## 4 Discussion
clinical spinal surgeons to consider the biomechanics of cortical trajectory systems during retrofit surgery. By observing the deformation of the lumbar spine, the results showed that there was no significant difference between groups. There was no significant difference in overall spinal stability between the use of a traditional pedicle screw system, CBT system, or both. However, the CBT system implantation was slightly more stable than the traditional pedicle screw system. In addition, lumbar spine stress distribution was observed after implantation of the traditional pedicle screw system or CBT system. The results show that there are obvious differences in stress distribution under four different load conditions. There was no significant difference in stress distribution in the lumbar spine with different implants. The CBT system was used clinically with a finer screw diameter (approximately 4.5 mm in diameter) than the traditional pedicle screw system (approximately 6.5 mm in diameter). The CBT system and traditional pedicle screw system showed similar stability in mechanical studies [15; 32]. The results of this study may be inconsistent with those of previous studies [16], mainly because the cage for transforminal lumbar interbody fusion is implanted in the model of this study. Therefore, cage implantation may make the whole spinal structure more stable. There were also clinical studies involving a traditional pedicle screw system and CBT system using cage implantation [33]. The CBT system was used to achieve stability comparable to that of the traditional pedicle screw system. In addition, the stress of the lumbar L3 and L4 showed low stress under four different loading conditions. This can be explained by the stress shielding effect (When the difference in elastic modulus between the screw system and the bone is large, the stress shielding effect will be more obvious. The force is transmitted mainly by the screw system, so the stress on the bone is reduced.). With the high rigidity of the implanted screw system, higher stresses occur in the adjacent vertebrae (the lumbar L2 and L5). Peak von Mises stress on the endplate was observed and higher stress was found in the endplate implanted on the upper side of the traditional pedicle screw system. The CBT system is used in the lower endplate of L2 with lower peak stress. The CBT system may reduce the stress on the endplate (Group 3 and Group 4 also exhibit less stress on the endplate than Group 1). However, more clinical evidence is needed to confirm whether the CBT system can reduce the injury of the adjacent lumbar segment. The CBT is mainly based on screw implantation of the cortical bone to achieve stability. Therefore, observing the upper vertebral body of the implant system in this study, peak von Mises stress in the lumbar L3 could be observed with lower stress on the lumbar vertebral body implanted in the cortical trajectory. The screw of the CBT system is fixed to the cortical bone, and [PERSON]'s modulus in cortical bone is bigger than [PERSON]'s modulus in cancellous bone. Thus, the lumbar L3 used the CBT system with minor deformation in the spine lumbar vertebral body. According to [PERSON]'s law, stress is proportional to strain, so there is less stress in the lumbar vertebrae implanted with a CBT screw. In addition, observing the lower vertebral body of the implant system in this study, the screws of CBT screws are inserted from the bottom up and from the inside out. Therefore, the vertebra of the spine is only partially supported by screws. As a result, the vertebral body (the lumbar L4) deforms greatly and produces high stress. In Group 4, greater deformation occurred in the lumbar L4 because of the placement of screws in the lumbar L4 with cancellous bone and the use of CBT in the lumbar L3. Therefore, Group 4 has higher stress in the lower vertebral body (the lumbar L4). In addition, distribution of von Mises stress on the screw system and cage was observed. High stress of the screw system mainly occurs near the joint of the screw and screw tulip.