Assessment of Auxetic and Nanocomposite Designs for Mimicking Natural Intervertebral Disc Behavior

Intervertebral disc degeneration remains a major contributor to chronic low back pain, yet existing disc replacement designs fall short in replicating the behavior of natural intervertebral disc. In this study, we explore the use of three auxetic geometries, namely reentrant, chiral and barbell geometries, combined with nanocomposite materials to assess their potential to mimic the performance of a healthy disc. Finite element analysis was performed on the L4-L5 lumbar segment under axial compression, comparing the response of these designs to that of a natural disc. Each design featured a Ti-6Al-4V endplates and an auxetic core made of ultra-high molecular weight polyethylene (UHMWPE), evaluated both without doping and with graphene nanoplatelet concentrations of 0.5%, 1%, and 1.5%. We analyzed interfacial mechanics using binary agreement mapping and evaluated strain energy and displacement to assess stress shielding and potential nerve impingement. Among the tested designs, the reentrant UHMWPE design most closely matched the natural disc’s contact pressure (52% agreement) and energy absorption, particularly over the nucleus pulpous. The chiral UHMWPE design best replicated sliding distance (24% agreement) and frictional stress (31% agreement), with regional agreement along the annulus fibrosus. All auxetic designs exhibited inward concave deflection, suggesting reduced risk of nerve impingement. While undoped UHMWPE performed better in pressure matching, graphene doping improved conformity in sliding distance and frictional stress. These findings underscore the potential of auxetic nanocomposite disc replacements to achieve more natural interfacial mechanics and functional performance than current commercial designs.