The finite element method is a mathematical analysis method that can be implemented to solve biomechanical problems. This technique was first applied to the medical field in the 1960s. Belytschko et al.  established the three-dimensional finite element model of the spine for the first time in 1972. For 40 years, the finite element method has been widely used in the field of spinal surgery. Finite element analysis of thoracolumbar fracture can provide very meaningful digital orthopaedic data for clinical practice, which is helpful for physicians to make appropriate surgical plans and engineering and technical personnel to improve material properties and optimize designs [13–17].
Based on previous related research [18–20], this study established a finite element model of an L1 burst fracture. Because the finite element model requires the internal structure to be as regular as possible during the meshing process, the model was repaired and slightly modified during the introduction process; however, the basic shape of the structure was basically retained. To simulate the true resetting process in the clinic, we divided the loading into two categories. First, in the process of restoring physiological lordosis, we loaded the displacement from 1 mm to 10 mm in the direction of 10° behind the Z-axis. When the displacement was 6 mm, the injured vertebrae recovered the physiological lordosis, and the leading edge height returned to normal. At this time, the whole vertebral body stress was concentrated on the upper intervertebral disc of the injured vertebra, and the upper intervertebral disc played a major role in the restoration of the injured vertebra under high stress, which can restore the height of the front edge of the injured vertebra, correct the kyphosis angle, and injure during the reduction. A certain reduction space was created in the vertebra, which was beneficial to reset the bone in the spinal canal. Second, in the process of opening and resetting, we applied the displacement load in the positive direction of the Z-axis and found that the place where the displacement changes the most was the posterior longitudinal ligament behind the injured vertebra. Due to the reduction of the posterior longitudinal ligament, a certain thrust was applied to the anterior bone mass, thereby pushing the bone mass in the spinal canal into the previously created reset space. By analysing the results of the two loads, we believe that there were two main forces in the reduction of the bone in the spinal canal. First, in the process of restoring physiological lordosis, the height of the vertebral body, especially the height of the anterior border of the vertebral body, was reset by the pulling force of the intervertebral disc (especially the annulus). At this time, there will be some reset space in the vertebral body. During the distraction process, the bone block in the spinal canal was pushed into the internal space of the vertebral body by the action of the posterior longitudinal ligament, thereby achieving the reset effect. This phenomenon also explains the poor effect of vertebral fracture reduction when the bone was turned over in the spinal canal or accompanied by posterior longitudinal ligament injury. Kose et al.  analysed the influencing factors of bone reduction in the spinal canal and found that the height of the vertebral body leading edge and the reduction of the vertebral wedge angle were substantial influencing factors, which is consistent with our research. In addition, the reduction effect of T12 and L1 is better than that of L2, and the reason for this discrepancy is related to the anatomy of the posterior longitudinal ligament. Hu et al.  found that the posterior longitudinal ligament exhibited the highest toughness in the thoracolumbar region. We believe that the main influencing factors of bone reduction in the spinal canal are the posterior longitudinal ligament and the intervertebral disc (especially the annulus fibrosus). The intervertebral disc plays an important role in reducing the height of the vertebral body and the angle of the wedge. The main function of the posterior longitudinal ligament is to push the bone in the spinal canal into the space created by the reduction of the vertebral body. The indirect reduction of the posterior ligament complex of the spine eliminates the need for extensive incisions to separate muscles, thereby reducing damage to muscle vessels. Furthermore, in this approach, there is less intraoperative bleeding and less trauma, which is conducive to the recovery of lumbar function. Thus, implementing this approach can enable the patient to get out of bed earlier after surgery, thereby satisfying the concept of rapid rehabilitation surgery . In clinical applications, we should pay attention to adjusting the angle of the nail to facilitate the reduction of the lordosis of the vertebral body. In addition, moderate overcorrection can significantly increase the reduction of the disc.
The finite element simulation has certain errors due to the influence factors, such as model simplification and assignment distortion. Due to the limitations in finite element model design experience, the finite element model is not perfect. The limitations in this study are that the effects of the surrounding tissues of the vertebral body (soft tissues, such as muscles) on the reduction process are not considered and that other types of fractures, such as endplate fractures, are not elaborated. In the future, we will continue to improve the process of generating and analysing the model so that the analysis results will be closer to the real environment of the human body.