The optimum knee flexion angle for skyline radiography is thirty degrees

There is wide variation in practice among orthopaedic surgeons regarding the use of skyline tangential patellar radiographs of the patellofemoral joint in patients with anterior knee pain. Various techniques are available for taking such radiographs and numerous radiologic parameters can be measured from them. There is no information as to which knee flexion angle is superior when taking skyline radiographs. The purpose of the current study was to compare the results of skyline radiographs taken at 30 degrees, 50 degrees, and 90 degrees knee flexion. Sixty-seven knees were studied prospectively. All radiographs were made anonymous and then were studied in random order by two observers. Five recognized patellofemoral parameters were recorded. All the parameters studied were measured most reproducibly when measured from the radiograph taken at 30 degrees knee flexion. The parameters were least reproducible when measured from the radiograph taken at 90 degrees knee flexion. In terms of detecting abnormality of the parameters measured, the radiographs taken at 30 degrees and 50 degrees were similar. The radiograph taken at 90 degrees knee flexion detected the fewest abnormalities. Skyline radiographic examination of the patellofemoral joint should be a mandatory part of the investigation of all knee problems. One radiograph, ideally taken at 30 degrees knee flexion, offers the best means of assessing the patellofemoral joint. There is no benefit to requesting multiple images at different angles of knee flexion.