A topology optimized spinal fusion cage for the use of weak but osteoinductive bioceramics as synthetic bone grafts

Spinal cages can be used to treat degenerated discs, by restoring height and stability in the segment and facilitating fusion between vertebrae. Bone grafts, often placed in the cage to enhance bioactivity, may be replaced with synthetic calcium phosphates (CPs). However, their inherent brittleness presents a challenge. This study aimed to design a cervical cage using topology optimization to accommodate bioactive but weak materials, while minimizing the amount of non-resorbable material, herein Ti6Al4V. A design space of a typical cage was subjected to 1Nm in flexion-extension, axial rotation, and lateral bending with 100 N preload in compression and compression-shear, resulting in an optimized design retaining only 21 % of the volume. The design was verified and experimentally validated using additively manufactured Ti6Al4V cages, applying ASTM-F2077 quasi-static testing with and without the inclusion of CP. The results showed no migration or cracking of the CP, and a mechanical performance in the upper range of approved cages. A significant increase in torsional strength and stiffness for the CP-containing cages of 11 % and 17 % was found, respectively. Overall, the study demonstrated the potential of topology optimization in creating spinal cages with reduced permanent material content and adequate mechanical properties for incorporating brittle materials like CPs.