Tapping test in patients with painful bone marrow edema of the knee

Background: Although bone marrow edema (BME) of the knee is a common phenomenon, physical tests to diagnose this condition have not been investigated. We hypothesized that a mallet test would be useful as a diagnostic aid and a screening tool. Methods: Seventy patients (36 female, 34 male) were investigated in this 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 nonaffected 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) and the affected quadrant (P < 0.0001 for the medial femoral condyle and the medial femoral plateau). A threshold value of VAS 2.0 for a tapping test to be positive in the distinction of BME affected quadrants of group 1 and all quadrants of group 2 sensitivity was 90.4%, specificity 83.7%, positive predictive value 73.4%, and negative, predictive value 94.6% was determined. Conclusion: The tapping test is a good screening instrument to diagnose BME in the knee.