The Cruciate and Collateral Ligaments

Abstract

The anterior and posterior cruciate ligaments are the major static stabilizers of the knee each commonly comprising two main bundles of collagen fibers. Complete tears of the anterior cruciate ligament are common but complete tears of the posterior cruciate are not. The two bundles of the anterior cruciate ligament, the anteromedial and posterolateral, are each preferentially loaded in different positions of flexion which then predispose to typical patterns of partial-thickness tear. Normal variants that can confuse MR interpretation include the triple bundle ACL, the anteromedial meniscofemoral ligament, and ligamentum mucosum. Specific secondary ACL injuries include bone contusions and collateral ligament injuries. The medial collateral ligament, comprising superficial and deep components, forms a rigid medial stabilizer and usually tears proximally, whereas the lateral collateral ligament complex, comprising the iliotibial band, the anterolateral and fibular collateral ligament, popliteus, and the biceps femoris tendon, forms a more dynamic stabilizer. The fibular collateral and anterolateral ligament tend to tear close to the tibial insertion. Small condensations of capsule extend the collateral ligament structures anteriorly and posteriorly. These are difficult to visualize on MR imaging edema and avulsion fractures can be attributed to these structures. Conventional radiographs of the knee are a mandatory component of the imaging assessment of the knee because small flake avulsion fractures can indicate major destabilizing injuries but are difficult to see on MRI.