Acute dislocation of the patella: MR findings.
PURPOSE: The authors describe magnetic resonance (MR) imaging findings of acute traumatic dislocation of the patella. MATERIALS AND METHODS: MR imaging at 0.1 T was performed in 25 patients with an acute patellar dislocation, 24 of whom underwent surgery. All patients were young male conscripts, aged 18-22 years. RESULTS: Signal intensity abnormalities of bone, seen as areas of decreased signal intensity on T1-weighted images, were found in all 25 patients (100%). The lateral femoral condyle was involved in all cases. Signal intensity changes also appeared in the patella in eight patients and in the lateral tibial plateau in three. Osteochondral fragments seen at plain radiography and cartilage fissuring noted at arthroscopy were not visible at MR imaging. Injury of the medial patellar retinaculum was seen at MR in each case. All patients had a joint effusion. Lateral subluxation of the patella was a common finding. CONCLUSION: Constant findings at MR imaging were (a) contusion of the lateral femoral condyle, (b) tear of the medial retinaculum, and (c) joint effusion.