Evaluating the effect of simulated osseointegration on the fatigue behavior of 3D-printed orthopedic implants

Interconnected porous lattices are commonly added to the surfaces of 3D printed orthopedic implants. These surface layers are typically 0.25-2.00 mm thick, facilitating a strong mechanical interlock between the synthetic implant and bone. Despite the biological benefits, the porous surface layers introduce stress concentrations at the solid-lattice interface, decreasing implant mechanical properties, which can lead to implant failure dependent on the level of osseointegration. This study explores how tissue regeneration inside the porous network influences the mechanical properties of implants. Samples were first manufactured via laser-powder bed fusion (L-PBF), with the gyroid lattice designed on the surface of the gauge section. PMMA-based bone cement was applied to surface porous tensile specimen as a surrogate to mimic bone growth into porous regions. The prepared samples were tested in monotonic tensile and tensile-tensile fatigue loading. The data demonstrates that ingrowth into the surface porous regions resulted in a 4-10% increase in UTS, 22-40% decrease in strain-at-failure, and a 25-38% increase in fatigue strength for surface porous groups. These results indicate that bone tissue regeneration could have shielding and load-sharing effects that decrease the impact of the local stress-concentrations introduced via surface porosity. The results highlight the importance of considering the implant-bone interaction as a composite when designing and testing orthopedic implants, which has been largely unexplored to date.