A variety of factors (disc pathology, lumbar spondylolisthesis, tuberculosis, tumors, trauma, etc.) can lead to lumbar instability, and spinal fusion is an important tool for reconstructing lumbar stability. The emergence of spinal biomechanics in the 1980s has further elucidated the importance of the posterior lumbar structure for lumbar stability and provided a theoretical basis for spinal fusion [1]. In recent years, the development of various fusion techniques has increased the fusion rate of the spine. The aim of this study is to deepen the understanding of the biomechanical characteristics of different fusion methods of the lower lumbar region through immediate and fatigue stability biomechanical studies of the currently commonly used clinical methods of lower lumbar fusion. I. Materials and methods 1. Materials (1) Experimental materials: 9 fresh specimens of adult lumbar 1-sacral vertebrae were used for this experiment, and the bone organism was excluded by X-ray examination before the experiment; intervertebral bone graft blocks were taken from the iliac crest of healthy cadavers with bone cortex on three sides, each block was about 2.5 cm (length) × 1.2 cm (width) × 1.1 cm (height) in size; the intervertebral fusion device (cage) was the agent of Huajiehao Company. The titanium TFC, with diameters of 16 mm and 14 mm, was selected according to the pre-experimental X-ray measurements; the posterior transpedicular internal fixation device was a short-segment CD internal fixation system distributed by the same company. (2) Specimen preparation: After obtaining the specimens, soft tissues such as fat and attached muscles were removed, while ligaments, joint capsule, intervertebral disc and bony structures were kept intact. The ends of the specimens were embedded with polymethacrylate and stored in a double plastic bag sealed in a -20℃ freezer for storage. Methods 1. Three-dimensional spinal motion test: The three-dimensional spinal motion tester used in this experiment (Figure 1) can simulate the spinal column in human motion characteristics, i.e., the test device can apply pure force dipole moment to the spinal specimen without affecting the free motion of the specimen after bearing. The sacrum of the specimen is fixed on the base and the loading disc is fixed at the L1 embedding end, and a pair of forces of equal magnitude, opposite direction and parallel to each other is applied to the specimen through the loading disc to form a pure force couple acting on the specimen specimen. By controlling the magnitude of the applied force, adjusting the orientation of the loading disc and the direction of the loading disc, the force couple moments of forward flexion/backward extension, left/right bending and left/right axial rotation are applied to the specimen, simulating the physiological activity form of the lumbosacral region and causing the corresponding motion of the lumbar spine. The three-dimensional motion images of the spine at zero and maximum load (8.0 N.m) were taken by two cameras at an angle to each other, and the marks attached to the scale were identified and positioned by a computerized image processing system. According to the theory of rigid body kinematics, the motion of any non-coherent three points on the rigid body can characterize the motion of the entire rigid body, so the two cameras at an angle to each other can be used to calculate the angle change between the segments, that is, the range of motion (range of motion, ROM). 2, fatigue test: fatigue group specimens placed on the 868Mini-MTS multi-axis experimental machine (Figure 2), with a speed of 400N / S, loaded to 200N load (load frequency of 1Hz); left and right 10 ° each rotation, the number of fatigue for 1500 times. After completing the operation, the specimen was removed and then placed on the spine 3D motion machine for testing. 3. Experimental procedure: The three-dimensional motion test of L4-5 segments was performed on the same specimen for the following eight states: ① intact structure of the lower lumbar spine; ② unstable lumbar spine (i.e., total L4 laminectomy and inferior synovectomy with simultaneous L4-5 nucleus pulposus removal) [2]; ③ CD short segment internal fixation (CD); ④ CD short segment internal fixation with intervertebral bone graft (CD- bone block,Figure 3); ⑤ CD short-segment internal fixation with intervertebral TFC fixation (CD-TFC,Figure 4); ⑥CD fatigue; ⑦CD-bone block fatigue; ⑧CD-TFC fatigue. After completing the three-dimensional motion test in each fatigue state, the internal fixation device was reinstalled and the pedicle screws were checked for loosening to avoid affecting the test results in the next state; to avoid the bias of the experimental results (systematic error) caused by the different test sequences, the test sequences of different states were randomly changed. At the same time, the specimen was constantly sprayed with saline to ensure its wetness throughout the experiment to minimize the tissue degeneration caused by the experiment on the specimen. x-rays were required after the installation of CD internal fixation, CD-bone block and CD-TFC (Figure 5) to ensure that the endosseous position was satisfactory. 4. Statistical processing: The data collected in this experiment were mainly the range of motion (expressed as angular displacement), in which the experimental errors and coarse differences were corrected and processed, and all data were subjected to two-way categorical ANOVA (Student-Newman-Keuls method). Using the segmental motion of its own intact structure as the control group, a t-test (α=0.05) was performed on the means of the randomized paired design data for each treatment group to observe the statistical significance of ROM changes in each treatment group relative to the intact structure group; meanwhile, the CD-bone block group was compared with the CD-TFC group and the CD -bone block fatigue group and CD-TFC fatigue group, respectively, were subjected to paired data t-test to further explore the effects of the two fusion methods on spinal stability. III.RESULTS There was a significant increase in angular displacement ROM as an indicator of segmental instability. After applying a load of 8.0 N.m, the measured ROM at L4-5 in anterior flexion/extension, left/right bending and left/right rotation for the eight states of the lumbar spine are shown in the accompanying table and Figure 6. The results showed that the lumbar spine instability model was satisfactorily fabricated, and the ROM during various activities was significantly increased in the instability group compared to the intact structure group. The stability of the CD-block and CD-TFC groups was significantly higher than that of the normal lumbar spine in all six directions of activity. -The lumbar spine was unstable in the fatigue state of CD, and was significantly unstable in forward flexion and extension and left and right rotation, while it was not significantly different from the normal lumbar spine in left and right lateral bending; the stability of the CD-block in the direction of forward flexion and extension in the fatigue state was still better than that of the normal lumbar spine. The stability of the CD-bone block in the direction of forward flexion and extension is still better than that of the normal lumbar spine in the fatigue state, while there is no significant difference with the normal lumbar spine in the direction of left and right lateral bending and left and right rotation; the stability of the CD-TFC in the direction of posterior extension and left and right rotation is significantly higher than that of the normal lumbar spine in the fatigue state, while there is no significant difference with the normal lumbar spine in the direction of forward flexion and left and right lateral bending activities.The comparison of the stability of the CD-bone block and CD-TFC groups is shown in Figure 7. The stability of the CD-bone block fatigue group was not significantly different from that of the CD-TFC fatigue group in all directions, as shown in Figure 8, and the stability of the two groups was not significantly different in the direction of forward flexion and back extension and left and right lateral bending, but the stability of the CD-TFC was significantly better than that of the CD-bone block group in the direction of left and right rotation. IV. Discussion 1. Biomechanical comparison of PLF and PLIF procedures in the lower back The PLF (postero-lateral fusion) procedure was the most common fusion modality in orthopedics until the 1990s, but clinical and biomechanical studies found a higher incidence of pseudoarticular formation, which led to a decrease in the rate of performing the procedure [3]. 1944-1945 Briggs, Milligan and Cloward The PLIF (transforaminal lumbar interbody fusion) technique was first proposed in 1944-1945 and has since been refined through the efforts of many scholars. From a biomechanical point of view, 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 [4]. A functional spine unit (FSU) consists of two adjacent vertebrae and the intervertebral disc between them, and its center of motion is 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-block and CD-TFC groups and the CD-only group, and 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. 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 L4-5 and L5S1 single segment, the PLIF procedure should be selected as much as possible, which will help 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 [5].PLIF provides anterior spinal support, and PLF strengthens 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 bed of bone grafting anteriorly PLIF can also improve the success rate of PLF fusion by reducing the intervertebral motion and maintaining the intervertebral height. 2. Biomechanical comparison between the intervertebral application of cortical bone block and intervertebral fusion device (cage) 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 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, PLIF is still not widely used in China. In addition, there are still certain complications such as pseudarthrosis formation after PLIF [6]. In order to solve the problem of intervertebral fusion, various intervertebral fusion devices (stainless steel, bioceramic, titanium alloy, carbon fiber, polymer material) that can carry bone graft materials have been developed successively [7]. Although clinical studies on intervertebral fusion devices are flourishing, relatively few biomechanical tests have been performed, with mixed conclusions, and most of them focused on animal experiments.Brantigan et al [8] confirmed the good performance of Cage in a prospective clinical study; however, in a series of tests on animal specimens, some authors found that the biomechanical performance of TFC was superior to that of the pedicle plate structure, while some authors concluded that the results of the application of Cage were not significantly different from previous postoperative tests of PLIF with the application of cortical bone blocks [9]. Since the intervertebral fusion process relies mainly 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 mostly focused on the immediate stability of the spine after PLIF, but no biomechanical tests have been performed on the immediate and post-fatigue periods. Biomechanical tests showed no significant changes in lumbar stability after PLIF with different types of intervertebral fusion [10], so the application of TFC can be considered representative. In this experiment, there was no significant difference between the immediate stability of the spine in the CD-bone block group and the CD-TFC group, both of which were better than the stability of the normal lumbar spine, and this result was different from some literature reports [11]. (2) the quality of the intervertebral bone graft The three-sided cortical bone graft from the anterior third of the iliac crest should be strong enough, and the bone graft should fill the entire vertebral space as much as possible; (3) the tight integration of the bone graft with the vertebral body The experimental operation focused on the installation of posterior internal fixation instruments, especially the DTT, and 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 application 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, and also allowing the placement of autologous cancellous bone and biomaterials in the Cage to promote bone healing [12]. Theoretically, the application of these new Cages is more conducive to maintaining vertebral height, standardizing and simplifying surgical operations, and reducing complications. Given that the immediate stability of the lumbar spine in the CD-bone block group and the CD-TFC group in this study was not significantly different, and the stability after fatigue was better than that of the intact spine group, and the long-term efficacy of Cage needs to be further observed,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 ad hoc, 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.