chapter 10: Patellar Dislocation

Radiologic Features

At the time of acute patellar dislocation, the diagnosis may be obscure. As part of the initial evaluation, radiographs should routinely include the standard standing anteroposterior (AP) view, the lateral view, a patellar tangential view, and a "notch" view. These images may be completely unremarkable, but the trauma of dislocation may leave some residual lateral placement of the patella, either as an indication of baseline malalignment, or as a result of medial retinacular disruption. The tangential radiogram, therefore, may indicate malalignment (primary or secondary). Any of the other views may reveal a small fleck of bone that will be attached to articular cartilage (sheared off the patella or lateral condyle at the time of dislocation and relocation). A defect in the medial patella or lateral trochlea may be detected on the tangential view in some patients. If there is no articular injury, or injury is limited to articular cartilage, radiographs may be completely normal. The notch view may be helpful if a loose fleck of bone with overlying articular cartilage has been displaced and dropped into the intercondylar notch.

Of course, the astute clinician will be looking for other findings on the radiograph that are suggestive of intra‑articular lesions such as osteochondritis dissecans, calcified meniscus, avulsion of bone by the anterior cruciate ligament, or fracture.

The lateral radiograph is very sensitive for detecting tilt of the patella. A standing true lateral at 0 and 30 degrees of knee flexion, with posterior condyles overlapped, is extremely helpful for detecting malalignment in the patient who has experienced dislocation (see Chapter 4).

When the patient is first seen, CT, MRI, and arthrography are not usually necessary unless the clinician needs to know more about the extent of injury. Certainly, if meniscus or cruciate ligament damage is in question, MRI may be helpful. Serial transverse sections of the patellofemoral joint at 20 degrees of knee flexion should be done in every complete MRI study. Lance et al (29) noted that hemarthrosis, medial patellar and lateral femoral contusion, and retinacular disruption suggest patellar dislocation on MRI. Virolainen et al noted contusion of the lateral condyle, tear of the medial retinaculum' and joint effusion as the principal MRI findings following acute patella dislocation (30).





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