Auxetic femur meta-implants with tuned micromotion distribution

Stress shielding and micromotions are the most significant problems occurring at the bone-implants interface due to mismatch of their mechanical properties. Mechanical metamaterials with their exceptional behavior and characteristics, can provide an opportunity to solve the mismatch of mechanical properties between the bone and implant. In this study, four types of human femoral hip implants (one solid implant and three lattice-structured meta-implants) have been considered for stress and micromotion analysis. For this aim, at the first stage, a well-known auxetic 3D re-entrant structure was studied analytically, and precise closed-form analytical relationships for elastic modulus and Poisson’s ratio of the unit cell were obtained. The analytical solution was validated with the numerical solution based on beam elements. At the second stage, three lattice meta-implants made of 3D re-entrant unit cells with positive, negative, and graded distribution of Poisson’s ratio were analyzed and the stress and strain distributions in implant and the micromotion at the bone-implant interface were studied. The results of analytical solution for mechanical properties of the 3D re-entrant structure presented great improvements in comparison with previous analytical studies on this structure. Moreover, the implementation of the re-entrant structure in the hip implant provided very smooth results for stress and strain distributions in the lattice meta-implants and could solve the stress shielding problem which occurred in the solid implant. The lattice meta-implant based on the graded unit cell distribution presented smoother stress-strain distribution in comparison with the other lattice meta-implants. Moreover, the graded lattice meta-implant gave minimum areas of local stress and local strain concentration at the contact region of the implants with the internal bone surfaces. Among all the cases, the graded meta-implant also gave micromotion levels which are the closest to values reported to be desirable for bone growth (40 µm).