Abstract
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.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
