Tapered stem geometry provides superior initial fixation stability to cylindrical stem geometry in the setting of severe bone loss: A finite element analysis

Initial stability of cementless stems used in total hip arthroplasty (THA) is critical for subsequent osseointegration at the bone/implant interface. In the setting of revision THA, there is frequently less intact femoral bone available for fixation of the new stem. This study aims to compare the initial fixation stability between a cylindrical and tapered stem, designed for diaphyseal fixation in a revision THA setting, with increasing bone defect severity. Using finite element analysis, severe Paprosky Type III femoral bone defects were simulated. The cylindrical stem had a 13.3% higher construct stiffness as compared to the tapered stem for the least severe bone defect modeled. In contrast, for the most severe bone defect, the tapered stem showed a 12.3% higher construct stiffness than the cylindrical stem. At the bone/implant interface, the tapered design resulted in less than 20 mu m of maximum tangential micromotion for all the bone defect models, whereas the cylindrical stem exceeded 20 mu m for all and was as high as 70 mu m in the most severe case. Given the limitations of the models presented, the results suggest the tapered stem obtained superior initial fixation as compared to cylindrical stem with increasing bone defect severity.