An experimental study of bending stiffness in lattice metamaterials with and without cement embedding

Additive technologies have made it possible to create medical devices with inhomogeneous structure. The structures of such products, called metamaterials, allow not only to reduce the weight of the product, but also to control the effective physical and mechanical properties by varying the geometry of their microarchitecture. Controlling physical-mechanical properties helps prevent post-operative complications associated with implantation. This study focuses on determining the stiffness and strength properties of different types of the lattice metamaterials, which are subsequently used to develop a numerical model and design endoprostheses, as well as to investigate antibiotic elution from bone cement. The work presents an experimental testing of three metamaterial types with two variations of cross-sectional angle, both with and without embedded cement. The cross-sectional angles of the specimens were selected based on the results of numerical simulations. The studied specimens were fabricated using laser stereolithography on a photopolymer printer, followed by post-polymerization. Stiffness properties were determined through using in situ four-point bending tests. As a result, the stiffness and maximum load values were obtained. These values were normalized to those of solid specimens. The specimens with embedded cement were at least 28% stiffer than those without cement. A comparison of numerical and in situ experiments revealed qualitative agreement in the results for the first and third types of specimens when the cross-section rotation was varied. However, the maximum load observed of the in situ experiments for the strongest specimens, according to the numerical model, showed little difference compared to the maximum loads of other specimens types. The authors attribute this result to the dependence of changes in physical and mechanical properties on geometry during polymerization.