Influence of Nucleus Pulposus Geometry on FEM Simulation Results of the Lumbar Spine

Resumo

The lumbar region of the spine is a critical area for biomechanical studies due to its high incidence of pain and injuries. Computational simulations have become essential tools for understanding the biomechanical behavior of this region, enabling the analysis of stresses, displacements, and loads under various conditions. This summary addresses the computational modeling of the spinal region between the second lumbar vertebra (L2) and the first sacral vertebra (S1), the parameters used, and variations found in the literature, highlighting how these differences impact simulation results. The Finite Element Method (FEM) was employed in a linear elastic structural analysis.  The most relevant parameters are the mechanical properties of the intervertebral discs (IVDs), which can be represented in various ways, with elastic or hyperelastic properties. The complexity of IVD geometry, features such as the number of layers in the annulus fibrosus (AF), with or without fibers, and the size and positioning of the nucleus pulposus (NP), significantly affect the numerical model's results.  This study compares two L2-S1 models with different NP sizes and positions. Model 1 has an NP occupying 27% of the IVD, positioned 2/10 posteriorly and 4/10 anteriorly, as proposed by Kapandji (2002). Model 2 has an NP occupying 40% of the IVD, positioned more centrally, following recent studies (Qasim 2014; Von Forvel 2015; Ding 2021; Lu 2022).  Both models used identical material properties obtained from a literature review. The applied loads correspond to the weight of the upper body (≈60% of total body weight), combined with flexion, lateral flexion, extension, and torsion movements. Additional loads simulated lifting 15 kg from the ground in a non-recommended ergonomic posture (knees straight and spine bent), generating intense load concentrations in the lumbar spine.  The simulations were evaluated based on vertical (z) and posterior horizontal (y) displacements, as well as principal stresses in the AF and NP. This study contributes to the biomechanical understanding of the L2-S1 region and aims to support future numerical modeling research.