Mechanical simulation of muscle loading on the proximal femur: analysis of cemented femoral component migration with and without muscle loading

Objective. This study examines the effect of including muscle forces in fatigue tests of cemented total hip arthroplasty reconstructions. Design. An experimental device capable of applying the joint reaction force, the abductor force, the vastus lateralis force, and the tensor fasciae latae force to the implanted femur is described. Background. Current in vitro fatigue tests of cemented total hip arthroplasty reconstructions do not apply physiological muscle loads. Experimental and numerical studies report significant differences in stresses obtained in the cement mantle depending on the loads applied. The differing stresses may alter the outcome of an in vitro test. Methods. Ten femoral components were reproducibly implanted into proximal composite femurs. Five of these femoral components were tested using a loadprofile which included muscle loading, five were tested without muscle loading. The migration of each femoral component was monitored continuously during dynamic fatigue tests. Results. Clinically comparable migration amounts were found for both sets of femoral components, with the femoral components tested with muscle loading experiencing lower mean migration, lower mean inducible displacement, and less experimental scatter. Conclusions. The inclusion of muscle forces seems to stabilise the femoral component during the test. In vitro fatigue tests of cemented total hip arthroplasty reconstructions should include muscle loading to provide increased confidence in the results obtained.