overstretch SI joint ligaments. The mechanism usually involves either a shearing force (one side of the pelvis moving forward or backward relative to the other) or a distraction force (the two sides being pulled apart). Sacral fractures from high-impact injuries can also disrupt the joint surfaces and surrounding ligaments, leading to instability even after the bone heals.
Sports injuries deserve mention here too. Activities involving repetitive unilateral loading, like running, golf, or gymnastics, can gradually stress the joint beyond its capacity. This isn’t always a single dramatic event. Sometimes it’s cumulative microtrauma that slowly stretches the ligaments over months or years.
Connective Tissue Disorders
Some people are born with ligaments that are inherently too stretchy. Ehlers-Danlos syndrome (EDS) is the most recognized connective tissue disorder linked to SI joint instability. EDS is a group of inherited conditions characterized by joint hypermobility, overly elastic skin, and fragile connective tissue. Because the ligaments around the SI joint are structurally weaker in people with EDS, pathologic movement develops more easily and tends to be chronic rather than episodic.
Generalized joint hypermobility, even without a formal EDS diagnosis, can predispose the SI joint to instability. If you can hyperextend your elbows, bend your thumb to your forearm, or place your palms flat on the floor with straight knees, your SI joint ligaments may also be laxer than average. Clinicians use the Beighton Criteria, a simple scoring system that checks flexibility at several joints, to assess this.
Lumbar Spinal Fusion Surgery
One of the less intuitive causes of SI joint instability is prior back surgery. When segments of the lumbar spine are fused together, the vertebrae above and below the fusion have to compensate for the lost motion. The SI joint, sitting directly below the lumbar spine, absorbs a disproportionate share of that extra stress.
In a study of 317 patients who underwent lumbar fusion, 12% developed new SI joint pain afterward. The risk increases slightly with longer fusions: 11.1% after single-level fusion versus 14% after four or more levels. Perhaps more striking, studies of patients with persistent back pain after lumbosacral fusion have found that the SI joint is the actual pain source in 32% to 35% of cases. This means that what many patients assume is a failed back surgery may actually be a new SI joint problem created by the changed biomechanics.
Degenerative and Inflammatory Conditions
Like any joint with cartilage, the SI joint can wear down over time. Osteoarthritis causes the cartilage lining to thin and the joint surfaces to become irregular. As the cartilage deteriorates, the joint loses some of its inherent fit, and the ligaments must work harder to maintain stability. This gradual process often begins in middle age and accelerates with obesity, heavy manual labor, or prior injury.
Inflammatory conditions work differently. Ankylosing spondylitis, for example, causes chronic inflammation of the SI joint (sacroiliitis) that can eventually lead to fusion of the joint surfaces. In its early stages, though, the inflammatory process can weaken the surrounding ligaments and cartilage before stiffening sets in, creating a window of instability. The distinction matters because inflammatory sacroiliitis requires treatment aimed at the immune system, while mechanical instability is addressed through stabilization.
Biomechanical Imbalances
Leg length discrepancy is a straightforward mechanical cause. When one leg is longer than the other, the pelvis tilts to compensate, and the SI joint on the longer side bears more load. Finite element modeling has shown that peak stresses across the SI joint surfaces progressively increase as the discrepancy grows from 1 to 3 centimeters. Even a 1-centimeter difference creates measurably uneven loading, though not everyone with a small discrepancy develops symptoms.
Poor core stability amplifies all of these mechanical factors. The deep stabilizing muscles of the trunk, particularly the transverse abdominis and multifidus, work as a muscular “corset” that supplements the ligaments. When these muscles are weak or poorly coordinated, whether from a sedentary lifestyle, injury, or neurological issues, the ligaments bear more of the stabilizing load and are more likely to become overstretched over time. This is why core strengthening is a cornerstone of nearly every conservative treatment plan for SI joint instability.
How SI Joint Instability Is Identified
Diagnosing SI joint instability is notoriously tricky because no single test is definitive. Clinicians typically use a cluster of hands-on provocative tests, including the thigh thrust, Gaenslen’s test, compression, distraction, sacral thrust, and Patrick’s FABER test. Each one applies a specific force to the SI joint to see if it reproduces your pain. When three or more of these six tests are positive, the combination has a sensitivity of 91% and specificity of 78%, meaning it correctly identifies most people with SI joint problems while producing relatively few false positives.
Standard imaging often misses instability. X-rays and MRIs taken while you’re lying down may look normal because the joint isn’t under load. Research comparing pelvic measurements in standing versus lying positions has found meaningful differences in SI joint positioning, with the joint tightening under weight-bearing and relaxing when recumbent. A normal SI joint moves only 1 to 3 degrees, so a change of even 3 degrees represents a 100% increase in its range of motion. This is why some clinicians request weight-bearing imaging or use diagnostic injections (numbing the joint directly) to confirm the diagnosis.
Stabilization Without Surgery
Most people with SI joint instability start with conservative treatment. An SI joint belt, a narrow band worn low around the pelvis, compresses the joint and reduces excess movement. Research has found that these belts are as effective as larger lumbar braces for reducing pain and improving function, with higher patient satisfaction due to their smaller size and comfort. The theory is simple: stabilizing the base of the spinal column provides support to the structures above it, much like securing the foundation of a tower.
Physical therapy focused on strengthening the muscles that support the pelvis is the other major pillar of conservative care. Exercises targeting the deep core, gluteal muscles, and hip stabilizers can compensate for ligament laxity by creating a muscular support system around the joint. For people with hypermobility conditions like EDS, this muscular compensation is especially important because the ligaments themselves may never provide adequate restraint on their own.