The objective of this study is to compare the stress shielding effect of various conventional as well as modified additive manufactured porous materials used for spinal cages. A finite element study was performed by changing the design (fully porous and hybrid) and the materials (PEEK, CFR-PEEK, Titanium) of spinal cages. All the models were simulated under uniaxial compression, to study the stress shielding effect. The Finite Element Analysis results showed that the hybrid spinal cage transfers more stress to its adjacent vertebrae than the other design configurations under uniaxial compression. The hybrid titanium cage was most effective in reducing the stress shielding effect. The hybrid cage is stronger than PEEK & CFR-PEEK cages, however, due to the porous structure reduced stress shielding was observed.

Stress shielding has been a major concern with biomedical implants due to the vast elastic modulus difference between the host bone and the implant material. Titanium, which is the most widely used biomaterial, has an elastic modulus of 110 GPa; cobalt chrome has an elastic modulus of 240 GPa; similarly, stainless steel 316L has an elastic modulus of approximately 230 GPa, whereas the elastic modulus of human cortical bone is 20 GPa and that of cancellous bone is 3–5 GPa. Lattice/porous structures are used in biomedical implants to tailor the elastic modulus of the implant to match that of the adjacent bone resulting in a reduced stress shielding effect. The capability of manufacturing solid as well as lattice/porous structures of additive manufacturing (AM) has accelerated the acceptance of AM in biomedical implant applications. In this study, three different types of metal lattice/porous structures obtained through AM (Gyroid, Diamond, and Schwarz W) and their three different pore sizes (0.4, 0.5, and 0.6 mm) are investigated for tailoring the elastic modulus of a Ti6Al4V ELI (Extra Low Interstitial) material to match that of cancellous bone. Results showed a significant reduction in elastic modulus for all the studied lattice structure types.

With the advantages of custom-designed mechanical properties such as lightweight, mechanical strength, etc., additively manufactured lattice structures are in high demand for many industries, including biomedical, automotive, etc. For the ease of additive manufacturing (AM), various orientations of unit cells for lattice structures can also be taken into account. In this study, the orientational dependence of mechanical properties for a gyroid lattice structure is discussed. AM is used to manufacture metallic gyroid-structured parts made of Ti-6Al-4V at different unit cell orientations. Finite-element analysis (FEA) along with mechanical testing is used to study the relation between the Young's or elastic modulus and unit cell orientation dependence of the mechanical responses for gyroid lattice structures of different unit cell orientations. The study found that gyroid lattice structure with unit cell orientation of 60° exhibits better mechanical properties than 0° and 30°.

Additive Manufacturing (AM) is rapidly finding application in the manufacturing of spinal implants. The flexibility of manufacturing complex freeform structures combined with lattice structures has opened an era of newer types of spinal implants with better osseointegration surfaces and lesser stress shielding. AM is being utilized not only for off-the-shelf spinal implants but also for customized spinal implants also, manufactured using CT-scan/MRI data of patients. In the present study, efforts have been made to summarize work published in the field of metallic AM applications for spinal implants.