Design optimization of cementless hip prosthesis coating through functionally graded material

Many metals and its alloys that have been used in biomedical applications which carry the most applied load from the natural bone to the artificial joint. Consequently, this leads to causing stress shielding and bone osteoporotic. Therefore the optimization of the artificial hip materials is one of the challenging opportunities in prosthetic design. It is found from literature that there are contradictions due to the use of hydroxyapatite (HAP) as a coating material. In this study a finite element analysis and optimization method have been carried out in order to find a new design of the hip stem coating using functionally graded material (FGM). The using of FGM coating leads to diminishing stress shielding at the medial proximal region of the femur. In addition, it reduces the interface shear stress between the coating and bone that affects the long term stability of the hip implant. In this study the gradation of the Young's modulus of the coating material changed through the vertical direction. Then the optimal design is compared with HAP coating and with homogenous uncoated titanium stem. The optimal design, in the case of a coating material which consists of HAP at the upper layer of the coating graded to collagen at the lower layer, is increase the maximum von Mises stress in bone at the medial proximal region of the femur by 65% and 19% compared to homogeneous titanium stem and titanium coated with HAP, respectively. The maximum lateral shear stress is reduced by 23% and 12%. However, the maximum medial shear stress is reduced by 39% and 14% compared to homogeneous titanium stem and titanium coated with HAP, respectively.