Discussion Author: Michael V Huppmann
MRI is frequently performed to evaluate for internal derangements of the knee. Commonly evaluated structures include the following.
The normal meniscus is a fibrocartilagenous, C-shaped structure that is uniformly low signal on both T1 and T2 sequences. Intrasubstance degeneration is present when there is signal within a meniscus that does not disrupt an articular surface. A meniscal tear is diagnosed when high signal contacts either the superior or inferior articular surface. Meniscal tears can be classified as oblique/horizontal, vertical longitudinal, radial, or complex depending on their orientation. A bucket-handle tear is a common vertical longitudinal tear in which the inner free edge of the meniscus becomes displaced into the intercondylar notch.
The anterior cruciate ligament (ACL) is a low-signal structure running between the posterolateral intercondylar notch and the medial tibial spine. Some linear, striated signal may be seen in a normal ACL at its insertion on the medial tibial spine. ACL tears are diagnosed when the ACL is not visualized or high signal interrupts the continuity of the fibers.
The posterior cruciate ligament (PCL) is a homogenously low-signal, curved structure shaped like a boomerang. Diffuse, intermediate signal throughout the ligament is indicative of a tear.
The medial collateral ligament (MCL) runs between the medial femoral condyle and inserts on the tibia. It is closely associated with the medial joint capsule and the medial meniscus. The normal MCL is uniformly low-signal. MCL tears can be graded as follows:
Grade 1 â€“ high-signal (edema or hemorrhage) in the soft tissues medial to the MCL
Grade 2 â€“ a partial tear indicated by high-signal in and around the MCL
Grade 2 â€“ a complete tear indicated by complete disruption of the MCL fibers
The lateral collateral ligament (LCL) is made up of the biceps femoris tendon, the fibular collateral ligament, and the iliotibial band. All are uniformly low-signal when normal. The biceps femoris tendon and the fibular collateral ligament form a conjoined tendon, which inserts on the fibular head. A tear is indicated by disruption of the fibers.
The extensor mechanism is comprised of the quadriceps and patellar tendons. These are best evaluated on sagittal images as low signal bands inserting on the superior pole of the patella and originating from the inferior pole and inserting on the tibial tuberosity, respectively. Striated signal within the distal quadriceps tendon at its insertion is normal. High signal within the tendon may represent a partial tear. Complete disruption is indicated by discontinuity of the tendon. Associated signal representative of edema may be seen within the patella or surrounding fat.
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