Interbody fusion cages were first approved for use by the FDA in 1997. Since that time, they have become very popular for spinal fusion surgery. Other materials have been tried. Carbon fiber, porous tantalum, and even bone dowels have been used in place of the titanium cages.
Carbon fiber is a nonmetallic substance made from carbon (graphite). It has many, many uses from racing yahts to musical instruments. Skateboards used for downhill speed boarding made from carbon fiber are flexible but hard. This material is used for road bikes and mountain bikes for the same reason. It has been used to make fusion cages because of its high rigidity and low weight.
Tantalum is a hard, transition metal that is resistant to corrosion. It resists attack by body fluids. It is not perceived by the body as irritating, so it’s widely used in making surgical instruments and implants.
More recently, bone dowels have been tried in place of titanium cages. Bone taken from cadavers was machined to form threads for use in spinal fusion. A large hole was drilled in each dowel to allow bone to grow in and around the dowel. The dowels were used much like the titanium cages. There were problems though with bone fracture and cracking. The bone dowels did much better when reinforced with metal plates or screws.
Researchers try different materials but keep coming back to the titanium. It is light but strong and bears greater loads through the spine than other materials.