 The anatomical contoured edge of both the cup and the head reduces the incidence of psoas impingement. The iliopsoas tendon is stretched over the femoral head when the hip is extended. In the normal hip, the tendon runs over the front edge of the acetabulum, and femoral head, which it uses as a fulcrum giving some leverage advantage as the muscle contracts to lift the leg up when bringing the leg up into the bath or into a high car for instance. When the femoral head is resurfaced, the tendon has to rub over the hard edge of the resurfacing device. Until now, femoral resurfacing devices have had a symmetric rim, which tends to extend beyond the normal limits of a femoral head, particularly in female hips. This overhang can cause painful abrasion of the tendon. Most acetabular components are also symmetric in shape, unlike the natural acetabular rim contour, which has a recess where the tendon runs. The rim of the acetabular shell can also be a cause of tendon irritation. 