[Echography in the diagnosis of patellar instability]

The authors examined with US the femoropatellar relationship on the lateral aspect of the knee in 20 patients (40 knees) with clinical symptoms of patellar instability; 20 healthy subjects (40 knees) were also studied as a control group. US examinations were performed with a 7.5 MHz probe, positioned perpendicular to the lateral aspect of the knee on a plane passing through femoral epicondyle and the middle third of patella. The results were compared with those obtained by means of conventional radiologic techniques and CT. US was useful to show lateral shift of patella, also in fully extended or in 30 degrees flexed knees-i.e., in the cases where conventional radiology meets with the greatest difficulties. CT is much more accurate but certainly more expensive and often not available. With progressive flexion of the knee, the gradual medialization of patella can be followed, which will be complete at a variable degree of flexion according to the initial lateral shift.