9riginal Artich Plain Radiographic Findings in Anterior Cruciate Ligament Injury and Repair Stefan Tigges, M.D.,* B. J. Manaster, M.D., Ph.D., ~ and Gordon Carson, M.D.* *Department of Radiology, Emory Clinic, Emory University School of Medicine, Atlanta, Georgia *Department of Radiology, University of Utah Medical Center, Salt Lake City, Utah *Department of Radiology, Grady Memorial Hospital, Atlanta, Georgia Although magnetic resonance imaging is the most accurate imaging method of evaluating the anterior cruciate ligament, several plain radiographic signs suggestive of anterior cruciate ligament injury have been de- scribed. Plain radiographs also play an im- portant role in evaluating anterior cruciate ligament reconstruction. ~e t Wordi Knee; Anterior cruciate ligament; Injury; Fracture; Ligament reconstruction; Radiographs Address correspondence and reprint requests to: Stefan Tigges, M.D., Emory University Schoot of Medicine, Emory Clinic, 1365 Clifton Road, NE, Atlanta, GA 30322. 1070-3004/96/105-112/$3.00 Volume 3, Number 3 American Society of Emergency Radiology © 1996 T he anterior cruciate ligament (ACL) is one of the most important stabi- lizers of the knee (1). Although magnetic resonance imaging (MILl) un- questionably is the most accurate noninvasive method for evaluating this ligament, at least five indirect plain radiographic signs indicating ACL injury have been described. Many, if not most, patients with an ACL tear will have normal plain radiographs, but radiologists should be able to recognize those subtle findings that indicate ligamentous injury. In this article, we will review the normal anatomy and function of the ACE, mechanisms of injury, and plain radiographic signs ofligamentous injury. We will also discuss ACL repair prin- ciples and complications. NORMAL ACL ANATOMY, FUNCTION, AND MECHANISM OF INJURY The ACL is a cordlike structure that extends from the posteromedial aspect of the lateral femoral condyle to insert on the tibia anterior to the medial tibial spine (2). This course from the posterior aspect of the femur to the anterior tibia enables the ACL to limit anterior motion of the tibia with respect to the femur (Fig. 1). The ACL's secondary role as a restraint to hyperextension of the knee can also be partially be explained by its posterior to anterior course. Its oblique course from the lateral femur to the medial tibia may account for the ligament's secondary function of limiting internal rotation of the tibia (Fig. 1). The ACL is made up of multiple fascicles that are arranged in a spiral (2); this arrangement may serve to increase the strength of the ligament. The ACL also plays a minor role in resisting varus and valgus forces. A variety of mechanisms may result in ACL injury. In contact sports, a blow to the posterolateral aspect of the knee with the foot planted results in anterior and valgus forces which may cause an ACL tear. If the medial collateral ligament and medial meniscus are also torn, the injury is known as "O'Donahue's un- happy triad." The ACL also is frequently injured as a consequence of extreme internal rotation of the tibia with respect to the femur while the knee is ex- tended. Hyperextension may lead to rupture of the ACL with or without an as- sociated posterior cruciate ligament (PCL) tear. PLAIN RADIOGRAPHIC FINDINGS IN ACL INJURY In our experience, most patients with an ACL injury will have normal plain radiographs. However, on numerous occasions, we have correctly diagnosed Emergency Radiology • May/June 1996 Anterior Cruciate Ligament Injury • 105