Posterior Fixation for Different Thoracic-Sacrum Alignments Containing a Thoracolumbar Vertebral Fracture: A Finite Element Analysis

Thoracolumbar vertebral fractures are one of the most common fractures; however, there is a lack of mechanical analyses for what the posterior fixation is for different spine alignments.This study used a three-dimensional finite element model of a T1-sacrum. Three alignment models were created: intact, degenerative lumbar scoliosis (DLS), and adolescent idiopathic scoliosis (AIS). The burst fracture was assumed to be at the L1 vertebral level. Posterior fixation models with pedicle screws (PS) were constructed for each model: 1 vertebra above to 1 below PS (4PS) and 1 vertebra above to 1 below PS with additional short PS at the L1 (6PS); intact-burst-4PS, intact-burst-6PS, DLS-burst-4PS, DLS-burst-6PS, AIS-burst-4PS, and AIS-burst-6PS models. T1 was loaded with a moment of 4 Nm assuming flexion and extension.The vertebrae stress varied with spinal alignment. The stress of L1 in intact burst (IB), DLS burst, and AIS burst increased by more than 190% compared with each nonfractured model. L1 stress in IB, DLS, and AIS-4PS increased to more than 47% compared with each nonfractured model. L1 stress in IB, DLS, and AIS-6PS increased to more than 25% compared with each nonfractured model. In flexion and extension, stress on the screws and rods of intact-burst-6PS, DLS-6PS, and AIS-6PS was lower than in the intact-burst-4PS, DLS-4PS, and AIS-4PS models.It may be more beneficial to use 6PS compared with 4PS to reduce stresses on the fractured vertebrae and instrumentation, regardless of the spinal alignment.