[Practical value of the descending ray angle in the axial femoropatellar study. Sensitizing role of external rotation of the skeleton of the leg in the 30 degree F.P angle].

The descending ray, or so-called "inverted", axial femoropatellar view, used much less frequently than the classical ascending view, is described. This view is easy to perform, but requires very rigorous positioning of the patient and the equipment. All of the practical details are illustrated by photographs. The advantages of this technique concern patient comfort, particularly easy control of quadriceps muscle relaxation, good reproducibility of the view and the ease with which axial femoropatellar views can be obtained at 30 degrees with external rotation of the leg. This manoeuvre considerably improves the detection rate, by standard radiology alone, of transient external instability of the patella, there by reducing the indications for CT scan.