Investigation of mechanical properties of lattice structures

Abstract

Lattice structures are gaining significance in the field of implants and prosthetics due to their advantages in terms of weight in comparison to bulk materials. They also allow for bone cell ingrowth into the structure. Additionally, the mechanical properties can be deliberately tuned through the structure's porosity to closely mimic bone elasticity.However, numerous potentially beneficial configurations lack sufficient data in terms of their safe and purposeful implementation. Errors arising from the absence of data in the design of these structures can lead to implant failure or damage to surrounding tissues.In this study, a comprehensive experimental campaign was implemented, accompanied by numerical simulations focusing on the mechanical properties of lattice structures, in order to improve the design of such structures for their application in implants.To that end, seven differently designed lattice structures were manufactured by means of lithography-based 3D printing, using photopolymer, zirconium dioxide, and hydroxyapatite as underlying materials. Compression, tension, bending, and shear tests were carried out on all seven lattice designs and on all three materials.From the results, the modulus of elasticity and the failure strength were determined. The latter was assessed in terms of both complete failure of the lattice structure and the failure of the first substructure. The obtained data was statistically analyzed, and comparisons were made between the different designs and materials. As expected, it turned out that the zirconium dioxide-based lattices exhibit the highest stiffness, followed by the hydroxyapatite-based lattices, and the photopolymer-based lattices. For example, the bending stiffness, when averaged over all lattice designs, amount to 13 GPa (zirconium dioxide), 6.44 GPa (hydroxyapatite), and 180 MPa (photopolymer). As for the failure strength under bending load, the ranking among the three materials is as indicated above but the failure strengths are in the same order of magnitude: 88.8 MPa (zirconium dioxide), 48.1 MPa (hydroxyapatite), and 30.5 MPa (photopolymer), considering again values averaged over all considered lattice designs. Analogous observations were made for tension, compression, and shear loading.As for the practical relevance of the studied lattice designs, the structures were compared to literature-based properties indicating the osteoconductivity, focusing thereby on pore size and bottlenecks of the cells. It turned out that similarly good osteoconductivity characteristics can be expected for all considered lattice structures. On the other hand, the acquired data were compared with the mechanical loads on implants according to studies. On this basis, recommendations for specific cells for some use cases were given.In conclusion, this thesis presents key mechanical properties related to the studied lattice designs and used materials under different loading conditions in a comprehensive manner, and can be considered as benchmark for future studies in this field.