Abstract: BACKGROUND: Oblique lateral interbody fusion is an effective method for the current clinical treatment of lumbar degenerative diseases. However, the treatment for postoperative lateral displacement of the cage has not been determined yet. Therefore, using finite element analysis to analyze the biomechanics of different conditions after oblique lateral interbody fusion has become a choice for more and more researchers.  
OBJECTIVE: To evaluate the effect of lateral displacement of cage on the lumbar biomechanics after oblique lateral interbody fusion with finite element analysis, and to provide biomechanical basis for the treatment of this condition.
METHODS:  A healthy male was selected to undergo lumbar CT scanning, and the scanning data were obtained. The three-dimensional finite element model of L3-L5 was established by Mimics, Geomagic, SolidWorks and ANSYS Workbench software, and was set as normal control group. Oblique lateral interbody fusion Stand alone fixation group (SA) and oblique lateral interbody fusion with bilateral pedicle screws fixation group (BPS) were established. According to the degree of Cage displacement, the SA group was divided into five groups: the cage non-displaced model (SA0), the midpoint of the right edge of the cage shifted to the right 1/8 point (SA1), 2/8 point (SA2), 3/8 point (SA3), and 4/8 point (SA4) of the transverse diameter of the upper endplate of L5. On the basis of SA grouping, the models with bilateral pedicle screws were BPS0, BPS1, BPS2, BPS3 and BPS4, respectively. The same loading conditions were applied to different models to simulate the flexion, extension, lateral flexion and rotation of the spine. The range of motion of the lumbar spine and the stress peak of the cage and screw-rod system were observed under different working conditions.  
RESULTS AND CONCLUSION: (1) The range of motion of SA0, SA1 and SA2 was lower than that of normal control group in different motion states, while the range of motion of SA3 and SA4 was greater than that of normal control group in extension and right flexion states. (2) Under different conditions, the range of motion and the peak stress of the cage in the SA group and the BPS group were increased with the increase of the lateral distance of the cage. (3) When the cage was in the same position, the activity and the peak stress of the cage in all states in the BPS group were lower than that in the SA group. (4) In each state, the peak stress of the screw and rod system in the BPS group was BPS0 < BPS1 < BPS2 < BPS3 < BPS4. (5) It is concluded that when the left side of the cage was displaced after oblique lateral interbody fusion stand alone, the range of motion of the lumbar spine and the stress peak of the cage in various states were gradually increased with the increase of the displacement distance, showing a tendency of spinal instability. On this basis, combined with bilateral pedicle screw fixation, better biomechanical stability of the lumbar spine can be obtained.

Key words: oblique lateral interbody fusion, cage displacement, range of motion, finite element analysis, biomechanics