Biomechanical finite element analysis of superior endplate collapse after thoracolumbar fracture surgery

BACKGROUND: In the follow-up after internal fixation of thoracolumbar fractures, the imaging of some patients shows “crater-like” collapse of the superior endplate of the injured vertebra, with variable collapse area and depth, even involving the anterior edge of the vertebral body. Though many papers had described the phenomenon, but nearly no one did biomechanical research about this. So we did this research in a creative way by using finite element model. METHODS: A healthy male volunteer was selected. The 64-slice thin-section spiral computed tomography images at the level of T11–L3 were collected. Data were imported into Mimics 15.0 medical image processing software to establish three-dimensional finite element skeletal models of T11 to L3 containing only three-dimensional surface elements without entities. The model was assigned values and verified. Then the pedicle screw-rod system was added to this model, and five models containing the screw-rod system with different defect sizes as well as five models that simulated the removal of the screw-rod system were derived at the same time (the defect volume was 1/5, 2/5, 3/5, 4/5, or 5/5 of the anterior vertebral column, respectively). Biomechanical analysis was then performed on this basis. RESULTS: After the removal of the internal fixator, as defect volume increased, the stress difference between the 4/5 defect group and the 5/5 defect group had the greatest magnitude of combined stress under the seven working conditions. When the volume of the collapse defect reached 4/5 of the anterior column of the vertebral body, the concentration of stress increased significantly, suggesting that the risk of continued compression or even refracture of the injured vertebra increased if the internal fixator was removed at that time. CONCLUSIONS: When the volume of the defect in the superior endplate of the injured vertebra reaches 4/5 of the anterior column, the removal of the internal fixator should be carefully considered to avoid refracture of the anterior column of the injured vertebra.