Biomechanics of extreme lateral interbody fusion with different internal fixation methods: a finite element analysis

BACKGROUND: Establishing a normal L3–5 model and using finite element analysis to explore the biomechanical characteristics of extreme lateral interbody fusion (XLIF) with different internal fixation methods. METHOD: The L3–5 CT image data of a healthy adult male volunteer were selected to establish a normal lumbar finite element model (M0). The range of motion (ROM) of L3–4 and L4–5, under flexion, extension, left bending, right bending, left rotation, and right rotation, together with L3–4 disc pressure was analyzed. Then the L4–5 intervertebral disc was excised and implanted with a cage, supplemented by different types of internal fixation, including lateral two-hole plate model (M1), lateral four-hole plate model (M2), VerteBRIDGE plating model (M3), lateral pedicle model (M4), posterior unilateral pedicle screw model (M5) and posterior bilateral pedicle screw model (M6). The ROM,the maximum stress value of the cage, and the maximum stress value of the intervertebral disc of L3–4 were analyzed and studied . RESULTS: The ROM of L3–4 and L4-L5 segments in the validation model under various motion states was basically consistent with previous reports. The lumbar finite element model was validated effectively. After XLIF-assisted internal fixation, the range of activity in L3–4 segments of each internal fixation model was greater than that of the normal model under various working conditions, among which the M5、M6 model had the larger range of activity in flexion and extension. After the internal fixation of L4–5 segments, the mobility in M1-M6 was significantly reduced under various motion patterns. In terms of flexion and extension, the posterior pedicle fixation model (M5、M6) showed a significant reduction,followed by M2. The maximal von mises cage stress of M1 was obviously greater than that of other models (except the left bending). Compared with M0, the intervertebral disc stress of M1-M6 at L3–4 segments was increased. CONCLUSIONS: It is recommended that the posterior bilateral pedicle screw model is the first choice, followed by the lateral four-hole plate model for fixation during XLIF surgery. However, it is still necessary to be aware of the occurrence of adjacent segment degeneration (ASD) in the later stage.