Test yourself—knee pain after falling

Abstract

The semimembranosus muscle originates at the posterolateral aspect of the ischial tuberosity. Distally, the semimembranosus tendon divides into five anatomically distinct tendinous branches, each inserting into the posteromedial region of the knee [1]. The insertion sites include the infraglenoid tubercle of the posteromedial tibial plateau, the posterior capsule, the posterior horn of the medial meniscus, and a portion of the capsule deep to the medial collateral ligament [2]. The function of the semimembranosus muscle is flexion and medial rotation of the knee [3]. The anterior cruciate ligament (ACL) attaches proximally to the posteromedial aspect of the lateral femoral condyle and courses anteromedially to insert anterior to the intercondylar eminence [4]. Functionally, the ACL can be divided into an anteromedial bundle and posterolateral bundle, which prevent anterior translation and internal rotation of the tibia, respectively [2, 4]. An avulsion fracture of the semimembranosus insertion site is a rare, but a highly specific, ancillary finding of cruciate ligament injury [2]. Radiographic findings demonstrate an avulsion fracture of the posteromedial tibial plateau, which is mildly superiorly displaced (Fig. 1). Magnetic resonance (MR) imaging of the knee confirms the avulsion of the semimembranosus central tendon at its insertion on the infraglenoid tubercle of the posteromedial tibial plateau (Fig. 2). Additional findings include a complete ACL tear and bone contusions in the lateral aspect of the knee joint (Figs. 3, 4). In a retrospective review of patients with acute knee injury, 100% of patients with abnormalities of the posteromedial tibial plateau of the semimembranosus insertion site had an associated complete ACL tear [2]. These abnormalities included fractures of the posteromedial tibial plateau or bone bruises and avulsions of the semimembranosus tendon insertion site. Some of these patients lacked bone contusions in the lateral compartment of the knee. This, as well as the lack of bone marrow edema in the medial femoral condyle, may help distinguish semimembranous avulsions from other bone marrow contusion patterns of the knee. Prior to this case, only three other studies of avulsion fractures of the semimembranosus insertion site and ACL tear had been described [2, 5, 6]. An avulsion fracture of the posterior medial plateau is not necessarily indicative of ACL injury, however, as a single case of a posterior cruciate ligament tear with semimembranosus avulsion has been reported [7]. The exact mechanism of injury of cruciate ligament tears and semimembranosus avulsion is debated. Cadaveric The case presentation can be found at doi:10.1007/s00256-010-1060-8.