Abstract: BACKGROUND: For lumbar degenerative spondylolisthesis, segmental instability, and discogenic diseases, lumbar fixation and fusion are generally required for non-operative treatment. Unilateral internal fixation and bilateral internal fixation are the two main internal fixation methods for the treatment of degenerative diseases of the lumbar spine.
OBJECTIVE: To explore the influence of different fixation methods before and after bone cement injection on the lumbar spine from the perspective of biomechanics by finite element method. 
METHODS: A total of 632 pieces of L1-S segments in the CT data of the National 973 Project of the “Digital Astronaut” were selected, and the interval between layers was 0.625 mm. The medical image processing software Mimics 10.0 was used to read in the scanned DICOM files to reconstruct the model of the lower lumbar spine L1-S. UG, MIMICS and other software were used to establish five kinds of human lumbar spine three-dimensional models with L3-5 normal, single fusion cage unilateral fixation and bilateral internal fixation, cage-bone cement perfusion unilateral fixation and bilateral internal fixation. Through the finite element analysis method, range of motion of L3-4 vertebral body joint, L3-4 fibrous annulus maximum stress, L3-4 nucleus pulposus internal pressure, the peak stress of the lower endplate of L4 and the peak stress of the pedicle internal fixation system were compared in the five models to simulate the human lumbar spine flexion, extension, lateral flexion, and rotation. 
RESULTS AND CONCLUSION: (1) Compared with the normal model, the pressure of the posterior extension of the fiber ring was increased by 54% in the simple fusion cage placement group. The pressure of the fiber ring was the highest when the single fusion cage was fixed in one side. In the fusion-cement perfusion group, bilateral internal fixation was reduced by 2%, 7%, 3%, and 3%, respectively, compared with that in the simple fusion-cement infusion group. In the fusion-cement perfusion group, the stress of the vertebral pedicle was reduced by 8%, 3%, 3%, 5% in bilateral internal fixation compared with that of the unilateral pedicle under flexion, extension, flexion, and rotation. (2) It is concluded that in the case of the support of the fusion device, there is no big difference in stability between unilateral and bilateral fixation. The stability of the lumbar spine can be basically guaranteed after the placement of the fusion device and the injection of bone cement.

Key words: bone cement, injection, posterior lumbar interbody fusion, finite element analysis, internal fixation method, biomechanics, fusion segment, stability