Design of Interbody Fusion Cages of Ti6Al4V with Gradient Porosity Using a Selective Laser Melting Process for Spinal Fusion Arthroplasty

Interbody fusion cage surgery with discectomy has been the major surgical method for treating back pain and degenerative diseases. However, highly rigid fusion devices cause great concentrations of mechanical stress, engendering stress concentration effects. In this study, various geometric structures with single porosity and gradient porosity were designed and 3D printed with selective laser melting. ANSYS software was applied to analyze the stress and strain distributions for different motion modes. For a disc fusion cage comprising three concentric rings surrounding a solid cylinder, with gradient porosity values of 60%-70%-80%-solid (from outside to inside), both the stress and strain showed closer agreement with the intervertebral disc stress and strain. Both the cage and the disc exhibited uniform stress distributions with bone, which would be able to avoid stress concentration. The patterns of the designed models were then printed using Ti-6Al-4V powders. The morphologies of the printed porous structures were examined by optical microscopy and scanning electron microscopy. The porosities of the printed specimens were investigated using the Archimedes method. The results revealed only minor geometric deviation between the computer-aided design patterns and the as-printed samples.