Optimum abductor moment arm for total hip arthroplasty

Introduction: To ascertain the optimal functional abductor moment arm of the hip for THA, we focused on the Trendelenburg sign. We investigate the various conditions associated with abductor moment arm to achieve a negative Trendelenburg sign postoperatively. Methods: We reviewed 30 patients (34 uncemented primary THAs; mean age, 56 years) at a minimum of two years postoperatively. The tilting angle of the pelvis while performing the Trendelenburg test (Trendelenburg angle) was measured using a magnetic sensor system. On the hip radiographs, the %FO was calculated by dividing the femoral offset, by the distance between the centers of the bilateral femoral heads, and by multiplying by 100. Results: The Trendelenburg angle averaged −4.3 degrees (−9 to −2.0) in all cases with a positive Trendelenburg sign, whereas it averaged +1.4 degrees (−2.0 to +12.0) when the Trendelenburg sign was negative. The %FO having a positive Trendelenburg sign (16.9 %; 10.0 to 22.5) were significantly shorter than those having a negative Trendelenburg sign (19.4 %; 13.5 to 24.7). The Trendelenburg angle correlated positively with %FO. Of the cases with a %FO value of more than 20%, about eighty percent of the cases had a negative Trendelenburg sign. Discussion: The optimum abductor moment-arm, when the Trendelenburg sign is negative, has not been described. This study indicates that a %FO of 20 % may be one of the factors taken into account when determining the suitable size and position for acetabular and femoral neck components.