Design and biomechanical evaluation of a 3D-printed lumbar interbody fusion cage with auxetic metamaterial structure

Interbody fusion cages are widely used to treat degenerative spinal disorders, but their clinical performance is often compromised by complications such as stress shielding and cage subsidence. To address these challenges, this study proposes a porous lumbar cage based on auxetic meta-biomaterials exhibiting a negative Poisson's ratio (NPR), designed to modulate load transfer at the bone-implant interface. Two cages with identical porosity were designed: an NPR cage based on a three-dimensional re-entrant honeycomb structure (3DRH) and a positive Poisson's ratio (PPR) cage using a conventional honeycomb (3DH). Their biomechanical behavior was evaluated in a validated L3-L4 finite element model under physiological loading, with a dense titanium cage included as a reference design. Both porous cages were fabricated by selective laser melting (SLM) for experimental testing. Finite element results showed that the NPR cage led to more uniform load distribution with the bone graft and reduced peak stresses at the superior and inferior endplates. It also exhibited higher strain energy density within the graft region, which may support mechanical stimulation relevant to bone remodeling. Mechanical testing indicated that the NPR lattice had higher apparent stiffness and a more evenly distributed deformation pattern than the PPR lattice. These findings suggest that NPR-based porous cage contributes to reducing the risk of stress shielding while maintaining high porosity, offering a potential approach to improve the biomechanical environment and postoperative fusion outcomes in lumbar interbody fusion cage.