Porous fusion cage design via integrated global-local topology optimization and biomechanical analysis of performance

Porous fusion cage is considered as a satisfactory substitute for solid fusion cage in transforaminal lumbar interbody fusion (TLIF) surgery due to its interconnectivity for bone ingrowth and appropriate stiffness reducing the risk of cage subsidence and stress shielding. This study presents an integrated global-local topology optimization approach to obtain porous titanium (Ti) fusion cage with desired biomechanical properties. Local topology optimizations are first conducted to obtain unit cells, and the numerical homogenization method is used to quantified the mechanical properties of unit cells. The preferred porous structure is then fabricated using selective laser melting, and its mechanical property is further verified via compression tests and numerical simulation. Afterward, global topology optimization is used for the global layout. The porous fusion cage obtained by the Boolean intersection between global structural layout and the porous structure decreases the solid volume of the cage by 9% for packing more bone grafts while achieving the same stiffness to conventional porous fusion cage. To eliminate stress concentration in the thin-wall structure, framework structures are constructed on the porous fusion cage. Although the alleviation of cage subsidence and stress shielding is decelerated, peak stress on the cage is significantly decreased, and more even stress distribution is demonstrated in the reinforced porous fusion cage. It promises long-term integrity and functions of the fusion cage. Overall, the reinforced porous fusion cage achieves a favorable mechanical performance and is a promising candidate for fusion surgery. The proposed optimization approach is promising for fusion cage design and can be extended to other orthopedic implant designs.