Irreducible lateral patellar dislocation: the importance of impaction fracture recognition.

Irreducible lateral patellar dislocation may occur in the older patient with a previous history of patellofemoral arthritis. The only subtle finding on physical examination to suggest this diagnosis will be positioning of the knee in less flexion than a typical lateral patellar dislocation, anterolateral position of the patella and internal rotation of the patella from the coronal plane. That is, the patella is dislocated laterally but the lateral border comes to lie in a position of variable degrees of anterior displacement relative to the medial patellar border. Plain x-rays may reveal the rotation of the patella along the vertical axis and an anterolateral rather than lateral positioning of the patella. Computed tomographic scanning is of benefit if the diagnosis is suspected or if an initial attempt at closed reduction is unsuccessful. Open reduction is recommended, if a single closed reduction attempt is not successful, to prevent any potential worsening of the patellar impaction fracture. A laterally dislocated patella that displays internal rotation about the vertical axis or the "flipped patella" sign is pathognomonic of an irreducible patellar dislocation and suggests patellar impaction on a lateral femoral condylar ridge osteophyte. Open reduction is easily achieved through a vertically oriented quadriceps tenotomy without the need for medial repair.