Ex vivo mechanical evaluation of anatomically and mechanically conforming patient-specific lumbar spinal fusion cages designed by full-scale topology optimization

Background: Intervertebral disc degeneration may require spinal fusion surgery using interbody cages. Implant subsidence remains a significant complication, largely attributed to the stiffness mismatch between interbody cages and vertebral endplates. While porous cage designs aim to reduce their stiffness, they do not account for patient-specific anatomical and mechanical variability. This study aims to evaluate whether patient-specific, topology optimized interbody cages improve load distribution and decrease subsidence risk compared with off-the-shelf cages in a human cadaveric mechanical model. Methods: Sixteen functional spinal units (FSUs) were allocated by bone quality to topology-optimized devices (TOD) or off-the-shelf (OTS) cage groups. Both cage types were designed for a TLIF approach (TOD by a previously developed topology optimization method) with the same footprint and additively manufactured from medical-grade titanium alloy. Endplate contact was assessed using pressure-sensitive film. FSUs underwent axial compression to failure at 1 mm/min with optical displacement tracking. Failure (subsidence) force is defined as the first local maximum of the force-displacement curve. Results: Pressure film analysis demonstrated broader and more uniform endplate contact for TOD cages, whereas OTS cages exhibited concentrated edge loading. Failure (subsidence) force and integral volumetric bone mineral density (vBMD) were positively correlated for both cage groups. Paired comparisons showed a significantly higher failure (subsidence) force (increase of median force by 97.6%) for TOD implants (median force: 876.5 N) compared to OTS implants (median force: 443.7 N; p = .03). Conclusions: Patient-specific topology optimized interbody cages improved endplate conformity and load distribution, resulting in higher resistance to subsidence compared to conventional off-the-shelf cages. These findings suggest that patient-specific topology optimized interbody cages may offer a biomechanically advantageous solution for spinal fusion, especially in patients with compromised bone quality, with the potential to reduce implant subsidence and improve clinical outcomes.