RESUMEN
Objective To compare the biomechanical properties between the two-level fixation by im- plantation of pedicle-screws into the adjacent upper and lower vertebrae of the fractured vertebra and the three-level fixation by implantation of pedicle screws into the fractured vertebra and its adjacent upper and lower vertebrae in the treatment of thoracolumbar burst fractures.Methods Eight fresh frozen calf spines were used in this study.Each specimen was tested in four models:intact model as the control,L1 burst fracture model,two-level fixation model, and three-level fixation model.The L1 burst fracture model was created on a biaxial material testing machine (MTS858 Bionix test system,America).During the experiment,the flexion,extension,bilateral bending and axial rotation loadings were applied to the specimens and the range of motion(ROM)was measured with a three-dimensional laser analysis apparatus and the stiffness was calculated subsequently.One-way statistical analysis was used.Results The ROMs under six different loadings in the fracture model became larger obviously(P＜0.05)and the stiffness decreased(P＜0.05).The ROMs in both fixation models were smaller than those in the other models(P＜0.01)and the stiffness increased distinctly(P＜0.05).There were no significant differences in ROMs and stiffness between the two-level fixation and three-level fixation models(P＞0.05).Conclusion Two-level fixation provides similar biomechanical stability as three-level fixation does in the reconstruction of unstable thoracolumbar fractures.