1. Introduction
The additive manufacturing (AM) process has enabled the production of complex geometries [
1,
2,
3,
4] along with composite material structures [
5,
6], which are difficult to fabricate with traditional manufacturing techniques, such as machining and injection molding. This led to rapid development of the topology optimization process with advanced geometries via generative design and cellular materials. Topology optimization (TO) is a procedure that optimizes the material mass distribution within the already defined external volume [
7], resulting in light-weight structures and minimization of feedstock material usage. The main objective is to maintain the desired mechanical properties reducing the mass of the structure; however, it also creates additional advantages such as high porosity and high surface-area-to-volume ratio [
8]. There are two different approaches to achieve TO: the density-based and truss-based approaches. The density-based approach is identical to generative design [
9]. On the other hand, the truss-based approach utilizes periodic unit cells (lattice structures) in order to achieve the optimum mass distribution [
10]. Nowadays, there is a plethora of applications in several industries with topologically optimized products using both of the two aforementioned approaches, indicatively in the aeronautical, automotive, and biomechanical industries [
11,
12,
13].
The truss-based approach imitates the cell structures from natural tissues, such as bones, corals, foams, and so forth. Thus, this approach is suitable for topology optimization of structures in biomechanical applications, such as implants and tissue scaffolds. Specifically, in this approach, the selected volume fills with lattice structures with specific geometry and dimensions replacing the solid material. Besides the advantage of minimizing the mass of the product for usage in biomechanical applications, this procedure offers the above-mentioned additional advantages, which facilitate the tissue regeneration process and allow diffusion of oxygen and nutrients [
14,
15]. There is a plethora of different lattice structures, the simplest ones being the 2D lattice structures like honeycombs and prismatic ones [
16]. The geometrical complexity increases in 3D lattice structures, resulting in a vast number of diverse 3D lattice structures with further classification in strut structures and sheet structures [
17]. The most widespread strut structures are the Octet and the Voronoi, which directly emerge from structures found in nature. On the other hand, the most common lattice sheet structures are the sheet triply periodic minimal surface (TPMS) structures such as Gyroid, Schwarz Diamond, and Neovius. According to Gibson et al. [
18], all lattice structures lead to reduction of the mechanical properties. The reduction of the mechanical properties in lattice structures depends mainly on the relative density and on the actual geometry of the applied lattice structure. Relative densities less than 50% enhance the size effect, which has an increasing influence on the mechanical performance as the relative density is reduced. However, not all lattice structures have the same mechanical behavior; specifically, lattice structures that exhibit stretching-dominated behavior are less affected by the size effect compared to lattice structures that exhibit bending-dominated behavior [
19]. Moreover, Al Ketan et al. [
20] propose lattice structures that have stretching-dominated behavior for biomechanical applications which receive increased stresses.
In addition, there are several studies that have tried to combine additive manufacturing techniques with orthopedic implants through customization and lattice structures. Mahmoud et al. [
21] have gathered the majority of these studies in a comprehensive review, summarizing the use of AM technologies to produce orthopedic implants from lattice structures and functionally graded materials. Gabbrielli et al. [
22] and España et al. [
23] investigated the possibility of using lattice structures in acetabular cups for mechanical and biological advantages. Moreover, Hazlehurst et al. [
24] suggested manufacturing a hip implant consisting of cubic lattice structures in the whole internal body of the implant and this led to severe reduction of the implant’s stiffness and strength. Furthermore, Limmahakhun et al. [
25] and González et al. [
26] investigated innovative designs in order to optimize the utility of lattice in orthopedic implants and increase their mechanical strength and this was achieved by functional gradation of the lattice structures.
The aim of this research was to extract an innovative design for a bioinspired hip implant utilizing topology optimization tools. Therefore, the current paper proposes a novel hip implant design with advanced lattice structures in order to address the reduction in strength and handle the in vivo loadings.
Figure 1 portrays a flowchart of the topology optimization process through lattice structures depending on the mechanical behavior. Initially, a hip implant was designed according to international medical standards and then its mechanical behavior was examined under in vivo static loads through finite element analysis (FEA). Furthermore, specific regions of the implant, where topology optimization was essential, were replaced by bioinspired lattice structures that have been shown to exhibit stretching-dominated behavior, such as the Voronoi strut structure and the Gyroid and Schwarz Diamond sheet structures. These TPMS structures were selected due to the promising mechanical strength that was exhibited in the aforementioned studies [
19,
20]. In addition, Voronoi structure was selected because it displays similar behavior to the trabecular internal structure, while demonstrating high mechanical strength [
27,
28]. These three lattice structures were examined and evaluated for their mechanical performance under the same in vivo loads. In addition, further design optimization was applied to lattice structures through functional gradation in order to improve the mechanical properties of a hip implant.
4. Conclusions
In this paper, an orthopedic hip implant was designed according to international standards and then a topology optimization of its geometry was performed. The topology optimization process was implemented via bioinspired lattice structures, namely, Voronoi, Gyroid, and Schwarz Diamond structures, which are derived from nature having superior mechanical performance. Moreover, topology optimization occurred with the implementation of these lattice structures in regions of low stress, in order to achieve the optimal mass distribution within the existing volume. Furthermore, functional gradation of the implemented lattice structures was performed. In particular, it was observed that the hip implant that contained Schwarz Diamond structures revealed the best mechanical behavior, both in simple topology optimized implant and in functionally graded implant. The factor of safety of the functionally graded Voronoi was 1.01 and for the design approach containing the TPMS structures of functionally graded Gyroid and Schwarz Diamond were 1.79 and 2.08, respectively. Topology optimization leads to a reduction of weight of 38% compared to the solid version of the hip implant, therefore less construction material is needed. Moreover, for the intramedullary stem, it has a 50% mean porosity to facilitate the process of tissue regeneration through diffusion of cells, oxygen, and other nutrients. Future work will focus on the fabrication of these implants, utilizing additive manufacturing methods and the evaluation of their mechanical properties will be measured.