The collateral ligaments are best visualized by coronal MR images (Fig. 6-27). The medial collateral ligament appears as a narrow low–signal-intensity band extending from the medial epicondyle of the femur to an attachment on the anteromedial aspect of the tibia 5 to 6 cm below the joint line. It is overlaid at its tibial attachment by the tendons of the pes anserinus, which are separated from it by an intervening anserine bursa that is not visualized unless it is inflamed. Deep to the tibial collateral ligament, the medial capsular ligament, sometimes called the deep portion of the tibial collateral ligament, has femoral and tibial attachments close to the joint interval and deep attachments to the medial meniscus, referred to as the meniscofemoral and meniscotibial or coronary ligaments. Valgus and rotary stresses can injure the medial capsular ligament or the tibial collateral ligament, usually in that order (39). In a complete rupture (i.e., grade III injury), MRI can show discontinuity, serpentine ligamentous borders, and edema within adjacent connective tissues. In a partial tear (i.e., grade II injury) or in the case of microtears confined to the ligament substance (i.e., grade I injury), the ligament may show no discontinuity, but the overlying subcutaneous fat typically demonstrates edema and hemorrhage, which is indicated by moderate signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Injury to the tibial collateral ligament is commonly associated with injuries to the anterior cruciate ligament and medial meniscus.
FIGURE 6-27. Tibial collateral ligament tear (MCL). A: Coronal T2-weighted fat suppressed sequences of grade I injury. There is edema (short arrows) overlying the intact fibers of the MCL (long arrow). B: Normal MCL (arrow).
The lateral collateral complex commonly refers to the lateral supporting structures of the knee, whose main components are the iliotibial tract, the lateral collateral ligament, the long head of the biceps femoris, and the popliteofibular ligament. These structures are best seen on axial and coronal MR images as a low–signal-intensity band extending somewhat obliquely from the lateral femoral epicondyle to the fibular head. The lateral collateral ligament is usually injured by varus and rotary stresses to the knee, although its frequency of injury is less than that of the tibial collateral ligament. The MRI findings of the injured fibular collateral ligament are similar to those for the tibial collateral ligament.