A method of assessing the severity of cam type femoro-acetabular impingement in three dimensions

Femoroacetabular impingement is caused by abnormal morphology of either the femur or acetabulum or both. Diagnostic criteria currently include an alpha angle of over 50° on a lateral radiograph. In this study, CT scans of symptomatic hips (n = 37) were compared with normal hips (n = 34) obtained from CT colonoscopy procedures. The femoral head described in terms of a three dimensional (3D) alpha angle and a 3D head neck margin (epiphysis) angle '3Dμ' using a semi-automated algorithm. In normal hips 70% have a maximum 3Dα angle of more than 50° at some point around their femoral head (mean 53° ± 5°, range 42° - 64°), while in cam hips, it was significantly larger (mean 69° ± 10°, range 54° - 94°, p<0.001). The 3Dμ also varied significantly and had a reverse relationship to that of the alpha angle: cam hips have an articular extent that crossed over spherical limit of the hip joint (mean minimum 41° ± 7°) while the articular margin of normal hips always remained within the spherical limit (mean minimum 49° ± 6°). This semi-automated algorithm provides an objective measure of the femoral head in health and disease. It can reliably distinguish cam hips from normal, enabling cam hips to have their cam quantified and their surgery planned objectively.