IS Case 406: Anterior cruciate ligament tear
Imaging Sciences URMC 2010
Publication Date: 2010-08-27
Coronal (Fig. 1) and sagittal (Fig. 2) fat-saturated, long TR images of the knee show high-signal within the anterior cruciate ligament with irregular contour, poor visualization and interruption of the fibers at the femoral attachment. A small joint effusion and soft tissue edema about the knee are also noted.
The normal anterior cruciate ligament (ACL) should have a taut, straight anterior margin with low signal intensity of the fibers on all pulse sequences. The proximal attachment is at the lateral femoral condyle posteromedial fossa. The ligament courses inferomedially and attaches anterolateral to the anterior tibial spine. On midsagittal images, the ACL should be parallel or near parallel to Blumensaat's line, approximately 55 degrees from the tibial plateau.
Direct imaging signs of acute ACL tear include poor or nonvisualization of the ACL, amorphous edematous mass with focal high T2 signal, irregular contour with wavy redundant fibers, and interruption of the fibers. The MR diagnosis of ACL tears are usually made by direct signs. Indirect signs may be useful when edema obscures the ACL and includes the MRI "drawer" sign where there is anterior subluxation of the tibia relative to the femur by 7 mm or greater on the lateral sagittal images. Bone injuries may also be indirect evidence of ACL tear such as bone contusions at the lateral femoral condyle and the posterior aspect of the lateral tibial plateau. Concavity of the lateral femoral sulcus greater than 1.5 mm, the "lateral femoral notch sign" on radiograph made also be seen on MRI.
Injuries to the ACL may occur in the mid portion (most common in contact sports), the femoral attachment (most common with skiing injuries) or injury to the osseous attachments. Associated injuries with ACL tears include medial collateral ligament and medial meniscus tears. The three parts of the O'Donoghue's Triad.
When differentiating chronic from acute ACL tear, one study found that the most useful MRI finding of an acute tear was edematous soft tissue in the intercondylar notch. Nonvisualization of the ACL was uncommon in chronic tears. The most useful finding to diagnose chronic ACL tear is abnormal course/focal angulation of the ligament without edema.
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