Patellar dislocation MRI imaging

Dislocation of the patella is another cause of post-traumatic haemarthrosis. The diagnosis if often not suspected by clinician or patient. Dislocation may result from a relatively innocuous twisting injury. MRI will show a characteristic appearance with subcortical oedema at the anterolateral aspect of the femoral condyle at the site of patellar impaction. There may be corresponding oedema of the medial aspect of the patella or signs of a medial patellar osteochondral fracture with or without loose body. The medial retinaculum is usually torn


After an acute trauma to the knee, severe pain and effusion may limit the clinical examination, and functional tests may be precluded. Some patients may be aware that patellar dislocation has occurred, or the patella is still displaced at clinical examination (Fig 9a). However, the exact mechanism of injury may not be elucidated if the patient does not recall the event or if the patella has spontaneously returned to its normal position. Thus, patellar dislocation is not suspected beforehand in as many as 50% of the patients undergoing imaging. Marked knee effusion and internal derangement of the knee may be present.