Comparative Finite Element Analysis of Posterior Short Segment Fixation Constructs with or without Intermediate Screws in the Fractured Vertebrae for the Treatment of Type A Thoracolumbar Fracture

Abstract Conventional short-segment stabiliztion for thoracolumbar fractures initially involved inserting pedicle screws in the vertebra one-above and one-below the level of injury, creating a four-screw construct connected by rods spanning. However, the reported early implant failure rate and loss of correction rate are quite high. Therefore, six-screw short-segment posterior fixation, which involves additional augmentation at the fractured vertebrae has been proposed to reduce the rates of kyphosis recurrence and implant failure. Yet, little is known about biomechanical changes to the spine, strongest advantage and the appropriate indications. The objective of this study was to investigate the biomechanical properties that are associated with augmentation of intermediate screws at the fracture vertebrae in relation to the severity of type A thoracolumbar fracture using finite element analysis. Short-segment stabilization models with or without augmentation screws at fractured vertebrae were established based on finite element model of moderate compressive fractures (AO Spine A1 and LSC grade-2 ), severe compressive fractures (AO Spine A1 LSC grade-3), and burst fracture(AO Spine A3 and LSC grade-2 ). The spinal stiffness, stresses at the implanted hardware, and axial displacement/micro-motion of the bony defect were measured and compared under mechanical loading conditions. Our results suggested that more severe fractures might benefit more from rigid fixation with six-screw constructs. Augmentation of intermediate screws at the fracture vertebrae is recommended for patients with higher wedge-shaped or burst fractures to reduce the risk of hardware failure and postoperative re-collapse of injured vertebrae.