MR imaging of injuries of the extensor mechanism of the knee.

Rupture of the extensor mechanism of the knee must be diagnosed early in order to be successfully treated. However, the clinical signs of this condition may be concealed by hematoma or hemarthrosis. The noninvasiveness and accuracy of magnetic resonance (MR) imaging make it useful in detection of such ruptures. Normal quadriceps tendons have a laminated appearance on MR images. The normal patella has signal intensity similar to that of bone marrow and cortex, while normal patellar tendons have homogeneous low signal intensity with all sequences. Patellar fractures, bone bruises, and avulsion of the tibial tubercle manifest as changes in marrow signal intensity. The laminated configuration of the quadriceps tendon allows distinction between partial and complete tears; both may be accompanied by edema and hemorrhage, which manifest as increased signal intensity on T2-weighted images. Ruptures of the patellar tendon also manifest as increased signal intensity on T2-weighted images.