## 4 Discussion
### Limitations of the Study  
The difficulties encountered during the experimental tests on multi-segment spine specimens led to the use of an animal model due to the limited availability of human specimens. Porcine spine specimens were chosen as a physiologically normal model that remains unaffected by deformity.  
The porcine lumbar spine exhibits a high degree of anatomical and structural similarity to the human lumbar spine, particularly in terms of geometric parameters of the vertebrae [44,45]. Research by [PERSON] et al. [45] has demonstrated that the biomechanical behavior of the human lumbar spine closely resembles that of the porcine low thoracic and lumbar spine. Comparing measurements of the geometry of human and porcine spines, it was revealed that the mean total spine length differed by a mere 0.1 mm between the two species (569.4 \(\pm\) 17.67 mm for humans and 569.5 \(\pm\) 16.19 mm for pigs). Although there are observable differences between the human and porcine spines in terms of the curvature of their respective spinal segments, such as cervical lordosis, thoracic kyphosis, and lumbar lordosis, the heights of the vertebrae in the corresponding spinal regions are similar. Furthermore, when comparing the width and height of the vertebral canal in individual vertebrae between human and porcine spines, it can be concluded that porcine vertebrae serve as a representative model for transpedicular stabilization techniques [5,45,46].  
During the development of the testing protocol in this study, it was noticed that conducting tests on such long spinal segments (Th7-L5) could occasionally result in buckling. It was observed that the absence of active support from the muscular system could compromise the dynamic stability of the system under analysis. Therefore, to prevent the undesirable phenomenon of buckling, parts of the ribs and muscles surrounding the isolated specimens were not removed. Furthermore, the number of load cycles for each test case was limited to 20 cycles. This limitation was implemented to reduce the potential risk of losing dynamic stability, which could lead to secondary instability.  
It should also be emphasized that numerical simulations are rarely performed on such long sections of the spine. As the literature shows, long sections of the spine are described most often with simplified material parameters characterizing individual osseous and soft tissues [24,29,30,47,48]. This is mainly due to the available resources of devices with high computing power. The introduction of nonlinear characteristics in numerical models is associated with an increase in computation time and demand for computer processing power. In order to optimize the time of calculations performed on the cluster, the presented solution to the research problem was simplified in some respects, which obviously was a certain limitation of the numerical analysis. In order to verify the correctness of the adopted assumptions and the obtained results, the model was validated against the literature data.  
The prepared geometric model representing the shape of the thoracolumbar spine accounted for some simplifications by removing details in the vertebral geometry. Another simplifying assumption was the selection of material parameters. Due to the fact that the loads acting on the spinal column are transmitted primarily by the IVD, the focus was put on representing its structure as accurately as possible. The accuracy with which the IVD was reproduced meant that the osseous tissues, ligaments, and muscles were described with simplified linear material parameters.