MR Evaluation of the “Arcuate” Sign of Posterolateral Knee Instability

OBJECTIVE. The purpose of this study was to evaluate associated knee injuries using MR imaging in patients with the “arcuate” sign, a term referring to avulsion fracture of the proximal fibula on conventional radiographs.MATERIAL AND METHODS. MR imaging of 18 cases (17 patients, both knees in one patient) with the arcuate sign on conventional radiographs was retrospectively interpreted to evaluate the associated meniscal, ligamentous, and bony injuries. In 12 cases, MR findings were correlated with surgical results.RESULTS. In all cases, avulsed bony fragments from the proximal pole of the fibula were attached to the fibular collateral ligament, the biceps femoris tendon, or both. Tear of the posterolateral capsule was seen in 12 cases (67%). Injury of the cruciate ligaments was noted in 16 cases (89%): injury to both the anterior cruciate ligament and posterior cruciate ligament was seen in nine cases (50%), injury to only the anterior cruciate ligament was seen in four, and injury to the posterior cruci...