Finite Element Analysis of A Hip Joint Prosthesis with An Extramedullary Fixation System

Hip joint prostheses generally enjoy intramedullary stems, with multitudinous designs and shapes, for fixation into the femur. There are patients, however, who are not able to use such prostheses, for instance, due to their very narrow medullary canals. This study is designed to investigate the feasibility of using an extramedullary fixation system for hip joint prostheses. The proposed design is based on Dynamic Hip Screw (DHS) which is originally used for the treatment of femoral neck fractures. A voxel-based finite element model of the femur is developed from QCT images of a cadaver. The model is validated by simulating an experimental in-vitro test which investigated the mechanical behavior of proximal femoral bone under compression loading. It is then used to assess the performance of a DHS-based extramedullary fixation system of the hip prosthesis, in comparison with that of a conventional long-stem design. Muscle and joint forces, extracted from a musculoskeletal analysis of the gait cycle, are applied to the model. The resulting stresses in the components of the two designs are analyzed to examine the possibility of their fracture. Also, the strain energy density in the periprosthetic bone is studied to investigate the long-term performance of the designs considering bone remodeling behavior. Results indicate very high stresses in the screws of the DHS-based system, larger than their endurance limit. Also, the DHS-based fixation is found to impose a large amount of stress shielding throughout the fixation site, raising the risk of bone failure or implant loosening due to bone remodeling. It is concluded that the DHS-based design, in its original configuration, might not be appropriate for hip arthroplasty. Suggestions have been given for the design of a practically successful extramedullary hip implant.