Femur Auxetic Meta-Implants with Tuned Micromotion Distribution

Abstract

Stress shielding and micromotions are the most significant problems occurring at the bone-implants interface due to a mismatch of their mechanical properties. Mechanical 3D metamaterials, with their exceptional behaviour and characteristics, can provide an opportunity to solve the mismatch of mechanical properties between the bone and implant. In this study, a new porous femoral hip meta-implant with graded Poisson’s ratio distribution was introduced and its results were compared to three other femoral hip implants (one solid implant, and two porous meta-implants, one with positive and the other with a negative distribution of Poisson’s ratio) in terms of stress and micromotion distributions. For this aim, first, a well-known auxetic 3D re-entrant structure was studied analytically, and precise closed-form analytical relationships for its elastic modulus and Poisson’s ratio were derived. The results of the analytical solution for mechanical properties of the 3D re-entrant structure presented great improvements in comparison to previous analytical studies on the 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).

Auxetic Metamaterials for Biomedical Devices: Current Situation, Main Challenges, and Research Trends

“…To achieve this goal, the authors obtained analytical relationships for the mechanical properties of the three-dimensional re-entrant cell, developed four types of implants (solid implant; meta-implant with a positive Poisson's ratio; meta-implant with negative Poisson's ratio; and meta-implant with gradient cell distribution) and studied them using finite element analysis (FEA) [21].…”

Innovative Surface Modification Procedures to Achieve Micro/Nano-Graded Ti-Based Biomedical Alloys and Implants

“…Mechanical 3D metamaterial with a porous structure is among the best materials for bone implants, with a graded Poisson's ratio distribution to optimize stress and micromotion distributions. In this regard, Ghavidelnia et al [220] analytically designed an auxetic 3D re-entrant structure with tailored elastic modulus and Poisson's ratio, and which has great potential to solve the stress shielding problem. Kolken et al [221] produced a novel meta-implant by 3D printing technology.…”

Auxetic Structures for Tissue Engineering Scaffolds and Biomedical Devices

“…The optimization of mechanical properties such as the elastic modulus, stiffness, mobilization, and Poisson's ratio is important to the development of bone replacement [80][81][82][83][84]. Therefore, researchers have been developing a surgical prosthesis having mechanical properties suitable for bone tissue by performing analyses, simulations, and experiments under various conditions [14,15,17,[85][86][87]. Among them, meta-implant studies that can tune the mechanical properties using auxetic structure have been attracted.…”

“…Sanami et al [14,15] explored the auxetic hip prosthesis for the purpose of an improved strain distribution. Recently, Ghavidelnia et al [17] introduced a porous femoral hip meta-implant with a graded Poisson's ratio distribution. Their implant generated a smoother stress-strain distribution, minimum areas of local stress, and a local strain concentration at the implant contact region.…”

“…In particular, many parts of the body, such as muscles, ligament tissues, vascular tissues, skin tissues, and bone tissues, are composed of negative Poisson's ratio (auxetic) tissues, so auxetic structures have been used for tissue engineering research to replace these tissues [12,13]. In addition, auxetic structures have been used for bio-prostheses, stents, hip stems, and surgical screws because of their optimized compressive strength, shear stiffness, and elastic modulus [14][15][16][17].…”