Effect of Design and Material on the Biomechanical Performance of Intervertebral Cage

Intervertebral disc injury is one of the most common spinal degenerative disorders. When a disc is damaged, the use of an intervertebral cage to fill the void between two vertebrae is a common practice. An ideal cage should have sufficient strength to withstand the in-vivo mechanical loads, and at the same time, preserve the normal function of the surrounding tissues, e.g., muscles and ligaments. This study investigated the effects of the cage material, i.e., PEEK or titanium, and geometrical design, i.e., bullet-shaped or kidney shaped, on these biome chanical features. First, the loads applied to each level of an intact spine was estimated using a coupled musculoskeletal-finite element (MS-FE) model at three flexion angles (20,40, and 60 degrees). The loads found for L4-L5 le vel were then applied to the FE models of different cage designs, having different mechanical stiffness. For each cage, the resulting displacements were applied to the coupled MS-FE model, as the boundary condition, to recalculate the loads and update the cage FE models. The maximum stress within the cage and the muscle and ligament forces were then assessed as the primary and secondary performance measures, respectively. The results indicated a significant effect for the geometry on the cage stress, with the bullet-shaped geometry demonstrating a superior performance. In terms of material, the PEEK cage, experienced a slightly lower stress due to its lower stiffness compared to titanium. However, considering the higher compressive strength of titanium and the relatively small difference in the maximum stresses, titanium could be a more conservative choice. The secondary performance measure was not affected significantly neither by the material nor the geometric design of the cage. It was concluded that the cage design with titanium material and bullet-shaped geometry has a higher biomechanical performance than the other designs examined.