Biomechanical comparison of posterior short-segment fixation with or without intermediate screws for thoracolumbar burst fractures under normal and osteoporotic conditions: a finite element analysis.

Abstract

Finite element analysis. To investigate the biomechanical response of posterior short-segment fixation with or without intermediate screws at the index vertebra in osteoporotic thoracolumbar burst fractures using finite element analysis. Spinal fixation in elderly patients with osteoporotic vertebral fractures is challenging because osteoporosis weakens the screw-bone interface, leading to screw loosening and loss of fracture reduction. Short segment fixation with intermediate screws has been proposed to reduce kyphosis recurrence and implant failure in unstable thoracolumbar fractures. However, the mechanisms by which intermediate screws enhance fixation strength in osteoporotic spines remain unclear. Six finite element models of T12 burst fractures were developed to simulate short-segment stabilization under normal or osteoporotic bone conditions, with/without augmentation screws at the fractured vertebra. Spinal stiffness, implant stresses, and axial displacement/micromotion of the bony defect were measured and compared under mechanical loading. Osteoporotic models exhibited a greater range of motion (ROM) than normal bone. All six-screw constructs reduced ROM across all motions compared with traditional four-screw models. Osteoporotic fracture models gained greater benefit from intermediate screw augmentation at the fracture vertebra, which also lowered axial displacement/micromotion. In six-screw models, rod stress increased while pedicle screw stress decreased. Intermediate screws at fractured vertebrae produced similar changes in stress distribution across all fixation models, regardless of bone quality. Our findings may facilitate implant selection for osteoporotic burst fractures, supporting the use of more rigid fixation sixscrew constructs to reduce the risk of mechanical failure and postoperative re-collapse.