Transient lateral patellar dislocation: diagnosis with MR imaging.

OBJECTIVE

Transient lateral patellar dislocation is frequently difficult to diagnose accurately on the basis of clinical findings. Accordingly, we studied the MR findings in 26 cases to determine if MR imaging is useful in establishing the diagnosis.

MATERIALS AND METHODS

A retrospective review of 1450 MR examinations of acutely injured knees was performed to determine the MR findings resulting from transient lateral patellar dislocation. A distinctive constellation of MR findings reflecting the known mechanism of injury indicative of transient lateral patellar dislocation was found in 26 patients. Patellar dislocation was not clinically suspected before imaging in 19 (73%) of these 26 patients; most patients had been referred for suspected injury of the cruciate ligaments or menisci. Axial, sagittal, and coronal MR images were obtained in all patients, and all had clinical follow-up. Correlation of MR findings with surgical findings was possible in six.

RESULTS

Specific components of the constellation of MR findings included disruption or sprain of the medial retinaculum in 25 (96%) of 26 patients, lateral patellar tilt or subluxation in 24 patients (92%), lateral femoral condyle contusion in 21 patients (81%), osteochondral injury in 15 patients (58%), and joint effusion in all 26 patients (100%). Concomitant injury to major ligaments or menisci was present in eight (31%).

CONCLUSION

Our findings suggest that patients with transient lateral patellar subluxation have a distinctive constellation of MR findings that can be used to distinguish this entity from other common knee injuries.