True dislocation of the sacroiliac (SI) joint is extraordinarily rare and requires the kind of force you would experience in a serious car crash, a fall from a significant height, or a high-speed collision. The joint is locked in place by some of the strongest ligaments in the body, and in everyday life it moves less than a millimeter. What most people describe as a “dislocated SI joint” is almost always SI joint dysfunction, a painful but mechanically different problem where the joint becomes irritated, inflamed, or subtly altered in how it handles load. The distinction matters because the causes, treatments, and urgency are very different.
Why the SI Joint Is So Difficult to Dislocate
The SI joint sits where your sacrum, the triangular bone at the base of the spine, wedges between the two large hip bones. Unlike a ball-and-socket joint like the shoulder, which trades stability for range of motion, the SI joint is built almost entirely for stability. The sacrum is shaped like a keystone in an arch, and it is physically wedged between the hip bones on either side. That wedging alone resists a tremendous amount of shearing force.
On top of that geometry, the joint is wrapped in layers of dense ligaments. The interosseous sacroiliac ligament, which fills the space behind the joint, is considered the strongest of all the SI joint ligaments and provides structural stability in multiple directions.1PubMed Central. The sacroiliac joint: an overview of its anatomy, function and potential clinical implications Additional ligaments on the front and back of the joint reinforce it further. The joint resists shear loads not through its own surfaces so much as through this combination of bony wedging and heavy ligamentous bracing.2PubMed Central. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain
The result is a joint with almost no meaningful motion. A systematic review of movement studies found that the maximum rotation measured at the SI joint was about two degrees, and the minimum was a hundredth of a degree.3PubMed. Movement of the sacroiliac joint: Anatomy, systematic review, and biomechanical considerations A separate literature review put the range of translational movement at less than one millimeter, with no difference between healthy and symptomatic joints.4PubMed. A literature review of biomechanical studies on physiological and pathological sacroiliac joints: Articular surface structure, joint motion, dysfunction and treatments To put that in perspective, the joint barely moves at all even when it hurts. Displacing it enough to qualify as a dislocation takes forces well beyond what normal activity, exercise, or even a hard fall on your backside can generate.
When True Dislocation Actually Happens
Genuine SI joint dislocation does exist in the medical literature, but it belongs to the world of trauma surgery, not outpatient clinics. It is most often reported after high-energy events: vehicle collisions, falls from height, or high-speed sports impacts. Even within those high-energy scenarios, dislocation of the SI joint without associated fractures of the pelvis is uncommon and tends to appear in younger patients whose bones and ligaments have different properties than adults’.5PubMed. Bilateral Sacroiliac Joint Dislocation in an Adolescent after a Skiing Accident
When it does happen, the consequences are severe. Anterior dislocation of the SI joint combined with pelvic fractures is associated with hemodynamic instability, meaning dangerous drops in blood pressure from internal bleeding, along with injuries to nearby blood vessels and organs. The condition carries high rates of both mortality and long-term disability.6PubMed Central. Severe multiple injuries in a 15-year-old boy with pelvic fracture complete anterior dislocation of the sacroiliac joint and rupture of the internal and external iliac arteries: A case report of a rare injury These patients are in emergency departments and operating rooms, not chiropractor offices. If someone walked into a clinic complaining of back pain and was told their SI joint was “dislocated,” the biomechanics of the joint make that diagnosis implausible without a history of catastrophic trauma.
What People Actually Mean by a “Dislocated” SI Joint
When a therapist, chiropractor, or even a physician says your SI joint is “out of place” or “subluxated,” they are almost certainly describing SI joint dysfunction. This is a real clinical entity that causes genuine pain, but it does not involve the joint surfaces physically separating or displacing in the way a dislocated shoulder does.
SI joint dysfunction is a broad term covering situations where the joint is a source of pain, whether from abnormal loading, inflammation, ligament strain, or changes in how surrounding muscles stabilize the pelvis. The tricky part is that research has not been able to pin down a reliable structural signature for the condition. One literature review found that surface morphologies of the joint did not appear to change regardless of whether dysfunction was present, and that the relationship between joint structure and pain remains unclear.4PubMed. A literature review of biomechanical studies on physiological and pathological sacroiliac joints: Articular surface structure, joint motion, dysfunction and treatments In other words, a painful SI joint often looks structurally normal on imaging, which makes the idea that it has “slipped” or “shifted” hard to support with evidence.
This does not mean the pain is imaginary. It means the mechanism is subtler than a bone going out of place. Irritation of the joint capsule, inflammation of the surrounding ligaments, altered muscle firing patterns, and sensitization of the nerves around the joint can all produce significant pain without any measurable positional change.
How SI Joint Pain Feels
Whether the underlying problem is dysfunction, inflammation, or something else, SI joint pain has a fairly recognizable pattern. In a study of 50 patients with confirmed SI joint pain (confirmed by diagnostic injection), the vast majority reported buttock pain, and about three-quarters also had pain in the lower lumbar area. Half described some pain extending into the leg, and about a quarter had pain traveling below the knee. A smaller number reported groin pain.7PubMed. Sacroiliac joint pain referral zones
That leg pain component is one reason SI joint problems get confused with sciatica. People feel sharp or aching pain in the buttock that radiates down the leg and assume it must be a disc problem or pinched nerve. Research comparing patients with SI joint-related leg pain to those with confirmed nerve root compression found some distinguishing features: SI joint patients were more often female, tended to have a shorter history of symptoms, and more often reported groin pain or a specific fall onto the buttocks before symptom onset. Nerve root compression patients, on the other hand, more commonly showed muscle weakness and positive nerve-tension signs. However, the overlap was significant enough that imaging was still needed to tell the two apart reliably.8PubMed Central. Sciatica-like symptoms and the sacroiliac joint: clinical features and differential diagnosis
Who Is Most Likely to Develop SI Joint Problems
SI joint dysfunction is not reserved for any single population, but certain groups show up in the data more often. Athletes are one. A systematic review and meta-analysis covering over 2,800 athletes found that the mean prevalence of SI joint pain or dysfunction was roughly 11% across all athletes studied. Among athletes who specifically had low back pain, that number jumped to about a third.9PubMed. Prevalence and Risk Factors of Sacroiliac Joint Pain in Athletes: A Systematic Review and Proportional Meta-Analysis One older study of elite rowers found SI joint dysfunction in over half the team, with sweep rowers (who rotate to one side) affected more than scullers (who row symmetrically).10PubMed. Sacroiliac joint dysfunction in elite rowers Repetitive asymmetric loading seems to be a risk factor, which makes intuitive sense given the joint’s role in transferring force between the spine and the legs.
Pregnancy is another well-known trigger. The hormone relaxin, along with other hormonal changes, loosens pelvic ligaments to prepare for delivery. That increased laxity can make the SI joint less stable and more prone to pain. Interestingly, though, a systematic review found that the direct link between relaxin levels and pregnancy-related pelvic girdle pain was supported by only low-quality evidence, with significant bias in how pain was assessed across studies.11PubMed Central. Pregnancy-related pelvic girdle pain and its relationship with relaxin levels during pregnancy: a systematic review So while the hormonal explanation is plausible and widely accepted, the science behind it is less clean than popular accounts suggest. Mechanical factors like weight gain, postural changes, and the demands of carrying a growing fetus likely contribute as much or more.
People with connective tissue disorders like Ehlers-Danlos syndrome (EDS) are also at higher risk. Their ligaments are inherently stretchier and less effective at stabilizing joints, and the SI joint is no exception. A case series of EDS patients who underwent SI joint fusion showed that while many were satisfied with the surgical outcome, the fact that fusion was pursued at all reflects how disabling SI joint instability can become in this population.12PubMed Central. Sacroiliac joint fusion in patients with Ehlers Danlos Syndrome: A case series
How SI Joint Problems Are Diagnosed
Diagnosing the SI joint as the source of pain is one of the more frustrating exercises in musculoskeletal medicine. There is no single test that reliably confirms it. Physical therapists and physicians use clusters of provocation tests, which are hands-on maneuvers designed to stress the joint and reproduce the patient’s pain. Several of these tests, including the distraction test, compression test, thigh thrust, and sacral thrust, have shown sensitivity and specificity above 60% when used in combination.13PubMed Central. Specificity, sensitivity, and predictive values of clinical tests of the sacroiliac joint: a systematic review of the literature
But even combined, these tests are better at ruling the SI joint out than ruling it in. A meta-analysis found that when a provocation test cluster comes back positive, the clinician can be only about 35% confident that the SI joint is actually the pain source. A negative result, however, lets the clinician conclude with about 92% certainty that the SI joint is not the problem.14PubMed. Diagnostic Accuracy of Clusters of Pain Provocation Tests for Detecting Sacroiliac Joint Pain: Systematic Review With Meta-analysis So if provocation tests do not reproduce your pain, the SI joint probably is not to blame. If they do, you still need further workup.
Imaging adds some information but has its own limits. Standard X-rays can show advanced changes like erosions or fusion of the joint, but they often miss early problems entirely. MRI is much more sensitive at picking up inflammation and is recommended when X-rays look normal but suspicion remains high.15Canadian Association of Radiologists Journal. Imaging the Patient With Sacroiliac Pain CT scans can also help characterize bony abnormalities.16PubMed. The sacrum: pathologic spectrum, multimodality imaging, and subspecialty approach
The closest thing to a gold standard is a diagnostic injection: a physician uses fluoroscopy (real-time X-ray guidance) to inject a local anesthetic directly into the SI joint. If your pain disappears temporarily, the joint was the source. Although no true gold standard exists for SI joint pain, diagnostic blocks have been widely accepted as the most useful confirmatory tool available.17PubMed Central. Use of Diagnostic Injections to Evaluate Sacroiliac Joint Pain
Treatment Options for SI Joint Pain
Most people with SI joint dysfunction start with conservative management, and many improve with it. A review of physical therapy interventions found that specific stabilizing exercises targeting the muscles around the pelvis and a nonelastic sacroiliac belt were the most consistently effective approaches, both in pregnant and nonpregnant adults. Manual therapy techniques like muscle energy technique, where the therapist guides you through gentle resisted contractions, and soft-tissue mobilization also showed benefit in some studies.18Journal of Women’s Health Physical Therapy. Identification and Effectiveness of Physical Therapy Interventions for Sacroiliac Joint Dysfunction in Pregnant and Nonpregnant Adults
The evidence for pelvic compression belts on their own is mixed. One randomized trial that added a belt to a stabilization exercise program found no clear benefit of the belt over exercises alone.19Journal of Women’s Health Physical Therapy. Effectiveness of Adding a Pelvic Compression Belt to Lumbopelvic Stabilization Exercises for Women With Sacroiliac Joint Pain: A Feasibility Randomized Clinical Trial Belts may help some people feel more supported, but they do not appear to be a game-changer when used in addition to a proper exercise program.
When conservative treatment fails, SI joint fusion surgery becomes an option. Minimally invasive approaches have become the dominant technique. A multicenter analysis of patients who underwent minimally invasive fusion with triangular titanium implants found that pain scores improved by an average of about six points on a ten-point scale, and roughly 96% of patients said they would choose to have the surgery again.20PubMed Central. One-year outcomes after minimally invasive sacroiliac joint fusion with a series of triangular implants: a multicenter, patient-level analysis A broader systematic review of SI joint fusion literature covering over 400 patients found that minimally invasive patients reported higher satisfaction rates and lower reoperation rates compared to open surgery patients. The mean reoperation rate after minimally invasive fusion was about 6%, versus about 15% for open procedures.21Journal of Neurosurgery: Spine. Surgical and clinical efficacy of sacroiliac joint fusion: a systematic review of the literature
A meta-analysis comparing different minimally invasive fusion techniques found that lateral triangular implants produced the largest pain improvements (about 4.8 points on a 10-point scale), with very low rates of complications like implant malposition or wound infection.22PubMed Central. Minimally Invasive SI Joint Fusion Procedures for Chronic SI Joint Pain: Systematic Review and Meta-Analysis of Safety and Efficacy Surgery is not a casual decision, but for people who have exhausted conservative options and have confirmed SI joint pain via diagnostic injection, the outcomes are generally favorable.
SI Joint Pain After Spinal Fusion Surgery
One population that deserves special mention is people who have had lumbar spinal fusion, particularly fusions that extend down to the bottom of the lumbar spine. When the lowest lumbar segments are fused and can no longer move, the SI joint has to absorb more of the forces that the fused segments used to handle. A biomechanical study confirmed this adjacent-segment stress effect, showing that fusion constructs ending at the sacrum increased the mechanical demands on the SI joint.23Journal of Neurosurgery: Spine. A biomechanical investigation of the sacroiliac joint in the setting of lumbosacral fusion: impact of pelvic fixation versus sacroiliac joint fixation This is a recognized phenomenon in spine surgery: you fix one problem and the joint next door takes the strain.
On the reassuring side, if SI joint fusion itself ever becomes necessary, a finite element study found that fusing the SI joint with triangular titanium implants produced minimal changes in hip contact stress, less than 5% in most motions. That suggests the risk of creating a new adjacent-segment problem at the hip by fusing the SI joint is low.24PubMed Central. Effects on hip stress following sacroiliac joint fixation: A finite element study
Inflammatory Conditions That Affect the SI Joint
Not all SI joint pain is mechanical. Inflammatory conditions, especially axial spondyloarthritis (which includes ankylosing spondylitis), target the SI joint directly. In these conditions, the immune system drives chronic inflammation at the joint, leading to erosion, sclerosis, and eventually bony fusion over years. Radiographic changes on X-ray are graded on a scale from 0 (normal) to 4 (complete fusion), but these changes often lag behind symptoms by years, which is why MRI is now the recommended first-line imaging tool for suspected inflammatory SI joint disease.15Canadian Association of Radiologists Journal. Imaging the Patient With Sacroiliac Pain
Research on joint anatomy in spondyloarthritis patients has found a strong relationship between certain SI joint shapes and symptomatic disease. Patients with mechanical joint disease had atypical joint anatomy in about 80% of cases, compared to roughly 38% in healthy controls. Women had a significantly higher proportion of atypical joints overall. Two particular joint forms, the iliosacral complex and the crescent-shaped ilium, were significantly more common in symptomatic patients.25Oxford Academic. Joint anatomy in axial spondyloarthritis: strong associations between sacroiliac joint form variation and symptomatic disease This line of research raises an interesting question: whether some people are anatomically predisposed to SI joint trouble because of the shape of their joint surfaces, not just how they use them.
If your SI joint pain started gradually, is worst in the morning and improves with movement, and you are under 45, an inflammatory cause is worth investigating. The pattern is quite different from mechanical dysfunction, which tends to worsen with activity and improve with rest. Getting the distinction right matters because the treatments diverge sharply: inflammatory SI joint disease responds to specific medications that target the immune process, while mechanical dysfunction is managed with the physical therapy and stabilization approaches described earlier.