Sacroiliac (SI) joint pain is a frequently overlooked cause of lower back and pelvic discomfort, yet it contributes to as much as 15-30% of all cases of chronic low back pain. The SI joints, located where the sacrum meets the iliac bones on either side of the pelvis, play a critical role in force transmission between the upper body and legs. Dysfunction or inflammation in this region can lead to significant discomfort and disability.
In this post, we will explore the anatomy and biomechanics of the SI joint, common symptoms, diagnostic challenges, and the current best practices for clinical evaluation and diagnosis.
Anatomy and Function of the SI Joint
The sacroiliac joints are two small, strong joints situated at the posterior aspect of the pelvis. They connect the sacrum—a triangular bone at the base of the spine—to the iliac bones of the pelvis. These joints are reinforced by a network of strong ligaments, including the anterior, interosseous, and posterior sacroiliac ligaments, which limit movement and provide stability.
Despite their limited range of motion (typically 2-4 mm of translation and 1-2 degrees of rotation), the SI joints are essential for:
- Load transfer during gait and lifting,
- Shock absorption between the spine and lower extremities,
- And pelvic stability, especially during pregnancy and postpartum.
Dysfunction of the SI joint can be caused by trauma, repetitive stress, inflammation, or biomechanical imbalances.
Common Causes of SI Joint Dysfunction
SI joint pain can result from a variety of mechanisms:
- Trauma: Falls on the buttocks, car accidents, or sudden twisting injuries.
- Pregnancy and postpartum changes: Hormonal changes and ligamentous laxity increase SI joint mobility.
- Leg length discrepancies or scoliosis: These asymmetries can stress the joint over time.
- Arthritic changes: Especially ankylosing spondylitis or osteoarthritis, which can inflame or degrade the joint surfaces.
- Post-lumbar fusion surgery: Biomechanical stress may shift to the SI joints after spinal fusion, increasing the risk of dysfunction.
Symptoms of SI Joint Pain
The symptoms of SI joint dysfunction can vary and often overlap with other conditions like lumbar disc herniation or hip pathology. Common complaints include:
1. Localized pain
- Usually felt on one side of the lower back, just below the lumbar spine.
- Can radiate into the buttock, groin, or posterior thigh (but typically not below the knee).
2. Pain with prolonged positions
- Sitting, standing, or lying in one position for too long can aggravate symptoms.
- Patients often report needing to shift positions frequently.
3. Pain during transitional movements
- Rising from sitting, getting out of a car, climbing stairs, or turning in bed may provoke sharp or aching pain.
4. Instability or stiffness
- Some patients describe a feeling of instability or locking in the pelvis.
- Stiffness is more common in cases of degenerative arthritis.
5. Tenderness to palpation
- The PSIS (posterior superior iliac spine) and surrounding area may be tender.
Diagnostic Challenges
SI joint dysfunction is notoriously difficult to diagnose due to its symptom overlap with lumbar spine and hip conditions. There is no single “gold standard” test, but a combination of clinical evaluation, patient history, and diagnostic imaging can help rule in or out the SI joint as the pain generator.
Clinical Evaluation and Diagnosis
1. Patient History
- A detailed history can point to SI joint involvement. Key factors include:
- Pain pattern and radiation
- Previous pregnancies or pelvic injuries
- History of lumbar fusion or pelvic asymmetry
2. Physical Examination
Multiple provocation tests are used to assess SI joint pain. A diagnosis is considered more likely if three or more of the following tests are positive:
a) FABER Test (Patrick’s Test)
- Flexion, abduction, and external rotation of the hip.
- Pain in the posterior pelvis suggests SI involvement.
b) Thigh Thrust Test
- Posterior shearing force applied through the femur in a flexed and adducted hip position.
- Reproduction of pain indicates possible SI dysfunction.
c) Gaenslen’s Test
- Patient lies near the edge of the table, with one leg hanging off and the other leg pulled toward the chest.
- Pain indicates sacroiliac stress.
d) Compression Test
- Downward pressure applied to the iliac crests while the patient lies on their side.
- Pain response may indicate inflammation or instability.
e) Distraction Test
- Force applied outward on the ASIS (anterior superior iliac spines) in a supine patient.
- Stresses the anterior SI ligaments.
A cluster of at least three positive tests out of the five has a reported sensitivity of 91% and specificity of 78% for SI joint-mediated pain.
3. Imaging
Imaging studies are often used to rule out other causes and confirm suspicion.
a) X-ray
- May show degenerative changes or joint space narrowing.
- Limited value in diagnosing non-degenerative SI pain.
b) MRI
- Can identify inflammation, sacroiliitis, or signs of spondyloarthropathies (such as in ankylosing spondylitis).
- STIR (Short-TI Inversion Recovery) sequences are particularly useful for detecting inflammation.
c) CT Scan
- More sensitive than X-ray for detecting joint erosions, sclerosis, and ankylosis.
- Useful in evaluating structural damage in chronic conditions.
4. Diagnostic Injections
The most definitive diagnostic tool for SI joint pain is a fluoroscopically guided intra-articular injection of local anesthetic into the SI joint.
- A ≥75% reduction in pain after the injection is considered diagnostic.
- Some practices follow this with a second injection for confirmation (double-block paradigm).
Differential Diagnosis
Because SI joint pain mimics other musculoskeletal and neurological conditions, clinicians must rule out:
- Lumbar disc herniation
- Facet joint syndrome
- Piriformis syndrome
- Hip osteoarthritis
- Trochanteric bursitis
- Lumbar radiculopathy
- Spondyloarthropathies
Conclusion
Sacroiliac joint pain is a complex and underdiagnosed condition that requires careful clinical reasoning for accurate identification. A detailed patient history, a focused physical examination using SI provocation tests, and confirmatory diagnostic injections are the most reliable approach to diagnosis. Imaging can support the clinical picture, especially in suspected inflammatory or degenerative cases.
Identifying SI joint dysfunction is essential for developing a tailored treatment plan, which may include manual therapy, exercise rehabilitation, dry needling, shockwave therapy, or in some cases, image-guided interventions. Left untreated, SI joint pain can become chronic and significantly impair function and quality of life.
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