Abstract: BACKGROUND: Vertebral replacement can restore the stability of the anterior column of the spine and correct deformity, but the traditional expandable titanium alloy replacement has a large elastic modulus and imaging artifacts, which increases the risk of adjacent segment degeneration. In theory, polyetheretherketone is more suitable for osteoporotic patients because of its similar density to bone, radiolucency and intraoperative adaptability, but there is still a lack of sufficient evidence.  
OBJECTIVE: To investigate the biomechanical performance of posterior short-segment screw fixation combined with a novel expandable polyetheretherketone vertebral body replacement in osteoporotic vertebrae using finite element analysis.  
METHODS: Four finite element models of T12 vertebral body replacement were developed based on different screw fixation methods and vertebral body replacement materials. Model 1: Conventional pedicle screws through the fractured vertebra combined with an expandable titanium cage (M1). Model 2: Conventional pedicle screws through the fractured vertebra combined with a polyetheretherketone-vertebral body replacement (M2). Model 3: Cement-augmented pedicle screws across the fractured vertebra combined with a polyetheretherketone-vertebral body replacement (M3). Model 4: Cement-augmented pedicle screws through the fractured vertebra combined with a polyetheretherketone-vertebral body replacement (M4). The range of motion, maximum von Mises stress of the fixation system, and adjacent segment endplates in T11-L1 segments were compared among the four models under identical boundary conditions.  
RESULTS AND CONCLUSION: (1) Comparing M1 and M2, the range of motion values of M2 under four motion states were higher than those of M1, but the differences were not significant. The maximum stress on the vertebral body replacement in M2 under the four motion states was reduced compared to M1. Additionally, M2 showed a decrease in the maximum stress on the adjacent segment endplates compared to M1. (2) Among M2, M3, and M4, the fixed segment’s maximum range of motion was lowest in M4. The maximum stresses on the screw-rod system and vertebral body replacement under the four motion states were also lowest in M4. The maximum stress on pedicle screws under four motion directions was significantly highest in M3. For the adjacent segment endplates, M4 had the lowest maximum stress across all directions. However, the differences among the three were not statistically significant. (3) In vertebral body replacement for osteoporotic vertebrae, the biomechanical performance of the expandable polyetheretherketone-vertebral body replacement is similar to that of the expandable expandable titanium cage and may perform better in reducing subsidence. Posterior short-segment screw fixation combined with the expandable polyetheretherketone-vertebral body replacement provides good stability, with cement-augmented pedicle screws through the fractured vertebra demonstrating the best biomechanical performance and the least impact on the fixation system and adjacent endplates.

Key words:  finite element analysis, thoracolumbar spine, osteoporosis, vertebral body replacement, polyetheretherketone, biomechanics