Posterior fixation for different thoracic-sacrum alignments containing a thoracolumbar vertebral fracture: a finite element analysis.

OBJECTIVE: 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.

METHODS: This study employed 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 (A1B1) PS (4PS) and A1B1 PS with additional short pedicle screws 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 4Nm assuming flexion and extension.

RESULTS: The vertebrae stress varied with spinal alignment. The stress of L1 in IB, DLS-B and AIS-B increased by more than 190% compared to the each nonfractured models. L1 stress in IB, DLS, and AIS-4PS increased to more than 47% compared to each nonfractured model. L1 stress in IB, DLS, and AIS-6PS increased to more than 25% compared to the each nonfractured model. In flexion and extension, stress on the screws and rods of IB-6PS, DLS-6PS, and AIS-6PS was lower than in the IB-4PS, DLS-4PS, and AIS-4PS models.

CONCLUSIONS: It may be more beneficial to use 6PS compared to 4PS to reduce stresses on the fractured vertebrae and instrumentation, regardless of the spinal alignment.