Characterization of additively manufactured lumbar interbody fusion cages based on triply periodic minimal surfaces

The advent of laser powder bed fusion (LPBF) allows the fabrication of open-porous bone implants such as lumbar interbody fusion (LIF) cages. However, common LIF cages feature massive support structures and cavities for bone substitute material to meet regulatory requirements with respect to mechanical and biological properties. Since the use of autologous bone substitutes involves significant costs and risks, research is being conducted into alternative designs. This is where the present study comes in by exploring the potential of triply periodic minimal surfaces (TPMS) for metallic LIF cages without supports and cavities in a holistic approach. Specifically, various homogeneous and graded scaffolds with pore size of 400-1100μm and volume fraction of ≤0.25 are designed before being produced from Ti-6Al-4V ELI powder using LPBF. Morphological characterization demonstrates a high process fidelity with a maximum of 8.7% target/actual deviation for the volume fraction. This promotes structural integrity so that the compressive strength of 69MPa or 20.6kN complies with the minimum requirements of ASTM F2077. Based on in vitro cell tests, a mineralization process including bone matrix formation is observed in all variants, with the homogeneous scaffold with 1100μm pore size proving to be particularly beneficial. In conclusion, the results encourage the further development of TPMS based LIF cages without support structures and cavities.