MR imaging appearance of the extensor mechanism of the knee: functional anatomy and injury patterns.

Trauma to the extensor mechanism of the knee, a common clinical problem, can be accurately evaluated with magnetic resonance (MR) imaging. The extensor mechanism consists of the quadriceps muscle and tendon, patella, patellar tendon, and patellar retinacula. Injuries of these structures can be classified into partial and complete tears. Acute injuries are associated with edema, hemorrhage, and fluid collections; chronic injuries often demonstrate redundancy, atrophy, and retraction of the affected structures. MR imaging is useful in differentiating partial and complete tears and in evaluating tissue edema and hemorrhage. It also allows detection of unsuspected nondisplaced patellar fractures and chronic conditions due to repetitive trauma. Transient dislocation of the patella is an often clinically unsuspected entity for which MR imaging can serve an important diagnostic role. A detailed understanding of the functional and anatomic relationships of the extensor mechanism can greatly assist in interpretation of MR images of the traumatized knee.