The frog-leg lateral radiograph accurately visualized hip cam impingement abnormalities

Radiographic evaluation of the anterolateral femoral headneck junction is essential in diagnosing cam femoroacetabular impingement. We hypothesized the frog-leg lateral radiograph can accurately assess femoral head-neck offset abnormalities associated with cam femoroacetabular impingement. We reviewed the radiographs of 61 hips treated for cam impingement and 24 asymptornatic control hips. To characterize the anatomy of the femoral head-neck junction, the femoral head sphericity, the a-angle of Notzli et al, and headneck offset were measured on all radiographs. Asphericity of the femoral head was detected more frequently in hips with impingement on all radiographic views when compared with control hips. The average a-angle was greater in hips with impingement on all views, with the greatest difference between hips with impingement and control hips seen on the frog-leg lateral view (65 degrees for hips with impingement versus 47 degrees for control hips). The average head-neck offset was decreased in hips with impingement on all views. The greatest difference between groups was seen on the frog-leg lateral view (6.6 mm for hips with impingement versus 9.3 mm for control hips). The frog-leg lateral radiograph provides accurate visualization of the femoral head-neck offset in patients being evaluated for femoroacetabular impingement. Level of Evidence: Level II, diagnostic study. See the Guidelines for Authors for a complete description of levels of evidence.