PRIOR LATERAL PATELLAR DISLOCATION - MR-IMAGING FINDINGS

PURPOSE: To illustrate the constellation of magnetic resonance (MR) fi ndings that suggest prior patellar dislocation. MATERIALS AND METHODS: A computer search of all MR examinations of the knee with a coded dia gnosis of patellar dislocation performed between February 1988 and Oct ober 1990 disclosed 22 cases (11 male and 11 female patients, aged 1-7 0 years). RESULTS: Eighteen cases (82%) demonstrated hemarthrosis, med ical retinacular disruption, and contusion of the lateral femoral cond yle. In 11 cases, the patella was displaced from the trochlear sulcus, but in no patient was there complete dislocation at the time of imagi ng. Axial short-inversion-time inversion-recovery (STIR) images demons trated subtle subchondral and cancellous signal intensity abnormalitie s that were often poorly appreciated on T2-weighted images. In 11 case s, patellar dislocation was not suspected before MR imaging. CONCLUSIO N: Hemarthrosis, medial patellar and lateral femoral contusion, and re tinacular disruption suggest prior lateral patellar dislocation. Becau se of its sensitivity to subtle intraosseous signal abnormality, axial STIR imaging is a valuable adjunct in the evaluation of the acutely i njured knee.