Journal of Bone and Joint Surgery, Jan 2010

"Is the frog-leg lateral hip radiograph reliable for assessing the alpha angle in femoroacetabular impingement?"
It is well established that radiographic assessment of the anterolateral aspect of the femoral head is a basic tool for evaluating cam-type femoroacetabular impingement. Computed tomography and magnetic resonance imaging, as well as plain lateral radiographs, have been proposed as options for imaging this type of lesion. The alpha angle is a reliable indicator of cam-type femoroacetabular impingement and can also be used to assess the success of corrective surgery.
The authors studied the alpha angles of 32 consecutive patients with femoroacetabular impingement. The angle was measured on frog-leg lateral radiographs using templating tools and was compared with measurements obtained by computed tomography, in order to assess the reliability of measuring the alpha angle on the plain frog-leg lateral radiograph in cam-type femoroacetabular impingement.
The authors concluded that measurement of the alpha angle on frog-leg lateral hip radiographs is not reliable. Several factors were held responsible for this, such as the radiographic projection, the position of the patient and the quality of the image. They also note that computed tomography may be necessary for accurate measurement of the alpha angle.