Sacroiliac (SI) Joint Injury from MVA

That location is the first clue. The "Fortin finger" sign — pointing with one finger to a spot one to two centimeters below and medial to the posterior superior iliac spine (just below and beside your belt line) — is one of the most specific physical findings in lumbar diagnosis, strongly suggesting SI joint origin. Central lumbar pain, by contrast, is felt in the midline or spreads symmetrically. SI pain also tends to worsen with single-leg activities (stairs, standing on one leg, getting in and out of a car) while purely lumbar pain is more likely to worsen symmetrically with flexion or loading.

The SI joint is built for stability, but that doesn't make it immune. Several crash mechanisms directly stress it. If your foot was on the brake at impact, the axial force traveling up through the leg transmits through the femur into the acetabulum and into the SI joint on that side — a very common driver-side injury. Side-impact crashes drive lateral force directly through the pelvis asymmetrically. And the posterior ligament complex (the strongest ligaments in the body) can still be sprained when the forces are high enough or when the joint is asymmetrically loaded, as in an angled collision.

This is the central challenge. Standard imaging rules out fractures, tumors, and inflammatory arthritis — it cannot directly diagnose SI joint dysfunction. The evidence-based approach uses the Laslett cluster of provocation tests: distraction, thigh thrust, compression, Gaenslen's, and sacral thrust. When three or more of these tests are positive, the research shows a high probability that the SI joint is the primary pain source. If the diagnosis remains uncertain despite positive provocation testing and an appropriate mechanism, a fluoroscopic-guided SI joint injection that eliminates pain by more than 75% serves as both diagnostic confirmation and therapeutic treatment.

SI joint pain frequently radiates into the buttock, posterior thigh, and occasionally below the knee — a pattern that closely mimics L5-S1 disc radiculopathy. The key distinction is that true SI joint referral rarely produces the specific calf, ankle, or foot distribution of nerve root compression, and it doesn't follow the tight dermatomal stripe of disc-related sciatica. Neurological signs (diminished reflexes, muscle weakness, sensory changes) point to nerve root involvement; their absence in the setting of buttock and posterior thigh pain with SI provocation testing points to the joint. Many patients come in already having been diagnosed with "sciatica" when the SI joint is the actual driver.

Most SI joint injuries from MVAs resolve over four to eight weeks with specific manipulation or mobilization, gluteal and pelvic stabilization exercises, and an SI support belt during the acute phase. Motor control retraining of the multifidus and deep gluteal stabilizers is particularly important because SI joint stability depends heavily on muscular control, not just ligament integrity. Injections (SI joint steroid injections or lateral branch blocks) are appropriate when conservative care has been given an adequate trial without sufficient improvement — not as a first step. A minority of patients with significant ligamentous disruption require longer timelines, and we coordinate with pain management when that threshold is reached.