# On the Finite Element Modeling of the Lumbar Spine: A Schematic Review
## 4 Discussion
Other hyperelastic models are used as well, though the Mooney-Rivlin and Neo-Hookean aproach are both effective in capturing the biomechanical disc response. Concerning the modeling of the annulus collagen fibers, they can be modelled in a number of different ways. Because these fibers provide important mechanical strength to the annulus, the most appropriate modelling approach is the isotropic linear elastic material model. Nonlinear material behaviour has been used as well. The final results of the computational simulations show that, when using the same modelling, the results of Von Mises stresses, displacements, and intradiscal pressure analysis are quite similar to each other. Different results can be observed depending on the constitutive model chosen for modelling based on whether more or less account is taken of the elasticity or viscoelasticity of the component, whether possible damage to the disc is considered, and other situations. All seven spinal ligaments must be modeled, as they are essential in stabilizing the spine. The most common modelling approach is a linear elastic approach using different Young's moduli as the strain rate changes. Modelling based on nonlinear behavior can be used as well, although the lack of experimental data poses challenges when using this approach. The literature presents a number of different approaches to modelling of the facet joints. Several studies suggest using hyperelastic behavior, while others use simple surface-to-surface contact or unidirectional gap contact. For loading conditions, Figure 4 shows the main loading conditions of the vertebral segments. The follower force is among the most important, and is often schematized by researchers based on the realization that one path line passes through all the rotational centers of the vertebrae. Lastly, the loading conditions of lateral bending, flexion, extension, and axial rotation are common to almost all of the studies we investigated. The validation process generally uses results obtained experimentally on samples of human vertebrae, although tests involving animal specimens can be a viable option. It is crucial to have the opportunity to computationally simulate the biomechanical behaviour of the spine because of the practical and economic difficulties in performing in vivo experiments. One important tool is statistical shape modelling (SSM), which is used to generate a 3D atlas of the spine that can be deformed based on specific boundaries to develop new anatomic models [63]. Among the different approaches used for validation [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64]. [PERSON] et al. [30] performed an experiment on human vertebrae and then simulated and verified the effect of the muscular forces on spinal loading. This was done in order to understand how the muscular loading impacted intradiscal pressure by pure flexion and extension, lateral bending, and lateral rotation tests. [PERSON] et al. [34] implemented ad hoc experimental tests to investigate the stresses incurred by the facet joints during normal bending and extension loading conditions. Meanwhile, other researchers [62] have pursued validation concerning the collagen fibers of the annular ground substance to better explain the stiffness and response of the fibers under different loading conditions. Several different experimental approaches have been used to verify the biomechanical response of the spine considering the response of the ligaments, disc, and facet joints [25; 62; 64]. Based on our literature review, general guidelines to modeling the lumbar vertebral body can be identified to improve the current scientific literature. Cortical and cancellous bone should be schematized through transversely elastic behavior in order to underline their true mechanical responses. Posterior bone segments can either follow the same modeling as cortical and cancellous bone or be schematized through linear isotropic elastic behavior.