chapter 10: Patellar Dislocation

Radiologic Features

Radiologic evaluation of the patient with recurrent patellar dislocations can help es­tablish the degree of traumatic articular injury, evidence of loose bodies, and predis­posing malalignment.

Standard anteroposterior, lateral, notch, and tangential (axial) radiographic views, as described in Chapter 4, will be adequate in many cases. If surgery is planned for re­current dislocations that cannot be managed nonoperatively, CT of the patellofemoral joint will give the best overall view of patellar behavior through a range of motion. If a pattern of tilt with minimal or no subluxation is found, lateral release alone may be suf­ficient. However, the majority of these patients have substantial subluxation, and the CT data can be helpful in understanding trochlear morphology as well as the amount of malalignment and associated tilt. This may help the surgeon develop an appropriate operative plan.

In the patient with recurrent patellar dislocations, there may be soft‑tissue calcifi­cation in the traumatized medial retinaculum (Fig. 10.4). Not infrequently, the medial border may show a defect where an osteochondral fracture has occurred during the dis­location (Fig. 10.5). Finally, the medial patellofemoral ligament may have pulled off a small fleck of bone at the time that it was torn. Sometimes this is visible on high‑qual­ity films (Fig. 10.6).

Osteochondral fracture with subsequent loose body formation is a frequent sequela of patellar dislocation. Ficat and Bizou (13) reported an incidence of 25%. This may not always be evident clinically if there is not the typical interference with mechanical function of the joint. A radiodense fleck seen in an unusual position should raise sus­picion of a loose body. These loose bodies may be of four origins: (1) from the carti­laginous surface of the patella, in which case they are generally small and invisible on routine radiograms; (2) from an osteochondral fracture of the medial patella as it passes over the lateral trochlea at the time of relocation; (3) the fragment may be pulled off by the medial patellofemoral ligament, in which case it is generally not free in the joint; or (4) a fragment from the lateral trochlea, sheared off at the time of patellar relocation.




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