With the development of the OLIF and the associated surgical instruments, OLIF is increasingly being used by spine surgeons. In contrast to lateral lumbar interbody fusion (LLIF) and posterior lumbar interbody fusion (PLIF), OLIF accesses the target disc from the window between the major abdominal vessels and the psoas, thus reducing the risk of lumbar plexus injury and paravertebral tissue destruction. The OLIF is characterized by shorter operation time, less bleeding, less postoperative pain, shorter hospital stay and faster postoperative recovery.
However, OLIF techniques are limited by the difficulty in removing lateral recess, ligament flava hypertrophy, and hyperostosis of facet joints directly from the lateral corridor. [30, 31] The improvement of symptoms after OLIF is based on the restoration of disc height, the increase of the foraminal area, the correction of coronal balance, and the indirect decompression of the neural elements by the wider implants[6, 32]. Joseph  et al. reported a complication rate of 20.2% (380/1885) for nerve injury in TLIF, while Abe et al. reported a complication rate of 1.2% (2/155) for nerve injury in OLIF, suggesting that the complication rate for nerve injury in OLIF is significantly less than in TLIF.
Subsidence depends on multiple factors related to the low quality of bone of patients, the damage to endplates during their preparation, overdistraction, multilevel fusion, small cages, and different types of instruments. Among the above-mentioned factors[34, 35]. many spinal surgeons believe that different types of instruments are one of the important factors for maintaining the stability of the surgical segment and reducing cage subsidence. Although OLIF is an effective and increasingly performed treatment, there is still no consensus regarding the ideal instruments. Different fixation methods have been reported in the literature, including CAGE alone, without the implantation of other internal fixation devices such as SA, UPS, and BPS. Relatively little has been reported on CBT with UPS-CTFS, and to our knowledge, few studies have used FE analysis methods to analyze biomechanics stability analysis of OLIF with various fixation options.
Degenerative diseases of the lumbar spine such as LSS mainly occur at the L4-L5 segment, and OLIF is also commonly performed at the L2-L5 segment. At the same time, due to the indirect decompression of OLIF CAGE, we believe that an intact model simulation could be used to achieve the ideal vertebral space size. Therefore, in this study, the L4-L5 segment fusion model was used to analyze the effect of different implants after OLIF surgery. Because OLIF CAGE can raise the height of intervertebral space, expand the size of intervertebral foramen, thus indirectly achieving decompression. Therefore, we believe that the surgery can indirectly restore the intervertebral height. After surgery, the intervertebral height is restored to a reasonable intervertebral height. We used patients with degenerative lumbar spine disease with relatively normal intervertebral height to implant OLIF CAGE and internal fixation device to simulate the postoperative patients with lumbar degenerative diseases.
3.1 SA group
Selvon St. Clair  et al. performed an experimental biomechanical study of the OLIF after cadaveric specimens, and obtained dynamic and static biomechanical data. They found that the lumbar spine was comparable to normal lumbar spine biomechanical performance standards after OLIF procedure, while ROM decreased by more than 50%, indicating that the fusion segment was stable enough to withstand a high degree of repetitive loading after OLIF procedure. The studies of Shasti Mark  et al. showed that compared to the model with internal fixation, the SA model had the greatest ROM in all directions and the greatest CAGE stress which may increase the potential risk of CAGE subsidence. Their studies have also shown that LLIF alone increases the stability of human spinal motion segments in all loading directions and that 30% of lumbar levels treated with LLIF showed CAGE subsidence on imaging . In addition, when the subsidence of CAGE occurs, the possible clinical symptoms are axial pain and neurological symptoms, which may be due to loss of indirect decompression space, collapse of the bone structure around the intervertebral body or degeneration. Tempel et al  included 297 patients undergoing lateral fusion with SA through a retrospective analysis of prospective data. They found that fusion subsidence was a significant predictor of postoperative revision with the Stand-Alone technique, and therefore recommended the implantation of internal fixation in cases with significant risk factors for fusion collapse. Most authors often combine the use of internal fixation devices to maintain the stability of surgical segment, reduce fusion loosening and promote intervertebral fusion. Therefore, in many cases, such as in patients with osteoporosis , SA does not ensure its stability and requires adjunctive fixation, such as pedicle screw to distribute the load over the implanted vertebrae and avoid subsidence of the implant. In our experiments the stress of Cage of SA was greater than the rest of the internal fixation for different motion loads, and the likelihood of postoperative CAGE subsidence was greater than the rest of internal fixation devices, but the relative ROM of SA was 25.58% of that of the normal model L4-L5 segment, reducing the relative ROM and limiting the ROM of fusion segment and providing a good fixation effect.
3.2 BPS group and UPS
BPS fixation features three column concept of spine stability and is therefore the gold standard for the treatment of degenerative and traumatic spinal disorders and is widely used as a posterior fixation device after OLIF CAGE implantation. In our study, the BPS showed the least ROM, CAGE stress, and screw stress in different motions, which also means that BPS provides the best biomechanical properties for OLIF. However, there are some disadvantages to BPS fixation, including damage to the paravertebral muscles during instrumentation and postoperative muscle atrophy, risk of nerve damage, vascular injury and increased operative time. In view of the relatively large size of the OLIF fusion and to reduce the risks associated with BPS during surgery, some surgeons choose UPS which can provide better stability and at the same time reduce the cost of the procedure to the patient. UPS causes less damage to the paravertebral muscles, less perioperative bleeding and overall lower cost of the implants. In our study, the relative ROM of the UPS was higher than that of the SA group, but ROM of the surgical segments was greater than that of other internal fixation models. The stress on the CAGE in the UPS group was higher than that of other groups, and there was an increased risk of CAGE subsidence. In a clinical trial, Aoki et al analyzed 1ss25 patients undergoing transforaminal lumbar interbody fusion and found that the incidence of loosening of the CAGE was higher in patients with UPS (8.3%) than in those with BPS (2.1%).
3.3 The UPS-CTFS group
Translaminar facet screw fixation was developed by Magerl in 1984 and is used clinically in patients with acute spinal trauma and degenerative spinal disease . Biomechanical studies have shown that the Translaminar facet screw fixation technique provides a similar degree of spinal stability compared to conventional BPS . Several studies have also demonstrated the biomechanical advantages of transforaminal pedicle screw fixation in terms of reducing flexion, extension and rotation[34, 46, 47]. Biomechanical and clinical studies have shown that UPS-CTFS can achieve similar stability and fusion rates as BPS[48–50]. In addition, the OLIF is long, and wide and can be adjusted, and the CAGE can pass through the entire vertebral body and be fixed in the endplate with relative stability on the bone surface. We therefore conjecture that relatively stable internal fixation can also be achieved with UPS-CTFS. In our study, we used the BPS group to compare with the UPS-CTFS group. The UPS-CTFS group had less ROM value than the BPS group in extension, left and right rotation and left and right bending; in terms of stress on the CAGE, the UPS-CTFS group had more stress than the BPS group in flexion, extension and left and right bending, but the two groups were similar in rotation. The Von Mises stress of the internal fixation device was higher in the UPS-CTFS group compared to the BPS group, with better s stress shielding Therefore, we conclude that UPS-CTFS can achieve similar fixation results as pedicle screws in OLIF and reduce operative time. It is worthwhile to put it into clinical use. In addition, the BPS is technically more complex and more invasive, requiring four pedicle screws and two titanium rods in a single-segment procedure, and causes damage to both lumbar muscles, which can be further reduced when combined with percutaneous pedicle screw fixation. However, the pedicle screw is more expensive than the facet screw. Therefore, considering biomechanical stability, technical difficulty and cost, UPS-CTFS may be an alternative to the OLIF BPS and UPS procedures
3.4 CBT group
For CBT technique, Santoni proposed in 2009 an inferior-inferior to superior-inferior approach to nailing, a new approach to nailing by increasing the contact between the screw and the cortical bone. In contrast to the pedicle screw technique, this technique requires the screw to be inserted caudally to cephalad in the sagittal plane and medially to laterally in the coronal plane, thus increasing cortical bone contact and maximizing tricortical fixation and increasing the holding screw strength. The article also points out that although the CBT screw is more resistant to axial extraction but less resistant to bending and rotation than the BPS. The use of CBT in osteoporotic patients has a better effect on the resistance to pullout force and can better prevent the occurrence of loosening of the internal fixation. In our study, we found that compared to the BPS, the ROM of the fusion segment was relatively low for the CBT screws in flexion and extension, while in flexion and rotation the ROM was greater than in the UPS-CFTS group, and the stress on the CAGE was greater in the CBT group than in the The BPS group. Although the CBT group had the highest stresses in all types of kinematic loads compared to the other models, they were still less than the yield strength of titanium of 897-1034 Mpa. From the above experiments, in terms of biomechanics, we believe that CBT screws can be an alternative to BPS, especially for patients with osteoporosis. However, the clinical use of this procedure still needs a long-term retrospective study for comprehensive analysis.
Our study is based on FE analysis and has several limitations. Firstly, the FE analysis was 3D modelled for CT data. We only selected the skeletal model, the rest models were made by later processing and no tissue model such as muscle skin was included, resulting in some experimental errors. Secondly, we only used the skeletal data of a single person for modelling and computing, ignoring the differences between different people. The description of provertebral disease was not taken into account in the experiment, so the experiment still requires to be validated by long-term clinical trials with large samples and cadaveric experiments.