Choice of Spinal Interbody Fusion Cage Material and Design Influences Subsidence and Osseointegration Performance

OBJECTIVE: The objective of the study was to quantify the effect of cage material (titanium-alloy vs. poly-etheretherketone or PEEK) and design (porous vs. solid) on subsidence and osseointegration. METHODS: Three lateral cages (solid PEEK, solid titanium, and 3-dimension-printed porous titanium cages) were evaluated for cage stiffness, subsidence compression stiffness, and dynamic subsidence displacement under simulated postoperative spine loading. Dowel-shaped implants made of grit-blasted solid titanium alloy (solid titanium) and porous titanium were fabricated using commercially available processes. Samples were processed for mechanical push-out testing and polymethylmethacrylate histology following an established ovine bone implantation model. RESULTS: The solid titanium cage exhibited the greatest stiffness (57.1 +/- 0.6 kN/mm), followed by the porous titanium cage (40.4 +/- 0.3 kN/mm) and the solid PEEK cage (37.1 +/- 1.2 kN/mm). In the clinically relevant dynamic subsidence, the porous titanium cage showed the least amount of subsidence displacement (0.195 +/- 0.012 mm), significantly less than that of the solid PEEK cage (0.328 +/- 0.020 mm) and the solid titanium cage (0.538 +/- 0.027 mm). Bony on-growth was noted histologically on all implant materials; however, only the porous titanium supported bony ingrowth with marked quantities of bone formed within the interconnected pores through 12 weeks. Functional differences in osseointegration were noted between groups during push-out testing. The porous titanium showed the highest maximum shear stress at 12 weeks and was the only group that demonstrated significant improvement (4-12 weeks). CONCLUSIONS: The choice of material and design is critical to cage mechanical and biological performances. A porous titanium cage can reduce subsidence risk and generate biological stability through bone on-growth and ingrowth.