Biomechanical analysis of unilateral biportal endoscopic lumbar interbody fusion: different heights of cage and osteoporosis

Background: By establishing a finite element model of unilateral biportal endoscopic lumbar interbody fusion (UBE-LIF), the biomechanical characteristics of UBE-LIF technology were analysed and evaluated. The risk of cage subsidence was also evaluated. Methods The finite element model of the L4-L5 vertebral body was constructed based on CT data from healthy adult male volunteers. According to the UBE-LIF surgical method, the fusio model with different cage heights of 8 mm, 10 mm and 12 mm was successively constructed. The flexion, extension, right lateral bend, left lateral bend, right axial rotation, and left axial rotation motions were simulated in 6 models with different bone conditions on the upper surface of L4 with 500 N followed load and 10 Nm torsional torque. The range of motion(ROM), Pedicle screw-rod system stress and endplate stress of each model under different working conditions were observed and analysed. Results The ROM of the 12mm model was the lowest, simultaneously, the maximum stress of the pedicle screw-rod system of the 12mm model was the lowest. However, the 12mm model has the largest endplate stress. The maximum stress of the L4 inferior endplate was greater than that of the L5 superior endplate in the UBE-LIF surgical models (24.9%). Compared with the normal model, the ROM of the osteoporosis model increased by 4.7%, the maximum stress of the pedicle screw-rod system increased by 14%, and the maximum stress of the endplate increased by 7.5%. Conclusions The results show that the appropriate height of the cage should be selected during the operation to ensure the stability of the segment and avoid the risk of the subsidence caused by the high cage. This is especially important for patients with osteoporosis.