Hip contact stress and femoral neck retroversion: a biomechanical study to evaluate implication of femoroacetabular impingement

The current literature on femoroacetabular impingement (FAI) is focused on acetabular orientation and femoral head asphericity, with little emphasis on the effect of version of the femoral neck. A biomechanical model was developed to determine the causative effect, if any, of femoral retroversion on hip contact stress and, if present, delineate the type of FAI with femoral neck retroversion. Five pairs of cadaveric hips (n = 10) were tested by loading the hip in 90° of flexion and measured the peak joint pressure and the location of the peak joint pressure. The experiment was repeated after performing a subtrochanteric osteotomy and retroverting the proximal femur by 10°. Ten hips were successfully tested, with one hip excluded due to an outlier value for peak joint pressure. Retroversion of the proximal femur significantly increased the magnitude of mean peak joint pressure. With retroversion, the location of the peak joint pressure was shifted posteroinferiorly in all cases. In conclusion, femoral neck retroversion increases peak joint pressure in the flexed position and may act as a cause of femoroacetabular impingement. The location of peak joint pressure suggests a pincer-type impingement with retroversion. The version of femoral neck should be assessed as a possible causative factor in patients with FAI, especially those with pincer-type impingement.