Mechanical Performance of Conventional Threaded Cage Designs and Interbody Fusion Cages Designed by Integrated Global and Local Topology Optimization

INTRODUCTION Conventional designs of spinal interbody fusion cages have mainly focused on providing immediate strength to maintain disc height and shielding the bone grafts within the cage. Therefore, the geometrical features of these conventional designs show little distinction from each other and most of them fall into a category of a pipe shape with thick shells as outer walls as well as a hollow interior space that brackets the fill of grafting materials. Further division is defined by the threaded and non-threaded anchorage mechanism that cage devices rely on to form rigid bonds with vertebral bodies. Threaded designs may be utilized along the entire outer surface of cylindrical cages, whereas they are distributed only on two collateral sides perpendicular to the insertion plane in wedge or rectangular blocks and are later wedged into the endplates of the vertebral bodies. These hollow pipe designs guarantee sufficient reconstruction stiffness in arthrodesis and play a substantial role in stability for motion segments postoperatively. Nonetheless, the rigid shells may shield an implanted graft or ingrown bone tissue from sufficient mechanical stimulus, (known as “stress-shielding”) thus increasing the risk for decreased mineralization and bone resorption. A new design approach for lumbar spine interbody fusion cage has been developed by using topology optimization algorithms to define the structural layout and inner microstructures. This approach addressed the conflicting design issues of having sufficient stability while at the same time having enough porosity to deliver biofactors like cells, genes, and proteins and impart sufficient mechanical strain to maintain developing tissue. The interior architecture consists of microstructures with reserved channel spaces for potential cell-based therapies and drug delivery. The interconnected struts of the microstructures formed a network of stress transmission so that the strain energy from applied loads can be not only absorbed by appositional bone ingrowth between the interface of the device and vertebrae but also by regenerate bone tissues inside the new cage design. The present study simulated performance of both conventional and the newly designed cages under various loading conditions of compression, torsion, lateral bending and flexion-extension using voxel-based finite element methods. MATERIALS AND METHODS New Cage Design of Integrated Topology Opotimization The integrated global and microstructure topology optimization approach is used to design a spinal cage that meets design requirements of immediate stability following implantation, sufficient compliance to avoid stress shielding, and high porosity for biofactor delivery [1]. The global topology optimization algorithm is used to generate global density distribution under physiologic loading, which is shown in Figure 1a. Immediate stability is addressed by constraining the total displacement at the vertebral surface to be less than a desired target. Total porosity for biofactor delivery and sufficient compliance is input as a constraint for the global optimization. The result is a global volume fraction distribution, ensuring sufficient porosity for biofactor delivery and avoidance of stress shielding.