Simulation-Driven Optimization of Nanocomposite Auxetic Intervertebral Disc Substitutes Mimicking Native Mechanics

Intervertebral disc degeneration remains a leading cause of chronic low back pain, and current disc replacement designs often fail to reproduce the behavior of the native intervertebral disc. This study evaluated chiral auxetic disc replacements incorporating polymeric and nanocomposite cores and compared them with a native L4-L5 disc model and a representative commercial implant. The finite element model included cancellous vertebral cores with cortical shells and was subjected to 2.8 mm axial compression and 7.5 N·m pure moments in flexion, extension, and lateral bending. Interfacial pressure, tangential contact stress, and sliding distance were evaluated under compression using pointwise agreement mapping, while posterior anterior-posterior displacement and strain energy were evaluated across all loading modes. The ultra-high molecular-weight polyethylene (UHMWPE) auxetic design provided the strongest overall performance among the proposed designs, with 31.25% pressure agreement and 49.33% sliding-distance agreement with the native disc, compared with 1.01% and 11.47%, respectively, for the commercial design. Graphene-reinforced UHMWPE produced the highest tangential contact stress agreement (34.06%) but reduced pressure and sliding-distance agreement relative to UHMWPE. UHMWPE also provided the closest strain energy response among the auxetic designs and closely reproduced the native posterior displacement during flexion. However, its range of motion remained substantially lower than the native disc, revealing a tradeoff between favorable compressive behavior and rotational compliance. These findings identify the UHMWPE chiral architecture as the most promising design evaluated while demonstrating the need for further geometric optimization to reduce bending stiffness.