Femoral component offset. Its effect on strain in bone-cement

The magnitude of the offset of the femoral prosthesis strongly influences the mechanics of the hip following a total hip arthroplasty. An increased offset increases the moment arm of the abductor muscles. This reduces the abductor force required for normal gait and, consequently, reduces the resultant force across the hip joint. These factors are advantageous. However, increased offset also increases the bending moment on the implant, which could adversely increase the strain in the medial cement mantle. To evaluate the relative advantages and disadvantages of these conflicting results of increasing the offset of the femoral component the authors measured in vitro in cadaver femora the effect of differing offsets of the femoral component on strain in the cement mantle. After testing the intact femora, the authors cemented femoral prostheses in place and quantified the abductor force, resultant force, and strain in the cement mantle under loading conditions simulating single limb stance at different femoral offset levels. The reduction in both abductor and resultant force was substantial with increased femoral component offset, but the strain in the cement of the proximal medial portion of the cement mantle was not significantly increased.