After more than ten years of development and clinical verification, MIS-TLIF has become a reliable surgical method for minimally invasive treatment of degenerative diseases of the lumbar spine [9, 21]. It is well known that achieving reliable intervertebral fusion will determine the long-term effect of surgery. Modern intervertebral fusion is mostly achieved by implanting an intervertebral fusion Cage (Cage). As a permanent implant in the human body, Cage plays an important role in promoting intervertebral bony fusion and maintaining early biomechanical stability . At first, clinicians used two Cages to implant the intervertebral space . The advantage is that the biomechanical stability is high, but the implantation will cause more bleeding, prolong the operation time, and increase the risk of dural tear and the cost of surgery. With the further study, scholars found that although the single Cage oblique implantation was not as biomechanically stable as two Cage implants, satisfactory clinical efficacy could be obtained, and the implantation process was simpler, the trauma was less, and the operation cost was lower[24–25]. However, with the wide application of Cage, the complications related to Cage have become increasingly prominent, such as Cage displacement, subsidence, non-fusion and other problems, which have not been effectively solved. It has been reported that Cage sinking and displacement may be caused by stress shielding . Cage shape may also be an important factor leading to displacement. Zhao et al.  retrospectively studied Cage displacement in five spinal centers and found that the incidence of Cage displacement in rectangular Cage (3.11%) was significantly higher than that in renal Cage (0.28%). In addition, the position of Cage in the intervertebral space is an important factor affecting displacement. Currently, the Cage is generally implanted into the intervertebral space at an oblique angle of 45° in clinical. In recent years, some scholars began to advocate Cage placement horizontally in intervertebral space to reduce the risk of Cage displacement and subsidence [4, 28–29]. Theoretically, it is more difficult for the Cage to move when it is horizontally implanted because the Cage is parallelly placed against the posterior longitudinal ligament horizontally. Therefore, the Cage should be rotated if it is supposed to be removed from the original implant position. Clinically, fusion strategies and Cage implantation methods are mainly determined by the experience and habits of surgeons. Therefore, to provide some biomechanical evidence to surgeons in determining the fusion strategies and Cage implantation methods in MIS-TLIF, we conducted this finite element study.
The finite element method is widely used in the lumbar spine theoretical biomechanics, especially for spinal diseases and surgical curative effect analysis . In this study, the stability of the model was evaluated by measuring the lumbar spine mobility in each model. By comparing Model-A and Model-C, it was found that both bilateral pedicle screw fixation and unilateral pedicle screw + lamina articular process screw fixation could significantly reduce lumbar motion, and both fixation strategies achieved good fixation strength and biomechanical stability, which was consistent with previous research conclusions . The influence of Cage implantation position on biomechanical stability was analyzed, and the comparison between Model-A and Model-B showed that Cage placement horizontally in intervertebral space could maintain more spinal stability than Cage placement at an oblique angle of 45°. Theoretically, Cage intervertebral space horizontal placement has a larger end plate contact area, the larger the contact area, the greater the friction resistance between Cage and end plate, so better stability. Considering the fact that the lumbar gap is wide at the front and narrow at the back, it is difficult for Cage to contact the end plate when placed at an oblique angle of 45°, so the friction resistance between Cage and the end plate is reduced. The overall stability of bilateral pedicle screw fixation and horizontal fusion device implantation was optimal.
According to the test results in Figure 3, the fixation strategy of unilateral pedicle screw + lamina articular process screw bears greater peak stress and overall stress of the screw system. This may be closely related to the geometry of the internal fixation system in this model. It can be inferred that the risk of rod breakage is higher with this fixation strategy than with bilateral pedicle screw fixation if bone fusion is not achieved for a long time. Liu F et al.  found in a long-term follow-up study of different fixation strategies that the incidence of screw fracture in unilateral pedicle screw + laminar facet screw fixation was as high as 14.29% (4/28), significantly higher than that in bilateral pedicle screw fixation. Our research results also prove the risks of this fixed strategy.
The elastic modulus of polyetheretherketone fusion Cage is similar to that of normal human bone . The results of this study found that although the maximum peak stress of Cage was 47.86 MP in FL of the Model-A, but it was much smaller than the yield stress of human cortical bone of 108 MPa . Meanwhile, the overall stress of Cage placed at the intervertebral space parallel to the posterior longitudinal ligament was lower than that of Cage placed at an oblique angle of 45° under all working conditions. The results show that the use of Cage level placed in the intervertebral space does not increase the risk of subsidence. Since Cage is placed horizontally in the intervertebral space parallel to the posterior longitudinal ligament, it needs to rotate itself before exiting from the original implantation port. Therefore, the risk of Cage displacement and exit when placed horizontally in the intervertebral space is far less than when Cage is placed at an angle of 45°.
There are still some limitations in this study. The FE model is constructed by the CT images from healthy subjects without any spinal diseases. Therefore, the changes in the geometry of spines and implantations were not considered. This study does not evaluate the biomechanical changes in adjacent segments since intervertebral fusion could lead to adjacent segment degeneration (ASD). The paraspinal muscles are not considered in the whole investigation, which could slightly affect the stability of the lumbar spine.
In conclusion, according to the results of the finite element study, MIS-TLIF internal fixation strategy and Cage implantation mode have a profound impact on the biomechanics of the lumbar spine. Although the unilateral pedicle screw + facet screw strategy resulted in less surgical injury and more benefit, there was a higher risk of long-term rod breakage. The surgeon needs to be vigilant and choose carefully. The surgical strategy of bilateral pedicle screw fixation + fusion Cage horizontal implantation is relatively safe, suitable for clinical application, and is expected to become the standard implantation strategy of MIS-TLIF.