## 4 Discussion
Therefore, the clinical spinal surgeon must tighten the screws when fixing the screw system. This may cause loosening of the screw system during the movement of the patient. The high stress of the cage mainly occurs at the cage edge. Therefore, arc design can be used to avoid high stress in cage design. In addition, titanium alloy was used for the cage, traditional pedicle screw system, and CBT system in this study. The yield stress of titanium alloy is about 1100 MPa [34]. The stress value of the cage and screw system observed in this study is much lower than 1100 MPa, so the cage and screw system would not cause permanent deformation under the load in this study. In addition, the area enclosed by four screw tulips in each group was observed from the posterior of the spine (Figure 13). The CBT is used in Group 2, which is approximately half of the area surrounded by the traditional pedicle screw system. The area of group 4 was about half that of the traditional pedicle screw system. However, the area of group 3 was similar to that of the traditional pedicle screw system. Therefore, if a clinical surgeon wants to perform spinal revision surgery, the patient was originally treated with a traditional pedicle screw system. Clinical spinal surgeons may consider using CBT for revision surgery. This may allow patients to have smaller wounds [8; 35]. In addition to the advantage of a smaller wound, the clinician has a smaller area of muscle to open when using CBT screws. According to previous research results, it is pointed out that dysfunction of these muscles may be associated with increased pain [36; 37]. Therefore, postoperative pain is reduced, and minimally invasive surgery could be achieved. There are some limitations to this study. The material properties used in this study should be set as anisotropic on bone or hyperelastic material on the intervertebral disc by previous research [22]. However, it is not easy to solve using nonlinear analysis in finite element analysis. In addition, the intervertebral disc structure established in this study is quite complex (the intervertebral disc model of this study contains 5 nucleus pulposus, 4204 annulus fibrosis, and 35 annulus ground substances). In order to avoid the use of nonlinear material properties and thus lead to the influence of the main topic to be discussed in this study, the properties of the materials in this study were mainly selected by homogeneous, isotropic, and linearly elastic materials. In addition, the screw system used in the simulation model of this study uses screws with a diameter of 5.5 mm, so the screw size is not consistent with the actual clinical use. The purpose of the mechanical evaluation was to compare the contrast between the traditional pedicle screw system and the CBT system and to avoid the influence of the screw diameter. In addition, the shape of the cage was also represented by a simple square shape, mainly to avoid the high stress caused by the serrated cage, which made the analysis results difficult in this study. After such simplification, this study may have some differences from the actual situation. However, it will not seriously affect the trend of research results. Such simplification can make the topics concerned by this research have more obvious trends. In addition, only two lumbar vertebrae were used for simulation evaluation in this study. Clinical spinal revision surgery usually involves the operation of more than two lumbar vertebrae. Therefore, after the evaluation of this study, more biomechanical evaluations of vertebral fixation can be performed in the future. This study evaluated the effects of the CBT screw and traditional pedicle screw implantation on the lumbar spine by finite element analysis. Although the results analyzed  
Figure 13: The area surrounded by four screw tulips in each group was viewed from the posterior of the spine. in this study may be slightly different from the actual situation.