Effect of Internal Mechanical Environment of Porous Scaffolds on Mechano-driven Bone Ingrowth: A Numerical Study

Porous cages with lower global stiffness induce more bone ingrowth and enhance bone-implant anchorage. However, it's dangerous for spinal fusion cages, which usually act as stabilizers, to sacrifice global stiffness for bone ingrowth. Intentional design on internal mechanical environment might be a promising approach to promote osseointegration without undermining global stiffness excessively. In this study, three porous cages with different architectures were designed to provide distinct internal mechanical environments for bone remodeling during spinal fusion process. A design space optimization-topology optimization based algorithm was utilized to numerically reproduce the mechano-driven bone ingrowth process under three daily load cases, and the fusion outcomes were analyzed in terms of bone morphological parameters and bone-cage stability. Simulation results show that the uniform cage with higher compliance induces deeper bone ingrowth than the optimized graded cage. Whereas, the optimized graded cage with the lowest compliance exhibits the lowest stress at the bone-cage interface and better mechanical stability. Combining the advantages of both, the strain-enhanced cage with locally weakened struts offers extra mechanical stimulus while keeping relatively low compliance, leading to more bone formation and the best mechanical stability. Thus, the internal mechanical environment can be well-designed via tailoring architectures to promote bone ingrowth and achieve a long-term bone-scaffold stability.