Findings: A joint effusion with fluid-fluid level, a medial patellar retinacular tear, abnormal lateral patellar position, medial sided patellar chondral defect, and a bone bruise are seen on the spin density weighted axial image. The bone bruise is more obvious on the STIR coronal image.

Diagnosis: Transient lateral patellar dislocation.

Discussion: Transient lateral patellar dislocation is now well accepted as an injury that occurs in recreational and high-performance athletes. Although accounting for only 2 to 3 percent of all knee injuries, they account for 9 to 16 percent of injury in young athletes with hemarthrosis. Traditionally a clinical diagnosis, this injury is often difficult to differentiate from injuries that are more common. It is even more difficult in the acutely swollen, painful knee.

The specific injuries seen on MR images can be accounted for by the mechanism of injury. Patellar dislocation most commonly results from a twisting motion, with the knee in flexion and the femur rotating internally on a fixed foot. Consequently, the patella is pulled out of the trochlea and over the lateral femoral condyle. The medial patellar facet impacts against the lateral femoral condyle, producing trabecular microfractures of osteochondral injury or both seen on MR as bone marrow contusion or fracture (Fig 1).

Injury to the medial retinaculum, formed by the distal fascial investments of the vastus medialis, ranges from sprain to complete disruption or avulsion, characterized by loss of continuity of retinacular fibers (Fig. 2). Patellar position alone cannot be relied on to determine previous dislocation as some degree of lateral subluxation can be seen in normal knees imaged in extension.

It is important to make the proper diagnosis, as patients tend to have recurrent episodes of dislocation that lead to extensive damage of the articular surfaces of the knee. Suspicion should be high in all injuries with the associated mechanism of insult, patients with hemarthrosis secondary to acute injury, and young athletes, particularly females as they differ from males in the angle of their femora. Evidence of osteochondral patellar fracture or fracture of the medial patellar facet is associated with lateral patellar dislocation, and should not be overlooked. Finally, suspicion should remain high in any patient known to have weakness or paralysis of the vastus medialis.

Reference:

1. Kirsch MD, Fitzgerald SW, Friedman H, Rogers L. Transient
   Lateral Patellar Dislocation: Diagnosis with MR Imaging. American Journal
   of Radiology. 1993;161:109-113.
2. Delzell PB, Schils JP, Recht MP. Subtle Fractures About the Knee:
   Innocuous-Appearing Yet Indicative of Significant Internal Derangement.
   American Journal of Radiology. 1996;167:699-703.
3. Moore KL. Clinically Oriented Anatomy. Williams and Wilkins, 1992.