The rational design of meta-implants using a combination of auxetic and conventional microstructures

Abstract

With the aging population growing, the increasing prevalence of osteoarthritis (OA) seems inevitable. As a result, the number of joint replacements is expected to grow substantially within the next decade. Today’s hip replacements have the potential to function for more than 20 years, but due to risk factors such as aseptic loosening, the younger patients will most certainly outlive their prostheses. Aseptic loosening refers to the mechanical failure of the implant-bone interface, which may arise as the end result of stress shielding or improper design. In places where the bone is understressed for prolonged periods of time, bone resorption will set in, which can eventually lead to implant failure. Optimization of this bone-implant interface is therefore of utter importance to increase the implant’s longevity. Recent advances in additive manufacturing (AM) have enabled the fabrication of highly-complex micro-architectures, which can be designed to exhibit novel mechanical properties at the macroscale. The development of these mechanical metamaterials has paved the way for personalized, life-lasting implants. This explorative study aimed to investigate the mechanical properties of 3D AM (hybrid) mechanical metamaterials and their potential to improve the mechanical fixation of off-axially loaded AM meta-implants, using the mechanical responsiveness of bone. The mechanical properties of six different 3D AM mechanical metamaterials were assessed during axial compression, with the help of Digital Image Correlation (DIC). Combinations of mechanical metamaterials were made to obtain novel 3D hybrid mechanical metamaterials, for which the expansions and initiated strain distributions were determined using DIC, upon off-axial compression. Finally, five different meta-implants (femoral stems) were designed. A compression was applied at the femoral head to examine the strain distributions in the surrounding, bone-mimicking foam blocks. Hybrid meta-implant Type 2 showed the most continuous compressive strain distribution, with a considerably different lateral strain profile compared to the current generation of femoral stems. These studies therefore demonstrate the potential of applying hybrid mechanical metamaterials in off-axially loaded meta-implants, such as the femoral stem, to initiate bilateral strains and potentially stimulate osseointegration at the bone-implant interface.