Appropriate incidence angle for fundamental research on new skyline radiography development.

Our purpose in this study was to elucidate the relationship between the angles formed between the anterior patella, tibial tuberosity, and the knee joint cavity and the flexion angle, sex, and age of the subjects. We investigated 368 images of 280 patients ranging in age from 16 to 60 years (179 knees of 150 men, mean age 36.4 years; 189 knees of 130 women, mean age 41.4 years) who underwent lateral radiography of the knee. The tibial tuberosity on the lateral radiograph of the knee was defined as a reference point, and a line tangent to the anterior patella was used as a reference line. The angle between the reference line and the straight line from the reference point to the knee joint cavity (incidence angle) was measured. The average incidence angle was 19° (SD 1.8°). There was almost no correlation between the incidence angle and the flexion angle, and neither the knee flexion angle nor age had any influence on the incidence angle. There was a difference between the sexes in the average incidence angle, but this difference was less than 1°. Further study on the same patients is required for comparison of this technique with the conventional technique that uses the femur for reference.