Prior lateral patellar dislocation: MR imaging findings.

Abstract

PURPOSE: To illustrate the constellation of magnetic resonance (MR) findings that suggest prior patellar dislocation. MATERIALS AND METHODS: A computer search of all MR examinations of the knee with a coded diagnosis of patellar dislocation performed between February 1988 and October 1990 disclosed 22 cases (11 male and 11 female patients, aged 1-70 years). RESULTS: Eighteen cases (82%) demonstrated hemarthrosis, medical retinacular disruption, and contusion of the lateral femoral condyle. In 11 cases, the patella was displaced from the trochlear sulcus, but in no patient was there complete dislocation at the time of imaging. Axial short-inversion-time inversion-recovery (STIR) images demonstrated subtle subchondral and cancellous signal intensity abnormalities that were often poorly appreciated on T2-weighted images. In 11 cases, patellar dislocation was not suspected before MR imaging. CONCLUSION: Hemarthrosis, medial patellar and lateral femoral contusion, and retinacular disruption suggest prior lateral patellar dislocation. Because of its sensitivity to subtle intraosseous signal abnormality, axial STIR imaging is a valuable adjunct in the evaluation of the acutely injured knee.