Abstract: BACKGROUND: Oblique lumber interbody fusion has become a mature lumbar fusion method at present, and its method of assisting lateral screw-rod fixation under the same channel has attracted attention in recent years. However, there is currently no consensus on the optimal intraoperative placement of lateral screw-rod, and there is a lack of relevant biomechanical evidence.
OBJECTIVE: To establish various oblique lumber interbody fusion combined with lateral screw-rod fixation models, evaluate their impact on lumbar spine biomechanics, and provide biomechanical foundations for the implantation of lateral screw-rod.
METHODS: Using CT images of a healthy adult, the L4-L5 Stand-Alone oblique lumber interbody fusion model and lateral screw-rod fixation models with different screw placement positions were established by using Mimics, Geomagic and SolidWorks software, including nine double screw-rod fixation models and five single screw-rod fixation models. Ansys Workbench software was employed to analyze the range of motion, peak stress of internal fixation and cage under conditions of flexion, lateral bending, and rotation.
RESULTS AND CONCLUSION: (1) The range of motion of all internal fixation models was less than Stand-Alone oblique lumber interbody fusion model. Both single screw-rod and double screw-rod showed the least range of motion when placed close to the upper endplates of L4 and L5, and the former had more restrictions on the motion. (2) The peak stress of cage in all internal fixation models was lower than Stand-Alone oblique lumber interbody fusion model. Except for right rotation condition, double screw-rod was minimal cage stress when placed anterior superior and posterior inferior quadrants of L4 and L5, which was similar to that when placed proximaly to L4 lower and L5 upper endplates; screw-rod showed minimal stress when placed proximally to L4 lower and L5 upper endplates. (3) Under flexion-extension condition, double rod- screw exhibited minimal internal fixation stress when positioned close to the upper endplates of L4 and L5; single-rod fixation showed minimal stress when placed proximally to L4 lower and L5 upper endplates. (4) It is indicated that the position of lateral screw-rod is one of the important factors that affect the biomechanical properties of oblique lumber interbody fusion, double screw-rod performs better biomechanical properties than single-rod fixation. The best biomechanical stability can be obtained when double screw-rod is placed close to the upper endplate of the vertebral body. The risk of cage subsidence is greatly reduced when the double screw-rod is placed in the anterior superior and posterior inferior quadrants of the upper and lower vertebrae or adjacent to the upper and lower endplates.

Key words: oblique lumber interbody fusion, lateral screw-rod, screw-rod placement, finite element analysis, biomechanics