The sacroiliac joint (SIJ) is a complex, diarthrodial-amphiarthrodial joint that serves as the primary transition point for load transfer between the axial skeleton and the lower extremities. It is estimated that the SIJ is the primary pain generator in approximately 15% to 30% of patients with chronic non-radicular low back pain. Despite its prevalence, diagnosing SIJ dysfunction remains a challenge due to the lack of specific radiographic findings and the significant overlap in pain patterns with lumbar disc and facet pathology.

1. Functional Anatomy and Biomechanics

The SIJ is unique because it is only “true” synovial in its anterior third, while the posterior two-thirds are syndesmotic, consisting of strong interosseous ligaments.

• Innervation: The innervation of the SIJ is highly variable and controversial. The anterior joint is supplied by the ventral rami of L4 and L5. The posterior joint, which is the primary target for interventionalists, is supplied by the lateral branches of the posterior primary rami of L4 through S3.
• Movement: The SIJ is built for stability, not mobility. It undergoes two primary types of movement: nutation (the sacral base moves anteriorly and inferiorly relative to the ilium) and counternutation (the sacral base moves posteriorly and superiorly). Excessive or restricted movement in these planes can lead to ligamentous strain and articular pain.

2. Clinical Presentation: The “Fortin Finger Test”

Patients with SIJ pain typically present with unilateral pain localized below the L5 level.

• Pain Referral: Pain often radiates to the buttock (94%), groin (14%), and occasionally the lateral thigh. It rarely radiates below the knee, which helps differentiate it from L5/S1 radiculopathy.
• The Fortin Finger Sign: This is a high-yield clinical pearl. A positive sign is when the patient can point with one finger to the exact area of pain, which is typically within 1 cm of the posterior superior iliac spine (PSIS).
• Aggravating Factors: Pain is usually worse with transitional movements (standing up from a chair), prolonged sitting, or climbing stairs.

3. Physical Examination: The IASP Provocative Battery

Because imaging often shows age-related changes that do not correlate with pain, the International Association for the Study of Pain (IASP) recommends a cluster of provocative maneuvers. Research by Laslett et al. suggests that if 3 out of 5 of these tests are positive, the probability of SIJ dysfunction is high.

1. Distraction Test: The physician applies a posteriorly directed force to both ASIS (stresses the anterior SIJ ligaments).
2. Thigh Thrust: The patient is supine with the hip flexed to 90 degrees; the physician applies a downward pressure through the femur (stresses the posterior ligaments).
3. Compression Test: The patient is side-lying, and the physician applies a downward force on the iliac crest.
4. Gaenslen’s Test: The patient lies near the edge of the table, hugging one knee to the chest while the other leg hangs off the table in extension.
5. Sacral Thrust: The patient is prone, and a vigorous downward pressure is applied to the center of the sacrum.

4. Diagnostic and Therapeutic Injections

Like the facet joints, the gold standard for diagnosis is a fluoroscopically guided intra-articular injection of local anesthetic.

• The Injection Technique: The needle is typically directed into the inferior third of the joint, where the synovial portion is most accessible. A “positive” block is generally defined as $\ge$75%–80% pain relief for the duration of the anesthetic.
• Therapeutic Options: Corticosteroids are commonly used for inflammatory sacroiliitis. For patients with recalcitrant pain who respond only briefly to injections, Radiofrequency Ablation (RFA) of the lateral branches is an option.

5. Lateral Branch Radiofrequency Ablation (RFA)

RFA of the SIJ is more complex than facet RFA because the innervation is a “plexus” rather than a single predictable nerve branch.

• The Target: The L4 and L5 primary posterior rami and the S1-S3 lateral branches.
• Technique: Approaches include the “leapfrog” technique or the use of specialized bipolar or cooled RFA probes to create a continuous “strip” lesion along the lateral edge of the sacral foramina. This interrupts the sensory transmission from the posterior joint capsule.

6. PM&R Integration: The “Form and Force Closure”

From a physiatric perspective, SIJ pain is often a failure of the “closure” mechanisms of the pelvis:

• Form Closure: The stability provided by the interlocking bone shapes and ridges of the joint.
• Force Closure: The stability provided by the muscles and fascia (specifically the gluteus maximus, latissimus dorsi, and the thoracolumbar fascia).

A PM&R-led rehabilitation program focuses on strengthening the “posterior oblique sling” and the pelvic floor. If a patient has a “leg length discrepancy,” an orthotic or shoe lift may be indicated to balance the pelvic shear forces. This is a critical distinction for the boards—interventional treatments provide the window of relief, but biomechanical correction prevents the return of the shearing force.

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High-Yield Board “Fast Facts”

• HLA-B27: Always consider Ankylosing Spondylitis in a young male with bilateral SIJ pain and morning stiffness that improves with exercise.
• Goldthwait’s Test: Used to differentiate between lumbar spine and SIJ pain. If pain occurs before the lumbar spine moves during a straight leg raise, the SIJ is the likely culprit.
• Pregnancy: SIJ pain is highly prevalent in the third trimester due to the hormone relaxin, which increases ligamentous laxity.
• Imaging: The “vacuum phenomenon” (gas within the joint space) is often seen on CT and is usually a sign of degenerative joint disease rather than infection.