## 1 Introduction  
The traditional pedicle screw fixation system has been widely used in the treatment of lumbar diseases such as disc herniation and disc stenosis. The traditional pedicle screw remains the most commonly used for spinal surgery. However, for patients with osteoporosis, due to the lack of fixation force of pedicle screws, complications such as screw loosening, displacement, and prolapse easily occur [1]. In addition, interbody fusion may accelerate degeneration of adjacent segments of the spine, resulting in adjacent segment disease (ASD) [2]. As a result, the rate of revision surgery within 5 years is as high as 20% in patients undergoing lumbar fusion [3, 4]. When performing ASD revision surgery, the surgeon often needs to remove the original vertebra screw fixation, which increases the exposure of the patient's surgical wound. This not only increases surgical time, blood loss, and surgical trauma but also increases the risk of postoperative infection [4]. Therefore, revision surgery is often more difficult than primary surgery, and there are many problems such as intraoperative bleeding and surgical complications. In addition, whether the revision surgery uses the same approach as the previous surgery and whether retaining the original pedicle screw are common problems encountered by clinical spine surgeons [5]. Cortical bone trajectory (CBT) is an improvement, and development of traditional pedicle screw fixation in patients with osteoporosis can significantly reduce intraoperative blood loss [6; 7]. CBT screws are implanted closer to the midline spinous process than traditional pedicle screws and have less soft tissue dissection. Additionally, CBT screws are implanted in an inward downward upward direction, which is less likely to invade the adjacent intervertebral joints. In addition, the screw and cortical bone contact area are large, so it can increase the screw torque or pull force, avoiding a loose screw [8]. The advent of CBT internal screw fixation provides a new idea for revision surgery for ASD patients. A CBT screw can be used to achieve revision by combining it with a traditional pedicle screw. There are a number of spinal surgeons who perform this type of surgery, and they all have good results [9; 10; 11; 12]. Although CBT screws and traditional pedicle screws have reached similar revision surgery rates and fusion rates [13], CBT screws and traditional pedicle screws are implanted in different positions in the vertebral body of the spine. Thus, the combined structure of using a CBT screw and a traditional pedicle screw is more complicated. Therefore, more mechanical analysis is needed to evaluate the stability. Finite element analysis has been used to evaluate the mechanical outcome of CBT screw implantation [14]. One researcher evaluated the size of different CBT screw implants. The results showed that the ideal screw size for CBT is greater than 5.5 mm in diameter and 35 mm in length [15]. Other researchers also assessed the difference between CBT screw implantation and traditional pedicle screws using a finite element method [16; 17; 18]. However, the results of each study were inconsistent, so the difference between CBT screw implantation and traditional pedicle screws was still worth discussing. A finite element analysis model of a CBT screw combined with a traditional pedicle screw was also established [19]. The biomechanical evaluation was compared with the CBT screw system, traditional pedicle screw system, and universal spine system. The results showed that CBT screws did not exhibit better mechanical properties than traditional pedicle screw systems and universal spine systems. This previous study only established three vertebrae and two discs. Therefore, biomechanical evaluation of the vertebral body near the intervertebral disc was not performed. It is also important to observe the biomechanical effects of adjacent segments after the vertebral screw system is implanted in the vertebral body.