1. Biomechanical comparison of PLF and PLIF in the lower back PLF (posterior lateral fusion) was the most common fusion method in orthopedics until the 1990s, but clinical and biomechanical studies found a higher incidence of pseudoarthrosis, resulting in a decrease in the rate of implementation. From a biomechanical perspective, the closer the bone graft is to the center of motion of the spine or to the gravity transmission line, the better the fusion results. A functional unit of the spine consists of two adjacent vertebrae and the intervertebral disc between them, with the center of motion located within the disc. Therefore, intervertebral bone grafting is more conducive to bone healing than other bone grafting methods. The simple CD short-segment transcatheter internal fixation system designed in this experiment simulated PLF fusion, while CD-bone block or CD-TFC simulated PLIF. The results showed that there was no significant difference in the immediate stability of the reconstructed lower lumbar spine between the CD-bone block group, CD-TFC group and CD-only group. The stability of the CD group was not significantly different from that of the normal lumbar spine during left/right bending and left/right axial rotation, while the stability of the lumbar spine in all other motion states was better than that of the control group in all three groups. However, after fatigue, the stability of the lumbar spine in the CD group decreased significantly and tended to be unstable, while the other two groups showed no significant damage to spinal stability after fatigue testing. The repositioning and fixation of lumbar slippage and instability and implant fusion can achieve the requirements of spinal biomechanics and stability, and the application of pedicle screw-bar fixation system improves the spinal fusion effect; however, the lack of strong support from the anterior column for simple posterior short-segment internal fixation can easily lead to complications such as loss of repositioning effect and internal fixation failure. In clinical practice, it is recommended that for patients who have opted for PLF, the mobility of the lumbar spine should be limited under the protection of a brace during early postoperative functional exercise, and the amount of lumbar activity should be increased after initial bone healing has been confirmed at 3 months. For cases with significant slippage and severe spinal instability in single segments of L4-5 and L5S1, the PLIF procedure should be selected as much as possible, which helps maintain the repositioning effect and reduce correction loss while preventing the formation of pseudarthrosis. Recently, some scholars have proposed the implementation of a combined PLIF and PLF procedure for states of poor stability.PLIF provides anterior spinal support, and PLF enhances posterior column stability, and can achieve circumferential fusion of the anterior and posterior lumbar columns through a single posterior incision, allowing the necessary support of the anterior column while the posterior internal fixation is more than fractured and loosened.PLIF, in addition to providing a broad anterior implant bed, can The success rate of PLF fusion can be improved by reducing intervertebral motion and maintaining intervertebral height. 2. Biomechanical comparison between intervertebral application of cortical bone block and intervertebral fusion device in PLIF The theoretical basis of PLIF procedure is that intervertebral bone graft fusion is more in line with biomechanical properties, which is conducive to maintaining the height of the vertebral body and avoiding secondary neural canal stenosis. Clinical studies by many scholars have found significant relief of chronic low back pain symptoms in patients after PLIF. Due to the complexity of the operation, the PLIF procedure is still not popularly used in China. In addition, there are still certain complications such as pseudarthrosis formation after PLIF. In order to solve the problem of intervertebral fusion, various intervertebral fusion devices that can carry bone grafting materials have been successfully developed one after another. Although clinical studies of interbody fusion devices are ongoing, relatively few biomechanical tests have been performed, with mixed findings, and most have focused on animal studies. Prospective clinical studies have confirmed the good performance of the Cage; however, in a series of tests on animal specimens, some authors found that the biomechanical performance of the TFC was superior to that of the pedicle plate structure, while some authors concluded that the results of the Cage were not significantly different from those of previous postoperative tests of the PLIF with cortical bone blocks. Since the intervertebral fusion process relies heavily on the upper and lower endplate bones to provide a wide fusion space, and the endplate is not fully developed in animal models, experimental results for animals and humans are different. Previous biomechanical studies of Cage on human spine specimens are relatively few and have focused mostly on the immediate stability of the spine after PLIF, but no biomechanical testing has been performed in the immediate and post-fatigue periods. Biomechanical tests showed no significant changes in lumbar stability after PLIF with different types of intervertebral fusions, so the application of TFC can be considered representative. In this experiment, there was no significant difference between the immediate spinal stability of CD-bone block group and CD-TFC group, both of which were better than the normal lumbar spine stability. This result was different from some reports in the literature, and the analysis of the influencing factors may include: (1) the quality of the sublaminar bone while increasing the contact surface of the bone block with the upper and lower vertebral bodies as much as possible, paying attention to the preservation of the sublaminar bone. (2) The mass of the intervertebral bone graft is taken from the three-sided cortical bone in the anterior third of the iliac crest with sufficient strength, and the bone graft fills the entire intervertebral space as much as possible; (3) The tight combination of the bone graft and the vertebral body provides reliable compression of the fused segment, and if necessary, a hugger is applied to ensure the bone graft plays a significant role in the gravity transmission axis. (4) the application of lateral connecting rods in the experimental operation focused on the posterior internal fixation instruments, especially DTT installation, we observed two cases of DTT bending in the CD-block fatigue group, which indirectly confirmed the role of DTT in three-dimensional spinal fixation. The purpose of the Cage is to provide stronger anterior lumbar column support by pressing into the endplate through its threaded edge, effectively reducing the shear forces acting on the lumbosacral joints and pedicle screws, while also allowing the placement of autogenous cancellous bone and biomaterials in the Cage to promote bone healing. Theoretically, the use of these new Cages is more conducive to maintaining vertebral height, standardizing and simplifying surgical procedures, and reducing the incidence of complications. In view of the fact that the immediate stability of the lumbar spine in the CD-bone block and CD-TFC groups in this study was not significantly different, and the stability after fatigue was better than that in the intact spine group, and the long-term efficacy of Cage needs further observation, the author believes that the selection of intervertebral implants should vary from person to person, and the application of new types of internal fixation should not be pursued adamantly, and the clinical practice should The patient’s own conditions, expectations of surgical outcomes, economic status, and the operator’s proficiency in PLIF technology should be taken into account. The early biomechanical changes after lower lumbar fusion were simulated in this experiment, while the clinical process of spinal fusion is a dynamic one, and intervertebral stability will gradually increase as the bone healing is gradually completed. For now, it remains a challenge to properly model the entire spine, instrumentation, and loading conditions in vivo for a more accurate biomechanical assessment.