Abstract: BACKGROUND: Unilateral biportal endoscopic lumbar interbody fusion as a novel minimally invasive technique has been widely used in the treatment of lumbar degenerative diseases. However, as a postoperative complication, fusion cage subsidence cannot be completely avoided.  
OBJECTIVE: To analyze the biomechanical characteristics of unilateral biportal endoscopic lumbar interbody fusion technology by establishing a finite element model, as well as the risk of fusion cage subsidence under different heights of the fusion cage and osteoporosis conditions.
METHODS: Based on the CT data of healthy adult male volunteers, the finite element models of the L4-L5 vertebral body were constructed. According to the unilateral biportal endoscopic lumbar interbody fusion operation method, the models with different cage heights of 8, 10 and 12 mm were successively constructed (normal bone and osteoporosis). The flexion, extension, right lateral bend, left lateral bend, right axial rotation, and left axial rotation motions were simulated in models with different bone conditions on the upper surface of L4 with 500 N follower-load and 10 Nm torsional torque. The range of motion, pedicle screw-rod system stress and endplate stress of each model under different working conditions were observed and analyzed. 
RESULTS AND CONCLUSION: (1) Under the same motion state, the range of motion of the six kinds of lumbar fusion models was significantly lower than that of the normal lumbar model. The range of motion of six kinds of interbody fusion models was the largest under flexion and the smallest under extension. With the increase in cage height, the range of motion of the intervertebral fusion model decreased under the same bone and motion conditions. (2) Under the same bone and motion conditions, with the increase in cage height, the stress of the pedicle screw rod system of interbody fusion model decreased. The stress of pedicle screw rod system in six kinds of interbody fusion models was the highest under lateral bending movement and the lowest under flexion and extension. When the height of the fusion cage was the same, the stress of the pedicle screw rod system in the osteoporosis model was greater than that in the normal model. (3) Under the same bone and motion conditions, with the increase of cage height, the maximum stress of the L4 lower endplate (or L5 upper endplate) of the intervertebral fusion model increased. When the height of the fusion cage was the same, the maximum stress of the L4 lower endplate (or L5 upper endplate) in the osteoporosis model was slightly greater than that in the normal model. Under the same motion state, the maximum stress of the L4 lower endplate was greater than that of the L5 upper endplate in six kinds of intervertebral fusion models. The maximum stress of the L4 lower endplate (or L5 upper endplate) under the flexion was the largest and the maximum stress of the L4 lower endplate (or L5 upper endplate) under the extension was the smallest. (4) The results show that the higher the fusion cage is, the better it is, but it needs to be properly stretched to ensure the stability of the segment and avoid the increased risk of fusion cage subsidence caused by too high fusion cage.

Key words: lumbar, unilateral biportal endoscopic, lumbar interbody fusion, osteoporosis, finite element analysis, biomechanics