Mechanical characterization of porous hydroxyapatite 3D-printed bone scaffolds

Abstract

Porous implants offer a promising alternative to autologous bone grafts for treating critical-sized maxillofacial bone defects from traumatic injuries or congenital diseases. Optimizing these implants' mechanical performance necessitates understanding how geometrical design parameters impact their mechanical properties. While numerous studies have examined this relationship in porous scaffolds, few have focused on ceramic materials with Triply Periodic Minimal Surface (TPMS) geometry. Hydroxyapatite, a ceramic material, has emerged as a leading candidate due to its similarity to natural bone and osteoconductive properties. TPMS geometry provides open, interconnected porosity, enhancing bone formation and vascularization within the scaffold. This study investigates the impact of porosity and wall thickness on the mechanical behavior of ceramic TPMS scaffolds. Gyroid geometry, a specific TPMS, was designed with three porosity levels: 25%, 40%, and 55%, each corresponding to different wall thicknesses but equivalent pore sizes. VAT polymerization was used as 3D printing method to produce these samples. The relevance of two different mechanical testing techniques, compression and spherical macro indentation, was evaluated for assessing the mechanical behavior of porous ceramic bone graft scaffolds. An in-depth analysis of load-displacement curves from these tests provided insights into resistance, result variability, energy absorption abilities, and accumulated damage before failure. The study revealed two distinct failure mechanisms: catastrophic brittle failure under compression and progressive failure under indentation, where samples retained load-bearing capacity even after the main failure event. The samples with 40% porosity exhibited unexpectedly weak performance compared to other porosity levels, showing not only the lowest resistance and but also the poorest combination of properties, particularly in terms of resistance and energy absorption efficiency. A consistent correlation was found between accumulated pre-failure damage and resistance, indicating that higher pre-failure damage leads to lower resistance, regardless of porosity level. Regarding the relevance of the mechanical testing methods, indentation consistently showed lower variability in the results compared to compression. However, it did not effectively capture the differences in behavior across the three porosity levels, often resulting in non-significant differences in the computed mechanical properties. Further research should be conducted using in situ tests with micro-CT imaging to gain a deeper understanding of fracture mechanics under compression and indentation.