Abstract
Fixation of the femoral component in total hip arthroplasty is a surgical challenge in the best of circumstances. Achieving immediate fixation without breaking the femur is a challenge, even to highly skilled and experienced arthroplasty surgeons. Surgical procedures are available that offer cosmetic benefit or a slightly quicker recovery, but accepting this compromise for fixation should be avoided.
The mechanical environment of the femoral component is challenging. The implant is exposed to offset loading in two planes, which results in substantial bending stress and rotational loads in the frontal plane as well as bending stress and torsional loads in the sagittal plane. The proximal femur is built to transmit load through the surface fibers, not the central cavity, so efforts to change it to a centrally loaded structure are fraught with difficulty. The entire structure should be used. The least reliable material is the cancellous bone in the intertrochanteric metaphysis and the most effective is the cancellous bone of the femoral neck and cortical bone of the upper diaphysis. For best fixation of the femoral component, the cortical bone of the upper metaphysis should be contacted, and the diaphyseal cortical bone actually should be grooved by the femoral stem during insertion.
Approach to the hip is important in fixation of the femoral component. Approaches that require femoral neck resection and use of a short stem are especially treacherous. The posterior approach is attractive because it encourages preservation of the femoral neck, and it provides straight access to the medullary canal of the femur while protecting the abductor muscles.
Static axial load in the extended position is most readily resisted by the femoral component. Distal cortical contact is very important to prevent toggle. Torsional loads generated from offset loading of the femoral head in flexion and with front-to-back acceleration generated loads during rapid gait are the most challenging in securing fixation of the implant. Ideal conditions include preservation of the femoral neck, engagement of a rectangular stem in a rectangular cavity, and mechanical lock into grooves in the diaphyseal cortical bone. If all these conditions are met, femoral component fixation can be achieved every time without damaging the abductor muscle group, and with minimal danger of femoral fracture.
A posterior approach with femoral neck preservation and a rectangular shaped femoral component with diaphyseal engaging stem is safe surgery and good mechanics. The anterior approach with femoral neck sacrifice and round, short stems is unsafe surgery and bad mechanics.
