Surgical variables affect the mechanics of a hip resurfacing system

Recent clinical studies have linked failure to surgical variables of stemmed hip resurfacing systems. We used finite element analysis to investigate the effects of implant position, stem orientation, and extent of fixation both on the local stresses and strains associated with implant loosening, neck fracture, and stem fracture, as well as on the load transfer distribution in the bone-implant system. The range of peak stress in the cement was reduced from 11 to 13 MPa for the varus stem to 3.2 to 4.2 MPa for the valgus stem. The range of peak strain in the bone was also reduced from -0.35% to -0.45% strain for the varus stem to -0.19% to -0.27% strain for the valgus stem, but only when reamed cancellous bone remained exposed. Peak stresses in the stem were low for all cases. Additionally, the implant's load transfer distribution was generally insensitive to all variables examined and the femoral head was substantially unloaded. Our data indicate the local stresses and strains associated with implant loosening and neck fracture were reduced by placing the implant in a valgus orientation and covering reamed cancellous bone, but unloading of the femoral head, found for all variables examined, may lead to adverse bone remodeling.