chapter 4:imaging the patellofemoral joint

Imaging the Patellofemoral Joint

"The value of experience is not in seeing much but in seeing wisely. "

—Sir William Osler

Standard radiography is sufficient in the evaluation of most patients with patellofemoral pain. Imaging the patellofemoral joint with techniques such as computerized tomography (CT), magnetic resonance imaging (MRI), and radionuclide scan (bone scan), however, may be very helpful in understanding more complex and resistant patellofemoral disorders. There are significant limitations of standard radiographs, and the student of patellofemoral disorders should be ready to request other appropriate diagnostic studies when necessary.


Anteroposterior (AP) View

With a patient supine on the radiograph table, there is a natural tendency for the legs to roll into external rotation. This must be controlled if the examiner wishes to obtain consistent and reproducible radiographs. In general, a standing AP view is most appropriate so that medial or lateral articular cartilage loss ("joint space narrowing") may be detected.

Turning the feet to straight AP alignment may introduce internal rotation and distort the patellar alignment. Fick (1) has shown that there is an average 12‑degree change of rotation by aligning the feet in this manner. By obtaining standing AP views of the knees in normal alignment for the patient, the relationship of the patella to the femur will be shown as it exists under normal standing conditions. Nonetheless, because there is some lateralization of the patella normally with the knees in full extension, the standing AP radiograph does not really provide much meaningful information except when there is more extreme malalignment or a lesion in the patella. Standing posterior to anterior (PA) views are most helpful in determining if there is medial or lateral compartment narrowing or evidence of arthrosis. These should be taken in full extension and at 30 degrees of knee flexion (10 degrees caudal). Also, evidence of other problems in the knee, which may be confused with a patellofemoral disorder, might be discovered on the PA radiograph. Finally, the examiner can determine if there is an unusually high‑ or low‑riding patella by carefully evaluating the AP (or PA) radiograph (Fig. 4.1). Patellar and condylar measurements may be taken also, if desired, from an accurate AP (or PA) radiograph.

In cases of anterior knee trauma, one may wish to obtain oblique radiographs (2), angling the radiograph tube 45 degrees both medially and laterally. These projections can reveal details about patellar fractures that are not detectable on standard AP or lateral radiographs.


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