The sacroiliac joint is the structural bridge between the sacrum — the triangular bone at the base of your spine — and the iliac bones of the pelvis. It transfers load between your upper body and legs, absorbs shock, and stabilizes the pelvic ring. Dysfunction here accounts for 15–30% of all chronic low back pain cases and is frequently mistaken for disc or facet pathology.

• The SI joint is a paired joint on each side of the lower spine with very limited motion by design.
• Its primary job is load transfer and shock absorption, not wide range of movement.
• Dysfunction is often misdiagnosed as lumbar disc or facet problems.
• Causes include degeneration, pregnancy, trauma, and prior lumbar fusion.
• Non-surgical treatment is the first line of care for most patients.
• A clinical evaluation is the only way to confirm the SI joint as your pain source.

What Is the Sacroiliac Joint?

The sacroiliac joint — the SI joint — is a paired synovial joint on each side of the lower spine, where the lateral surface of the sacrum meets the auricular surface of the ilium. See our guide to the sacrum for a full anatomical breakdown.

Unlike most joints, the SI joint has very limited motion: roughly two to four degrees of rotation and one to two millimeters of translation. It is reinforced by some of the strongest ligaments in the body because its job is structural — a shock-absorbing bridge between the axial skeleton above and the lower extremities below.

How Does the SI Joint Function?

The SI joint serves three primary roles in the pelvis-spine system.

• Load transfer. Every step transmits ground-reaction force upward through the pelvis and into the lumbar spine. The SI joint distributes that force across a wide articular surface, protecting lumbar discs from peak stress.
• Shock absorption. The cartilage and ligamentous complex acts as a mechanical buffer. When the joint degenerates, compressive load increases on adjacent lumbar segments.
• Pelvic ring stability. Working with the pubic symphysis, the SI joints lock the pelvic ring into a stable structure essential for walking and single-leg activities.

When the SI joint is dysfunctional, coordination between lumbar flexion and hip motion breaks down, increasing strain on the facet joints and discs. Patients with lumbar facet syndrome sometimes have concurrent SI joint involvement because both structures share the same load-transfer pathway.

Why Does the Sacroiliac Joint Fail?

Degenerative changes accumulate over time — part of the broader pattern of spinal osteoarthritis. Pregnancy and hormonal changes loosen the SI joint ligaments, necessary for childbirth but producing lasting hypermobility and pain in some patients.

Prior lumbar fusion is a documented upstream cause. When lumbar motion segments are stiffened surgically, mechanical demand transfers to the sacroiliac joints and accelerates degeneration — adjacent-segment disease at the lumbosacral junction. Nearly 1 in 5 patients told they need spine surgery choose not to proceed, in part to preserve the natural mechanics of the pelvis-spine complex.

Trauma and asymmetric loading from falls, accidents, or repetitive occupational postures can displace the SI joint or strain its ligaments. Lumbar instability shifts pelvic tilt and adds indirect stress to the SI joint.

Expert Take

Patients with SI joint dysfunction frequently arrive after months of treatment aimed at lumbar discs or facet joints — treatment that delivered little lasting relief because the source was never correctly identified. The SI joint’s pain patterns overlap significantly with disc and facet pain. A thorough diagnostic workup — including provocative physical tests and, when indicated, a fluoroscopically guided SI joint injection — is the only reliable way to separate SI joint pain from other sources.

How Is Sacroiliac Joint Dysfunction Diagnosed?

Diagnosis relies on clinical history, physical examination, and targeted diagnostic injections. No single imaging finding confirms SI joint dysfunction — MRI and CT reveal structural changes but do not reliably correlate with pain.

Clinicians use a battery of provocative tests — the FABER test, the distraction test, the thigh thrust test, and Gaenslen’s test. When three or more are positive, specificity for SI joint origin increases substantially. A fluoroscopically guided injection of local anesthetic is the closest available gold standard. Our companion guide on sacroiliac joint dysfunction covers the full diagnostic process and available treatments.

What Are the Non-Surgical Treatment Options?

• Physical therapy focused on pelvic stabilization, gluteal strengthening, and lumbopelvic motor control.
• SI joint injections — corticosteroid and local anesthetic delivered under fluoroscopic guidance.
• Radiofrequency ablation of the lateral branch nerves for longer-duration relief when injections confirm the SI joint as the source.
• Prolotherapy and biologic options targeting ligamentous laxity to restore joint support without surgery.

Patients with chronic low back pain frequently have overlapping pathology across discs, facets, and SI joints. When the clinical picture includes concurrent disc pathology, the treatment plan expands. A clinical evaluation is the only way to know which structure is driving your pain.

How Does the SI Joint Relate to the Lumbar Spine?

The L4-L5 and L5-S1 disc levels — covered in our guide to the L4-L5 disc — sit immediately above the sacrum and are most directly affected by sacroiliac mechanics. When the SI joint is hypermobile, altered pelvic tilt increases flexion stress on these discs. Conversely, degenerated discs that reduce height alter the lumbosacral angle and increase SI joint load.

Patients who have had prior fusion procedures and develop new low back pain should have the SI joints specifically evaluated — a pattern addressed in our guide to failed back surgery syndrome.

Frequently Asked Questions

What does sacroiliac joint pain feel like?

SI joint pain is most commonly described as deep, aching pain in the lower back or buttock, on one side. It worsens with prolonged sitting, standing, or climbing stairs, and improves with movement. The pain can radiate into the posterior thigh, groin, or lower abdomen — mimicking disc-related sciatica. Provocative physical examination tests and diagnostic injections are used to distinguish it from other sources.

Is the sacroiliac joint the same as the hip joint?

No. The SI joint connects the sacrum to the ilium of the pelvis. The hip joint connects the pelvis to the femur. They are anatomically distinct but biomechanically linked — dysfunction in one alters loading patterns in the other.

Can sacroiliac joint dysfunction develop after spinal fusion?

Yes. When lumbar motion segments are fused, the mechanical load transfers to adjacent structures including the SI joints. Accelerated SI joint degeneration following lumbar fusion is a documented clinical pattern and one reason some patients explore non-surgical options that preserve natural spinal mechanics.

How is sacroiliac joint pain different from disc-related back pain?

SI joint pain is typically unilateral, below the belt line, and reproduced by provocative pelvic tests. Disc-related pain is more often central or bilateral and reproduced by lumbar flexion or extension. Both can coexist, and accurate diagnosis requires clinical evaluation rather than imaging alone.

Does the sacroiliac joint show up on MRI?

MRI can reveal structural changes — erosion, sclerosis, inflammation — but imaging findings do not reliably predict which patients have pain originating from the joint. A fluoroscopically guided diagnostic injection remains the most reliable confirmation method.

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.