A typical sign on a plain knee radiograph

Cobby et al use a depth cut-off of 1.5 mm, which is three times the standard deviation above the norm. 1 Warren et al conclude that a torn anterior cruciate ligament is suggested by a depth of more than 2.0 mm. 2 An association between a lateral femoral notch sign and a torn anterior cruciate ligament is found in 0% to 26% of patients, depending on the definition used, the population, and the mechanism of injury. 3 4 5 6 Based on radiographic results from four (>2.0 mm) of 124 patients with a torn anterior cruciate ligament, Yu et al found a sensitivity of 3.2%, specificity of 100%, and positive predictive value of 100% for complete tears of the anterior cruciate ligament. 6 Gentili et al found a sensitivity of 19% and a specificity of 100% on magnetic resonance imaging (MRI) of a group of 89 patients (54 torn and 35 normal anterior cruciate ligaments), using a depth of >1.5 mm. 7 On MRI, depending on the location and depth, Grimberg et al found a sensitivity of 49% to 66% and a specificity of 70% to 89%. MRI findings confirmed the tear, together with a posterior root tear of the lateral meniscus, a grade 1 sprain of the medial collateral ligament, and extensive bone marrow oedema of the lateral femoral condyle and proximal tibia. The lateral femoral notch sign is indicative of anterolateral rotatory instability of the knee and warrants further evaluation of the soft tissue by MRI. 1 Cobby MJ, Schweitzer ME, Resnick D. The deep lateral femoral notch: an indirect sign of a torn anterior cruciate ligament.