Patellar dislocation. A and B, Axial fat-suppressed, proton density–weighted images through patellofemoral joint show regions of increased signal, representing marrow edema beneath medial facet of patella (long arrow) and in lateral aspect of lateral femoral condyle (thick arrow). This pattern of osseous contusion indicates recent lateral patellar dislocation. Note hematocrit level in joint effusion (arrowheads).