Interface Micromotion of Cementless Hip Stems in Simulated Hip Arthroplasty

Problem statement: The design of hip prostheses has evolved over time due to various complications found after hip replacement surgery. The currently commercially available cementless femoral stems can be categorized into one of three major types, straight cylindrical, tapered rectangular and anatomical. Each type proposes a unique concept to achieve primary stability-a major requirement for bone healing process. Virtual analyses have been made on individual implants, but comparison between the three major types is required to determine the strength and weaknesses of the design concepts. Approach: Three types of implants were modeled in three dimensions-the straight cylindrical, rectangular taper and anatomical. The size of the three implants was carefully designed to fit and fill the canal of a femur reconstructed from a computed tomography image dataset. Hip arthroplasty was simulated virtually by inserting the hip stem into the femoral canal. Finite element method was used in conjunction with a specialized sub-routine to measure micromotion at the bone-implant interface under loads simulating physiological walking and stair-climbing. Another sub-routine was used to assign bone properties based on the grayscale values of the CT image. Results: All the three types of cementless hip stems were found to be stable under both walking and stair climbing activities. Large micromotion values concentrated around the proximal and distal part of the stems. Conclusion/Recommendations: The three major types of hip stems were compared in this study and all of them were found to be stable after simulated physiological activities.