Abstract: BACKGROUND: The biomechanical research on a single cortical bone track screw has been reported in many articles at home and abroad. There are not many reports on the biomechanical research on the entire motion unit of the vertebral body, cage, and screw rod, especially the stress distribution of the motor unit and the stability of the internal fixation device by the fixation method of cortical bone trajectory combined with pedicle screw.
OBJECTIVE: To summarize and analyze the biomechanical differences between traditional pedicle screw fixation and cortical bone trajectory combined with pedicle screw fixation in osteoporotic vertebral bodies. 
METHODS: The CT scan data of one osteoporosis volunteer (bone mineral density T value < -0.25 SD) were obtained, and the L3-S1 vertebral functional unit osteoporosis finite element model was established to simulate the fixation of traditional trajectory screws and cortical bone trajectory combined with pedicle screws after verification of validity. The stress distribution and stability of the two internal fixation methods on the motion unit were compared according to the force and activity of the vertebral body motion unit in the flexion, extension, lateral flexion and rotation states, respectively.
RESULTS AND CONCLUSION: (1) The results of finite element verification showed that the model could well simulate the physiological activity of lumbar vertebrae in osteoporosis patients. (2) In the flexion and extension states, the maximum stress of the internal fixator was similar to that of the traditional trajectory screw group, but the maximum stress of the nail rod system in the cortical bone trajectory combined with pedicle screw group was greater than that in the traditional trajectory screw group in the lateral flexion and rotation states. In lateral flexion, the cortical bone trajectory combined with pedicle screw group increased by 11.5% compared with traditional trajectory screw group, and in rotation, the cortical bone trajectory combined with pedicle screw group increased by 25.2% compared with traditional trajectory screw group. The maximum equivalent stress of cage in cortical bone trajectory combined with pedicle screw group was greater than that in traditional trajectory screw group, and the maximum equivalent stress of cage in cortical bone trajectory combined with pedicle screw group was 17% higher than that in traditional trajectory screw group during lateral flexion and 15% higher than that in rotation. (3) Two groups of model after in static load, bending forward, stretch, lateral flexion and rotation condition, intervertebral range of motion, two kinds of internal fixation devices can provide similar stability. (4) In general, both fixation methods can enhance the fixation strength of the lumbar fusion segment and provide similar stability, but cortical bone trajectory combined with pedicle screw internal fixation can further enhance the mechanical strength of the fusion segment and provide a better fusion stress environment.

Key words: cortical bone trajectory combined with pedicle screw, mechanical property, vertebral motor unit, finite element analysis