With the widespread use and development of transcatheter internal fixation techniques, improving pedicle screw placement techniques and improving pedicle screw revision is one of the current topics of discussion [1], while the internal fixation of lumbar 5-sacral 1 has its own specificity and revision is more difficult. This experiment intends to biomechanically evaluate the different methods of lumbosacral pedicle screw revision and provide a basis for clinical application. I. Materials and methods 1. Materials 2. Specimen preparation Seven fresh healthy young adult cadaveric lumbosacral spine specimens were examined by X-ray to exclude congenital malformations, fractures, tumors, osteoporosis and other spinal disorders before the experiment. The soft tissues around the vertebral bodies were removed, the intervertebral discs were severed from the vertebral body, the lumbar 5 was freed into a single vertebral body, and the sacral 1 vertebral body was kept intact during the sacral trimming process, sealed in a double-layer plastic bag, and stored frozen in an ultra-low temperature refrigerator at -20 degrees. The specimen was removed 24 hours before the test and thawed naturally at room temperature. 3.Vertebral arch screws were provided by J.H. Hua, with four specifications of 5.5mm/45mm, 7.0mm/55mm, 6.25mm/35mm and 8.0mm/45mm. The screws were lengthened at the end to facilitate fixture fixation during the experiment. The thread spacing is 1.2mm, depth 1mm (Figure 1). 4, polymethyl methacrylate (PMMA) bone cement provided by the Tianjin Institute of Synthetic Materials Industry, a room temperature self-consolidating agent composed of methacrylate/styrene copolymer phenol and methyl methacrylate monomer. Methods 1. Biomechanical testing of pedicle screws The screws were placed according to the conventional pedicle approach, and the individual vertebrae were fixed in a special fixture for biomechanical testing of pedicle screws on a U.S.-made 858 Mini-MTS multi-axis testing machine. According to the experimental design, different types of pedicle screws were inserted and a thread was reserved at the end, and then the specimen was placed on the MTS machine and screwed into the screw completely, while the torque data were recorded at each time and the maximum torque was finally obtained; then the screw extraction experiment was conducted and the maximum axial extraction force was set at 4000N, the maximum displacement was 5mm for the screw fixation group alone and 30mm for the screw plus bone cement fixation group. 30mm, along the long axis of the pedicle direction at a loading rate of 5mm/min screw extraction, when the load-deformation curve yielded (Figure 2), reached the maximum displacement or the vertebral body pedicle damage will be stopped. 2, experimental grouping lumbosacral vertebrae were evaluated separately for biomechanical experiments. 3, lumbar 5 experimental grouping Normal control group: 5.5mm/45mm pedicle screws were placed in 7 lumbar 5 vertebrae. Screw revision group alone: 7.0mm/55mm screws were placed in the normal control group after the completion of the experiment. Bone cement reinforced fixation group: After the completion of experiments in the first two groups, one side of the arch was randomly selected for revision with 5.5mm/45mm screws plus bone cement reinforced fixation, and the other side was revised with 7.0mm/55mm screws plus bone cement reinforced fixation. 4.Sacral 1 experimental grouping Normal control group: 6.25mm/35mm pedicle screws were placed in the 7-segment sacral specimen. Screw revision group alone: after the completion of the experiment in the normal control group, 8.0mm/45mm screws were placed for revision; cement-reinforced fixation group: after the completion of the experiment in the first two groups, 6.25mm/35mm screws plus cement-reinforced fixation were randomly selected on one side of the pedicle for revision, and 8.0mm/45mm screws plus cement-reinforced fixation were applied on the other side for revision. 5.Observations The maximum axial extraction force and the maximum lateral torque moment of the pedicle screws, as well as the damage to the vertebrae or the pedicle during the extraction process after the revision of the pedicle screw reinforced fixation. After the bone cement reinforced fixation group was extracted from the experiment, coronal or sagittal cuts were made along the long axis of the pedicle to observe the distribution of PMMA around the pedicle screws. 6. Statistical processing: The experimental errors and crude differences were corrected and processed, and all data were subjected to two-way categorical ANOVA (Student-Newman-Keuls method). The t-test for the mean of the randomized paired design data was performed between each treatment group and the control group and between different diameter screws in the same group, and the significance level was set at 0.05. III. RESULTS 1. Maximum pedicle screw holding force The maximum pedicle screw holding force of different groups is shown in Table 1. lumbar spine: after the revision of 7.0mm/55mm screws and the application of cement-reinforced fixation alone, a significant increase in pedicle screw holding force was obtained. The increase was more obvious with cement-reinforced fixation; among them, there was no significant difference in holding force between pedicle screws of different diameters and lengths after cement-reinforced fixation (P > 0.05). Sacral spine: after revision of 8.0mm/45mm screws alone, there was no significant difference in the holding force of pedicle screws compared with the control group; after applying cement-reinforced fixation, the holding force was significantly higher than that of the normal control group and the screw revision group alone, and there was no significant difference in the holding force between different types of screws (P > 0.05). 2.Maximum torsional moment of pedicle screws The torsional moments of pedicle screws in different groups are shown in Table 2. after the revision of 7.0mm/55mm screws alone in lumbar 5 pedicle, the torsional moment increased significantly, while in sacral 1 pedicle, after the revision of 8.0mm/45mm screws alone, the torsional moment did not change significantly. In the lumbosacral spine, the torsional moment was significantly reduced after cement-reinforced fixation (the screws could still be screwed out relatively easily after cement curing, Figure 3), and there was no significant difference in torsional moment between screws of different diameters and lengths (P>0.05). 3. Correlation analysis between the maximum holding force and the maximum torque of pedicle screws The experimental results showed that the screw holding force and torque of the screw revision group showed an increasing trend, while the screw holding force increased and the torque decreased in the bone cement reinforced fixation group. The maximum holding force and torsional moment of screws in the control group and the screw revision group were set as the independent variable and the dependent variable respectively, and the correlation coefficient r=0.85 was obtained from the statistical analysis, indicating that the torsional moment and holding force were positively correlated when the screws were combined with bone. 4. Damage during pedicle screw extraction and distribution of bone cement in the pedicle No damage to the pedicle or vertebral body was observed because protective parameters were adopted in the experimental settings of the normal control group and the screw-only revision group. Most of the extraction damage of the pedicle screws in the cement-reinforced fixation group was the extraction of the pedicle screws, i.e., the peeling of the bone-screw interface; only two cases of fracture of the junctional area between the pedicle and the vertebral body occurred in the cement-reinforced revision of the lumbar 5 pedicle (Figure 4). Under the present experimental conditions, the bone cement powder mixed with water in a ratio of 2:3 was easily injected and evenly distributed in the pedicle, and the screw texture traces were clear (Figure 3). With the widespread use of transforaminal internal fixation in spinal surgery, the number of cases of internal fixation failure has gradually increased. Moore et al. suggested [2] that the incidence of screw loosening and withdrawal during and after degenerative slippage was nearly 5.5%, while the failure rate of internal fixation after spinal fracture was as high as 9%. In recent years, articles on pedicle screw revision and reinforced fixation have been published at home and abroad, but few biomechanical studies have been reported on the evaluation and comparison of specific anatomical structures of the lumbosacral spine. 1. Selection of revision screw size Different sizes of screws present different biomechanical properties, and within the limits of the anatomical morphology of the pedicle and the mechanical needs of internal fixation, increasing the diameter or length of the revision screw can enhance the screw-bone bond [3]. The extraction force is related to the surface area of the cylindrical bone and is determined by the outer diameter of the screw and the depth of entry. When large-diameter screws are placed, the cancellous bone in the center of the pedicle is pushed into the peripheral relatively dense layer, and the threads can be placed into the peripheral relatively dense layer; extraction strength tests for pedicle screws of different diameters showed that the extraction strength of screws with larger diameters was greater than that of screws with smaller diameters, and the extraction strength increased with the increase of the outer diameter [4]. The longer the screw, the greater the fixation strength, and it was found that 60% of the screw fixation strength was within the pedicle, and the strength increased by 15-20% after reaching the cancellous bone of the vertebral body, by another 16% when reaching the anterior bone cortex but not penetrating it, and by 20-25% when penetrating the anterior bone cortex [5].Polly et al [6] concluded that a 2-mm increase in diameter is the most ideal method for pedicle screw revision; a 1-mm increase in diameter and a length increase of 5-10 mm is also a reliable means. However, the method of simply increasing the diameter of the screw to improve stability, which is commonly used in clinical practice, is strictly limited by the anatomical conditions of the pedicle, and the use of larger diameter pedicle screws also increases the risk of nerve root injury and pedicle fracture, and the maximum outer diameter of the screw should not exceed 80% of the outer diameter of the pedicle [7]. In the author’s opinion, screw revision is the best option to increase the diameter and length of the screw with accurate access points because the previous nail path has been damaged. 2. Differences in the holding force of lumbosacral pedicle screws and anatomical characteristics This experiment showed that when the lumbar 5 pedicle screws were revised, the holding force of the pedicle screws exceeded the strength of the original screws when the diameter was increased by 1.5 mm and the length was increased by 10 mm; the holding force of sacral 1 screws was generally smaller than that of the lumbar spine, and the result of simple screw revision was different from that of the lumbar spine, where the holding force only reached the level of the original screws. The pedicle screw holding force depends on the bonding strength of the thread-bone screw interface, and the quality of the bone around the screw becomes a key factor in determining the screw holding force, and high-quality bone can obtain a greater holding strength. In the lumbar spine, screw screwing compresses the cancellous bone to the strong cortical bone, and the bone density around the screw is relatively high, with a more satisfactory bone quality; the placement of thick screws can even cut into the surrounding bone cortex, increasing the holding strength. In the sacrum, due to the obvious widening of its arch, the cancellous bone content and the lack of strong bone cortex, the bone density around the screw is low and the bone strength is low after screwing, and the holding strength is relatively small, and it is often difficult to obtain satisfactory holding strength by increasing the diameter and length of the screw. Therefore, clinically, there are more improvement methods for internal fixation of sacral arch, such as increasing the length of screws to make them penetrate the anterior bone cortex, changing the direction of needle entry through S1 upper endplate and even L5 lower endplate or adding additional screws in S2. 3.Bone cement reinforced fixation revision Clinically, when the arch nail tract is severely damaged or osteoporotic, it is more difficult to revise simply with large diameter screws, and often add biomaterials to strengthen the stability of the arch screw. After reinforcement with bone cement, the bone screw interface is transformed into a stronger bone-adhesive-screw interface [8], which can significantly increase the pedicle screw holding force and has little relationship with the screw structure itself, and the screw extraction mainly relies on the bone cement-bone interface peeling. The key to revision is how to mix and inject the bone cement, so that it is evenly distributed in the pedicle and achieves the best combination with the screw. In clinical practice, the successful application of simple screw revision is always ideal, but the actual situation is often very complicated, such as osteoporosis, serious damage to the nail path, poor access to the needle position and the need for additional rerouting, etc. It is more difficult to use large diameter screws alone for revision, and biomaterials need to be added to strengthen the stability of the pedicle screws. In particular, the sacral spine has a special anatomical structure, with a wide pedicle and a large cancellous bone content. After the failure of the first screw fixation, the screw loosens and causes cancellous bone extrusion, which makes the nail channel significantly larger than the original screw, and it is difficult to obtain sufficient holding force with the limited increase of the screw diameter and length, so the significance of bone cement reinforced fixation in the revision of the sacral spine is more significant at this time. Since the strength of the bone-adhesive-screw interface exceeds the structural strength of the spine itself, the improvement of biomaterials should focus on controlling the histochemical reaction of the additive, and it is not very meaningful to increase the adhesive strength of the reinforcement excessively. The clinical application of common bone cement can produce a series of problems, such as damage to surrounding tissues (including the spinal cord and nerve roots) from polymerization heat, toxicity and carcinogenic effects from long-term in vivo retention, etc., which are gradually replaced by new biomaterials [9]. 4, correlation between pedicle screw holding force and torque moment Most biomechanical experiments use axial extraction force as an index to assess screw holding strength, but recent studies have found that lateral loading is often the main cause of early screw loosening [2], so the combination of holding force and extraction force better reflects the holding strength of screws. The results of this experiment showed that the screw holding force and torsional moment did not show absolute agreement, and the correlation between holding force and torsional moment was very different for different screw bonding interfaces. The correlation between the bonding strength and torque moment at the bone-screw interface alone is significant, which is consistent with previous studies (correlation coefficients of 0.83-0.925) [10]; in practice, physicians are also accustomed to judging the holding strength of pedicle screws by torque moment, which has some clinical utility. Bone cement reinforced fixation is the screw-cement-bone interface, and the torque moment is meaningless when the bone cement has not yet cured at the time of screw placement. When the bone cement is cured, the bone-cement bond is solid, while the screw-cement bond is not tight enough, so the screw can still be twisted easily; the holding force of the screw mainly relies on the anastomosis of the bone cement with the thread inlay after curing, depending on the strength of the bone cement itself and the bone-cement bond strength.