Modelling the influence of heterogeneous annulus material property distribution on intervertebral disk mechanics

Accurate modeling of the annulus fibrosus (AF) is a crucial aspect to study spine mechanics in silico. Numerical models require validation at both the microscale and organ level to be representative of the real system. Although the AF presents distributed material properties, its response is often modeled with a homogeneous stiffness distribution. The aim of this study was to investigate the influence of different modeling approaches on the numerical response of disk models, based on lamellae mechanics. A material mapping strategy was developed to define element-wise the local annulus material properties, based on prior findings of single-lamella mechanics and collagen distribution. Three modeling approaches were compared: homogeneous, radial and radial-circumferential distribution of material properties. The simulations showed a strong influence of the chosen modeling approach on the disk's tissue- and organ-scale mechanics. A homogeneous model with uniform, average lamellae stiffness predicted a substantially different internal stress distribution and organ-level response, compared to a model with heterogeneous material properties of the annulus lamellae. Finally, the study has indirectly highlighted that the organization of the mature disks could be a consequence of adaptation to the stresses induced by the applied loads, in order to evenly distribute the load over the entire structure.