Radio-anatomic position for the lateral radiographic view of the human patello-femoral joint.

Recent clinical and epidemiological studies have emphasised the importance of the patello-femoral joint (P-FJ) in osteoarthritis. X-ray examination of this joint for joint space width (JSW) assessment using the standard medio-lateral view is difficult. In 26 femora, the mean angle of inclination between the medial and lateral condyles measured at their inferior and anterior surfaces, relative to a line passing between the medial and lateral femoral epicondyles, was used to define the optimum alignment of the x-ray tube. The results showed that for medio-lateral radiography of the P-FJ, the central ray of the x-ray beam should be directed cranially, in a upward projection by 5 degrees and then anteriorly by 4 degrees, resulting in superimposition of the inferior and anterior condylar surfaces respectively. The value of these angles was confirmed by radiographs of the 26 femora and 10 post mortem joints. The margins for joint space width measurement were identified by embedding lead balls, within the cartilage of the cadaveric knees, and were defined as the median vertical ridge of the patella and the middle of the patella groove on the femur.