Abstract: BACKGROUND: Taking into account the stability of spinal fixation, the preferred approach for lateral lumbar interbody fusion is commonly the utilization of posterior bilateral pedicle screws, typically performed in two stages. An alternative method involving the posterior approach of unilateral fixation using lateral vertebral nail rods in the lateral decubitus position has shown potential for enhanced mechanical stability. This technique also offers additional advantages such as reduced operation time, lowered risks, and decreased costs, making it a promising area for further investigation. 
OBJECTIVE: To compare the biomechanical effect of lateral lumbar interbody fusion with four different types of posterior instruments. 
METHODS: A validated L3-5 finite element model was modified to simulate four different types of lateral lumbar interbody fusion: Model A: posterior bilateral pedicle screw fixation; Model B: posterior unilateral pedicle screw fixation; Model C: lateral bilateral screw fixation; Model D: lateral unilateral screw fixation. The stability in the range of physiological motion, and the stress difference of screw fixation and interbody fusion apparatus were compared among the models. 
RESULTS AND CONCLUSION: (1) When compared with the intact model, all reconstructive models displayed decreased motion range at L4-5. Model A had a more obvious range of motion decline. (2) In the flexion, lateral flexion and axial rotation, the differences between the peak stress of the lateral screw fixation and the peak stress of the posterior screw fixation were more than 67.74 MPa, 80.10 MPa and 43.95 MPa, respectively. (3) In terms of internal fixed stress distribution in different reconstructed models, the stress distribution of Model A and Model B screws was mainly concentrated in the body of the pedicle screw, while the peak stress of Model C and Model D screws was mainly concentrated in the tail of the screw. (4) It is suggested that Model A could obtain the best stability and reduce the risk of cage sinking and displacement. Model B could also provide better stability, and could be a choice for lateral fusion on the premise of ensuring stability and fixation. (5) Besides, Model C could achieve the same effect as Model D when the lateral road screw was selected. However, patients with lumbar instability should be cautious to choose unilateral lateral internal fixation, and excessive extension should be avoided to reduce the possibility of screw loosening and fixation fatigue. 

Key words: interbody fusion, lateral fusion, internal fixation, simulation, finite element, biomechanics, optimized analysis