Circumferential and axial strain in the proximal femur: effect of prosthesis type and position

Because the Charnley-Müller prosthesis inserted in a varus position has been found subject to loosening, circumferential and longitudinal strain patterns were determined in proximal cadaver femora that had been instrumented with strain gauge rosettes and implanted with total hip replacement femoral prostheses. Both Charnley and Charnley-Müller prostheses were inserted in the varus, valgus, and neutral positions. An intact femur was similarly studied. Strain patterns in the test femora were altered by implant insertion, mainly by reducing longitudinal strain. Prosthesis type did not alter strain patterns but prosthesis position did. Valgus position produced compressive axial strain laterally and tensile strain medially at the tip of the prosthesis which was the reverse of that seen with varus position. Valgus position produced biaxial compressive strain at the medial calcar whereas varus position produced circumferential tension and axial compression at that location.