Numerical study of functionally graded hip implants with different second-generation titanium alloys

Total hip Arthroplasty (THA) is performed every year at a very high frequency to improve the quality of life of thousands of patients all over the globe. Yet the expected service life of such a surgery remains unsuitable for patients under 50 years old. This is mainly related to stress shielding and the potential adverse tissue reaction to some of the elements of the market dominant implant materials. In this research, functionally graded (FG) implant designs of several titanium alloys layered with hydroxyapatite (HA) were proposed to provide lower implant stiffness compared to a solid stem to approach the requirements of human bone. Moreover, TNZT (Ti35Nb7Zr5Ta), and TMZF (Ti12Mo6Zr2Fe) second generation titanium alloys were studied as a replacement for the famous Ti6Al4V alloy to avoid the adverse tissue reactions related to aluminum and vanadium elements. The different FG models were numerically tested using a 3D finite element simulation after virtual implantation in a femur bone under the dynamic load of a patient descending stairs. The numerical study assesses the variation in stress distribution and strain energy in a femur bone for different FG hip stems as well as the axial stiffness of the hip stems. Results indicated an increase in strain energy and von Mises stress in the cortical and cancellous bones using FG hip stems. Additionally, the axial stiffness reduced for all FG hip stems relative to the commercial Ti6Al4V hip stem.