Abstract: BACKGROUND: The surgical treatment of severe lumbar fractures is mainly based on posterior short-segment fixation, and different pedicle screw fixation methods have differences in biomechanical properties.
OBJECTIVE: To simulate three different screw fixation techniques in the treatment of severe L1 vertebral fractures using three-dimensional finite element technology, and to compare and analyze the biomechanical characteristics of the upper and lower intervertebral discs, fractured vertebrae, and internal fixation devices during the simulated spinal movement.
METHODS: A three-dimensional model of the T11-L3 spine was established to simulate severely unstable fracture of the lumbar 1 vertebral body using the designed three pedicle screw fixation methods: Model A with 4 traditional pedicle screws (2 in each T12 and L2), model B with 4 Universal Spine System pedicle screws (2 in each T12 and L2), and Model C were bilateral intermediate fixation with 6 pedicle screws (2 in each of T12, L1, and L2). Restraint and load were applied to simulate the maximum stress and maximum displacement of the internal fixation, the displacement of the L1 vertebral body, and stress on the intervertebral discs superior and inferior to the injured vertebra during the anterior flexion, posterior extension, left-sided bending, right-sided bending and left-right rotation of the lumbar spine. 
RESULTS AND CONCLUSION: (1) The maximum displacement and motion range of the lumbar 1 vertebra with Model C in flexion and extension, left bending, right bending, left rotation, and right rotation were lowest in the tested motion states; the maximum stress of the screws and connecting rods was the smallest; and the maximum stress of intervertebral discs superior and inferior to the injured vertebra was the smallest. This was followed by the Model B, while the maximum displacement and motion range of the lumbar 1 vertebra and the stress and displacement of the screw connecting rod in the Model A were highest, but it showed the worst biomechanical stability and was more prone to adjacent segment degeneration. (2) These results show that bilateral intermediate fixation with six pedicle screws during the treatment of severe lumbar fractures is more dispersed in internal fixator stress and exhibits the least displacement and motion range of the vertebral pedicle screws and rods compared with four universal spinal system pedicle screws and four traditional pedicle screws. Thus, it can effectively reduce the occurrence of internal fixation failure and the adjacent segment degeneration.

Key words: severe lumbar fracture, finite element analysis, biomechanics, pedicle screw, posterior short segment, stress, adjacent segment