Radiographic diagnosis and accuracy in knee joint effusions.

Anteroposterior and lateral knee radiographs were obtained prior to arthrography in 200 patients. Presence and quantity of joint effusion were recorded, and radiologic criteria for the presence or absence of knee effusion were evaluated. Only the lateral projection was of value in assessing joint fluid. The fat pad separation sign was the most accurate indicator of fluid and effusions as small as 1-2 ml were recognized. There were accuracies of 88% in the diagnosis and 90% in the exclusion of knee effusions. Satisfactory lateral knee radiographs required radiologic demonstration of the entire suprapatellar area. Minimal knee flexion and a 5degrees cephalad angulation of the x-ray tube help in adequately visualizing this area.