The Truth About The SI Joint: Facts vs. Fiction

Over the last several weeks, I’ve been deeply immersed in the medical literature on sacroiliac (SI) joint pain and dysfunction. While I’d previously skimmed the research, something recently compelled me to dig in properly—to see, in a systematic way, what the scientific and medical communities actually say about the SI joint.

My curiosity was driven largely by a disconnect between theory and practice. In my clinical experience, SI joint pain has almost never turned out to be a true problem with the joint itself. Instead, it has consistently resolved relatively quickly through targeted work on surrounding muscles. That contrast made me want to understand the prevailing medical paradigm more clearly: was the research aligned with what I was seeing, or was there a deeper misunderstanding at play?

As I reviewed the literature, one theme emerged almost immediately—the SI joint is an extremely stable structure. Anatomically, it is a synovial joint and does contain synovial fluid, but its capacity for movement is remarkably limited. High-quality biomechanical studies show that SI joint motion measures only about two to four millimeters of translation and roughly one to four degrees of rotation. In other words, the joint barely moves at all.

Despite this, SI joint pain and dysfunction remain baffling to many clinicians. A common assumption—especially in physical therapy, orthopedics, yoga, and corrective exercise circles—is that pain in the SI region must be due to instability, excessive laxity, or abnormal joint motion. The narrative is familiar: “Your SI joint isn’t stable.”

However, both clinical outcomes and pain clinic research strongly contradict this idea. Studies from pain clinics that properly understand myofascial pain show that approximately 82 percent of patients presenting with pain are diagnosed with myofascial trigger points as the primary source. These patients are often treated successfully—and quickly—using established trigger point therapies, including older injection-based approaches. This strongly suggests that what is often labeled as “SI joint dysfunction” is far more likely to be muscular in origin.

Structurally, the SI joint itself is small, irregular, and extremely well-supported. It functions primarily as a shock absorber, transmitting forces between the upper and lower body, and it is reinforced by some of the strongest ligaments in the human body. The idea that it frequently “goes out” or becomes unstable simply doesn’t hold up under scrutiny.

My deeper dive into this topic was sparked by a conversation with a yoga anatomy instructor who claimed that the SI joint could essentially “rip apart.” That assertion sent me straight to the research. Cadaver studies show that altering the spacing of the SI joint requires roughly 400–500 pounds of force per inch—enough force to fracture the sacrum or pelvis. In real-world terms, this level of force occurs only in high-impact trauma, such as motor vehicle accidents. It is not something that can be generated through normal movement, exercise, or manual manipulation.

When clinicians describe SI joint pain, the symptoms they list are notably broad: low back pain, pelvic or buttock pain, hip or groin pain, pain radiating into the lower extremity, and difficulty sitting, sleeping, or walking. From a trigger point perspective, these referral patterns are immediately recognizable. The gluteus medius commonly refers pain directly into the SI region and sacrum. The gluteus minimus can mimic sciatica, sending pain down the leg. The piriformis, iliopsoas, and quadratus lumborum (QL) all produce referral patterns that overlap heavily with what is labeled as SI joint pain.

Movement studies provide even more compelling evidence. Using radiostereometric analysis (RSA)—a technique involving the implantation of tiny beads into pelvic bones—researchers were able to measure SI joint motion with extreme precision. These studies found that sacral nutation occurs when transitioning from lying down to sitting or standing, but crucially, there was no difference in SI joint movement between symptomatic and asymptomatic sides. In other words, joint motion had no correlation with pain.

Other studies examined stabilization devices, such as the Hoffman external fixation frame, used in severe cases to “stabilize” the SI joint. While some patients reported pain relief, precise measurements showed no change whatsoever in SI joint position or movement. The authors concluded that effective manipulation or stabilization was not dependent on altering joint alignment. This raises an important question: if the joint itself isn’t being stabilized, what actually is?

The most logical answer is the lumbopelvic–hip complex as a whole. These devices likely reduce muscular demand on structures like the lumbar spine, gluteals, and QL—again pointing toward a muscular rather than joint-based mechanism.

Orthopedic tests used to diagnose SI joint dysfunction fare no better under scrutiny. High-quality studies consistently show that these tests lack reliability and reproducibility. Many are provocative tests, meaning they reproduce pain without identifying its true source. The commonly used FABER (or figure-four) test is a good example. While often interpreted as stressing the SI joint, the position significantly shortens and compresses the gluteal muscles. Since SI joint spacing cannot meaningfully change during this movement, a “positive” test is far more plausibly explained by gluteus medius or minimus trigger points.

The same pattern appears in the Gillet test, frequently used by chiropractors to assess SI joint motion. Careful measurement shows minimal SI joint movement and no difference between painful and non-painful sides. The apparent motion clinicians think they feel is actually movement of the pelvis relative to the hip—not movement within the SI joint itself. Pain provoked during this test is most likely due to increased muscular load, particularly on the glutes and QL during single-leg stance.

Even in pregnancy—a population often cited as evidence of SI joint laxity—the data tell a consistent story. While the hormone relaxin does increase ligamentous laxity, studies show that pregnant women with moderate to severe pelvic pain have the same SI joint mobility as those with little or no pain. Again, there is no correlation between joint laxity and symptoms. A far more plausible explanation is increased load, altered posture, increased lumbar lordosis, and higher stabilization demands placed on the glutes and QL.

Leg length discrepancy is another commonly cited factor. Finite element modeling shows that even a one-centimeter discrepancy can increase load across the SI joint fivefold. But from a myofascial perspective, this increased load challenges the entire lumbopelvic–hip complex, particularly the stabilizing muscles. The joint is blamed, while the muscular system doing the stabilizing is overlooked.

Taken together, the medical literature paints a clear picture. The SI joint is stable, minimally mobile, and extremely resistant to structural disruption. Pain attributed to SI joint dysfunction rarely correlates with joint movement, laxity, or alignment. Instead, the evidence consistently points toward muscular overload, trigger point referral, and stabilization demands placed on the surrounding musculature.

The problem, it seems, is not the joint—it’s the paradigm.