The effect of abductor forces on femoral stem stresses in cemented prosthetic hip replacement

Abductor insufficiency has been reported as a clinical factor contributing to femoral stem fracture. The objective of this study was to investigate the stresses in the femoral stem as it is affected by abductor insufficiency.A two-dimensional plane stress finite element model of a femur with a cemented straight stem prosthesis, with the material properties of either Co-Cr-Mo alloy or Ti-6AI-4V alloy, was constructed. Each femur/prosthesis composite was then loaded, using a one-legged stance condition, accounting for both joint reaction and abduction forces. Stress profiles along the medial and lateral aspects of the femoral component were then calculated.The finite element models were also loaded using conditions representing abductor insufficiency with walking aids and without walking aids (Trendelenburg's gait). The abduction force was reduced to zero, and the joint reaction force reduced to superimposed body weight at a vertical angle to simulate Trendelenburg's gait with the pelvis tilted. To simulate the conditions with walking aids, the pelvis was maintained level, the joint reaction force was maintained constant, and the abductor force was varied. Stress profiles were again determined along the medial and lateral aspects of the femoral component.The prosthesis loaded under Trendelenburg conditions showed decreased femoral stem stresses, both medially and laterally, when compared to the normal physiological (one-legged stance) loading. The titanium alloy prosthesis demonstrated lower stem stresses in each loading condition when compared to the Co-Cr-Mo alloy prosthesis.The results of the study indicate that during the one-legged stance a patient with abductor insufficiency (who shifts his weight over his prosthesis) actually places less stress on his femoral component than the patient with normal abduction strength, and probably is not a major consideration in femoral component failure.