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 to facilitate mechanical interlocking with interfacing bone. Despite the biological advantages, the porous surface layers introduce stress concentrations at the solid-lattice interface that can lead to implant fracture, particularly if complete osseointegration is not reached. This study investigated how tissue regeneration within the porous network influences the mechanical properties of implants through simulating bone ingrowth using PMMA based bone cement. Bone cement was selected as a surrogate material because of its widespread use in orthopedic applications and mechanical properties reflective of bone tissue. Metal specimens were manufactured via laser-powder bed fusion (L-PBF), with a layer of gyroid lattice added to the surface of the gauge section. The gyroid layer was subsequently coated in bone cement to represent an osseointegrated implant. The prepared samples were tested in monotonic tensile and tensile-tensile fatigue loading. The data demonstrate that simulated ingrowth into the surface porous regions resulted in a 4-10% increase in nominal ultimate failure strength, 22-40% decrease in strain at failure, and a 25-38% increase in fatigue strength for surface porous groups in the coated variations. The increased tensile and fatigue strength suggest that load sharing between the gyroid region and regenerated bone can partially mitigate the detrimental effects of stress concentrations introduced by surface porosity. This underscores the importance of osseointegration not only for the bone-implant interface, but also the mechanical stability of the implant itself. The results also highlight the importance of considering the implant-bone interaction as a composite when designing and testing orthopedic implants. Incorporating this interaction into mechanical testing and design may more accurately reflect in-vivo conditions and subsequently expand the design space of porous structures to optimize biological integration while maintaining mechanical performance.