Introduction: Although bone marrow edema (BME) of the knee is a common phenomenon, physical tests to diagnose this condition have not been investigated thus far. We hypothesized that a mallet test would be useful as a diagnostic aid as well as a screening tool.

Methods: Seventy patients (36 women, 34 men) were investigated in this blinded controlled study. Group 1 consisted of patients with painful BME in the knee and group 2 of patients with a painful knee without BME. Pain provoked by a reflex mallet was assessed for each quadrant on a visual analog scale (VAS).

Results: The VAS score was 3.7 (±2.1 cm) for quadrants affected by BME (group 1), 1.59 (±1.44) in non-affected quadrants of the knee affected by BME (group 1) and, 0.85 (±0.85) in painful knees without BME (group 2). Pain on the tapping test was significantly correlated with the presence of BME in the affected knee (p< 0.0001) as well as the affected quadrant (p< 0.0001 for the medial femoral condyle and the medial femoral plateau).

Conclusion: The tapping test is a good screening instrument to diagnose BME in the knee.