A multiscale finite element model of fluid-microstructure interactions in human intervertebral disc compression

The human intervertebral disc (IVD) is a complex, anisotropic structure composed of the nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplates (CEPs), which together enable the spine to bear loads and accommodate multi-directional motion. Although experimental studies have revealed the nonlinear, time-dependent, and region-specific mechanical behavior of the IVD, capturing this complexity in computational models remains a major challenge. This study extends a previously validated biphasic finite element model of the AF - incorporating collagen fiber networks and interlamellar structures - into a full-scale IVD model that accounts for the heterogeneous, anisotropic, and fluid-solid coupled properties of all components. A multiscale identification strategy links experimental microstructural properties to macroscopic behavior, and an automated meshing approach enables systematic variations in geometric features. The model is validated against a broad set of experimental data under three compressive loading protocols: compressive creep-recovery, cyclic compression, and stepwise compression-relaxation. Numerical results reproduce global and regional IVD mechanics, including energy absorption, strain-rate sensitivity, and spatial strain heterogeneity, governed by fluid-microstructure interactions. The study also evaluates key experimental factors - such as preload duration, hydration, and geometry - highlighting their influence on the IVD response. These findings demonstrate the predictive strength of the multiscale biphasic approach, providing a robust computational foundation for advancing IVD biomechanics and supporting future clinical applications in spine health and degeneration. STATEMENT OF SIGNIFICANCE: This study introduces a validated multiscale finite element model that simulates the time-dependent behavior of the human intervertebral disc (IVD) by capturing key fluid-microstructure interactions. Building on previous modeling of the annulus fibrosus, this framework extends to the full IVD, integrating the biphasic, fiber-reinforced lamellar structure of the annulus and its coupling with the nucleus pulposus. Parameters are identified from experimental data and validated under multiple loading scenarios, including creep-recovery, cyclic compression, and stepwise compression-relaxation. The model reproduces global mechanical behavior and regional strain distributions, emphasizing the roles of fiber recruitment, fluid redistribution, and anatomical variation. This work enhances our understanding of IVD biomechanics and offers a predictive platform for investigating IVD degeneration and guiding repair strategies.