Sacroiliac (SI) joint dysfunction is a condition in which the joint connecting the sacrum to the pelvis generates pain due to abnormal movement — too much or too little — or progressive degeneration. It is one of the most frequently overlooked sources of chronic low back and buttock pain, regularly mistaken for a lumbar disc problem. Accurate diagnosis is essential because the treatment path differs significantly from disc-related pain.

What Is the Sacroiliac Joint?

The sacroiliac joint connects the sacrum — the triangular bone at the base of the spine — to the ilium, the broad upper bone of each side of the pelvis. There are two SI joints, one on each side. Together they form the structural bridge between the spine and the lower extremities, transmitting the upper body’s weight to the legs with every step. For a deeper look at the anatomy of this region, see our guide to the sacrum: anatomy, function, and common pain conditions.

Despite its load-bearing role, the SI joint has a remarkably small range of motion — approximately 2 to 4 degrees of rotation and 1 to 2 millimeters of translation. Its primary function is stability, not movement. It is reinforced by some of the strongest ligaments in the body, including the sacroiliac, sacrotuberous, and sacrospinous ligaments. When those ligaments are compromised — through trauma, pregnancy, repetitive stress, or degeneration — the joint either moves too much (hypermobility) or becomes restricted (hypomobility). Either state generates significant pain.

How Does SI Joint Dysfunction Develop?

SI joint dysfunction develops through several distinct mechanisms, each placing the joint under abnormal stress or disrupting the ligament support that keeps it stable.

• Trauma: A fall onto the buttocks, motor vehicle accident, or sudden force through the pelvis can strain or partially disrupt the supporting ligaments, creating abnormal joint movement.
• Pregnancy and postpartum changes: The hormone relaxin loosens pelvic ligaments in preparation for childbirth. In some individuals this laxity persists, producing chronic hypermobility.
• Leg length discrepancy: Even a small structural or functional difference in leg length alters pelvic mechanics and places asymmetric load on the SI joints over time.
• Prior lumbar fusion: Spinal fusion — particularly multi-level fusion — transfers mechanical stress to the SI joint, accelerating degeneration. This is sometimes called adjacent segment disease at the sacropelvic level. Understanding lumbar instability helps clarify how fusion alters load distribution throughout the spine.
• Degenerative joint disease: Like any synovial joint, the SI joint is subject to osteoarthritis. Cartilage breakdown over time leads to inflammation, stiffness, and pain. Our overview of spinal osteoarthritis and facet joint degeneration provides related context.
• Inflammatory arthropathies: Conditions such as ankylosing spondylitis and psoriatic arthritis preferentially target the SI joint, causing inflammatory sacroiliitis distinct from mechanical dysfunction.

Why Is SI Joint Dysfunction So Often Misdiagnosed?

Pain from the SI joint refers to the low back, buttock, groin, and posterior thigh — patterns that overlap almost exactly with lumbar disc herniation and piriformis syndrome. This anatomical ambiguity makes SI joint dysfunction one of the most commonly misattributed conditions in spine care.

Back pain is the leading cause of disability worldwide, and 30% of U.S. adults report recent low back pain. Among those with chronic low back pain, the SI joint is a frequently underappreciated contributor. When the wrong source is treated, the actual driver of pain remains unaddressed. Back surgery has roughly a 40% failure rate — and a portion of those cases involve undiagnosed SI joint pathology. Individual outcomes vary, and a clinical evaluation is the only way to know the true source of a patient’s pain. For a broader look at the many causes of chronic back pain, see our guide to the top causes of chronic back pain.

What Are the Symptoms of SI Joint Dysfunction?

Symptoms are characteristically one-sided and tend to worsen with activities that load or rotate the pelvis.

• Low back and buttock pain: Typically below L5, concentrated at or just below the posterior superior iliac spine (PSIS). Patients often identify the exact spot with one fingertip — a finding known as the Fortin finger sign.
• Groin pain: Anterior referral into the groin occurs in a meaningful subset of SI joint cases and is commonly mistaken for a hip problem.
• Posterior thigh pain: Pain can radiate down the back of the thigh, mimicking sciatica, though it rarely travels below the knee.
• Pain with position changes: Rising from a chair, rolling over in bed, climbing stairs, or standing on one leg are common aggravating activities.
• Prolonged sitting or standing intolerance: The joint is sensitive to sustained load in any fixed posture.
• Stiffness after rest: Morning stiffness that loosens with gentle movement is a hallmark of inflammatory sacroiliitis but also occurs in mechanical dysfunction.

How Is SI Joint Dysfunction Diagnosed?

No single imaging study confirms SI joint dysfunction with certainty. Diagnosis relies on a combination of clinical provocation tests, imaging interpretation, and — when necessary — a diagnostic injection.

• Provocation tests: A cluster of physical examination maneuvers — FABER, FADIR, Gaenslen’s, posterior shear (thigh thrust), and distraction — each stress the SI joint from a different direction. Three or more positive tests substantially increases diagnostic confidence.
• MRI and X-ray: Imaging identifies structural changes such as sacroiliitis, erosions, sclerosis, or joint space narrowing. Standard lumbar MRI often does not capture the SI joint adequately; dedicated SI joint sequences are more informative. Understanding lumbar spine anatomy helps contextualize where the SI joint fits within the broader spinal structure.
• Fluoroscopic diagnostic injection: An image-guided injection of anesthetic directly into the SI joint — the gold standard for confirming the joint as the pain source. Significant pain relief following the injection confirms the diagnosis.

Distinguishing SI joint dysfunction from lumbar facet syndrome is a common diagnostic challenge. Our resource on lumbar facet syndrome details how those two conditions differ in presentation and treatment.

How Is SI Joint Dysfunction Treated?

Treatment is matched to the underlying mechanism — inflammatory, mechanical, or degenerative — and to the severity of functional impairment.

Conservative Care

• Physical therapy: Targeted exercises address pelvic stability, gluteal strength, and hip mobility. For hypermobility, the emphasis is on stabilization rather than stretching. For hypomobility, manual therapy and mobilization techniques restore joint motion.
• SI joint belts and bracing: External compression bracing provides temporary mechanical support, particularly helpful during postpartum hypermobility phases.
• Anti-inflammatory medications: NSAIDs manage acute inflammatory flares and provide short-term symptomatic relief.
• Image-guided corticosteroid injections: Reduce inflammation and confirm the diagnosis simultaneously. Relief is typically temporary, lasting weeks to months.

Minimally Invasive and Procedural Options

• Radiofrequency ablation (RFA) of lateral branch nerves: Interrupts the nerve signals transmitting pain from the SI joint. Effective for a defined subset of patients; relief generally lasts 9 to 18 months before nerves regenerate.
• SI joint fusion: Minimally invasive triangular titanium implant fusion is appropriate for severe, refractory mechanical dysfunction that has not responded to conservative or injection-based care. This is a surgical intervention with its own risk and recovery profile.

When Disc Pathology Is Also Present

Some patients present with both SI joint dysfunction and underlying disc-related pain — particularly those who have had prior lumbar surgery that transferred load to the pelvis. In those cases, addressing only the SI joint leaves the disc component untreated, and vice versa. For patients whose chronic low back pain involves documented annular tears or disc degeneration alongside SI involvement, a thorough evaluation is necessary to determine which is the primary pain driver. Our overview of chronic low back pain outlines how disc-related and non-disc-related sources are differentiated clinically.

Is There a Connection Between SI Joint Dysfunction and Disc Disease?

The relationship between SI joint dysfunction and spinal disc disease runs in both directions. Lumbar disc degeneration alters the mechanics of the entire lumbar-pelvic complex, changing how load is distributed to the SI joints. Conversely, SI joint hypermobility increases stress on the adjacent lumbar segments, accelerating disc wear. Patients who have undergone lumbar fusion are at particular risk for developing SI joint pathology as load transfers away from the fused segments. Understanding this interplay — explained in detail in our guide to lumbar spine anatomy and common conditions — is essential for building a treatment plan that addresses the actual source of pain rather than only the most visible finding on imaging.

Frequently Asked Questions

How do I know if my back pain is from the SI joint or a disc?

The location and behavior of pain provide initial clues: SI joint pain is typically one-sided, concentrated below L5 near the dimple of the back, and worsens with activities like rising from a chair or climbing stairs. Disc-related pain often follows a nerve distribution down the leg below the knee. A clinical evaluation — including provocation tests and, when needed, a diagnostic injection — is the definitive way to distinguish the two.

Can SI joint dysfunction cause leg pain?

SI joint dysfunction can produce referred pain into the buttock and posterior thigh, but it rarely travels below the knee. Pain that extends down the calf and into the foot is more consistent with true radiculopathy from a disc problem or nerve root compression. A thorough evaluation determines the source.

Does SI joint dysfunction show up on an MRI?

Standard lumbar MRI is not optimized to capture the SI joint. Dedicated SI joint MRI sequences or CT imaging can reveal structural changes such as sacroiliitis, sclerosis, or erosions. However, imaging findings do not always correlate with pain severity. A diagnostic injection is considered the gold standard for confirming the SI joint as the pain source.

Can SI joint dysfunction resolve without surgery?

Many cases of SI joint dysfunction respond to physical therapy, bracing, and targeted injections without requiring surgery. For patients with severe, refractory dysfunction that has not responded to conservative care, minimally invasive SI joint fusion is an option. A clinical evaluation is the only way to determine the right pathway for a specific patient.

What is the connection between prior back surgery and SI joint pain?

Lumbar fusion — particularly multi-level fusion — transfers mechanical load to the SI joint, which can accelerate degeneration and produce new or worsening SI joint pain over time. This is sometimes called adjacent segment disease at the sacropelvic level. Patients with a history of spinal fusion who develop new low back or buttock pain should have their SI joints evaluated as part of a comprehensive workup.

If I have both disc pathology and SI joint dysfunction, which gets treated first?

Treatment sequencing depends on which source is identified as the primary pain driver and which is contributing secondarily. A thorough diagnostic evaluation — including provocation testing, imaging, and targeted injections — determines the correct order. Addressing only one source when both are present typically produces incomplete relief. A clinical evaluation is the only way to know the right approach for a given patient.

This content is for general informational purposes only and does not constitute medical advice, diagnosis, or treatment. It is not a substitute for evaluation by a qualified physician. Treatment decisions depend on your individual medical history and clinical findings. Schedule a consultation to discuss whether the procedure is right for you.