In patients with thoracolumbar ankylosing spondylitis (AS) fracture fixation, use of the transpedicular screw (TPS) compared with the cortical bone trajectory (CBT) technique provided better biomechanical strength under axial, rotational, flexion/extension, and lateral flexion loading conditions, according to results of an analysis published in Frontiers in Surgery.

The researchers sought to evaluate the biomechanical features of the TPS and CBT techniques in thoracolumbar AS fracture fixation, using finite element analysis.

Three-dimensional finite element models of the AS spine were developed based on computed tomography (CT) scans. The AS spinal fracture models used were CBT or TPS; an intact AS spinal model served as the control in 10 individuals. The fractured spine was fixed by screws using the traditional transpedicular pathway in 10 patients with AS in the TPS group. Further, the CPT method was applied to fix the fractures in 10 additional patients with AS in the CPT group.  


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The AS spine demonstrated comparable construct stiffness following posterior fixation by TPS and CBT methods under axial, rotational, and flexion/extension loading conditions. Although the traditional TPS method demonstrated better construct stiffness, a significant difference was observed only under the rotational loading condition with TPS vs CBT (21.0±0.8 N/mm vs 15.6±0.7 N/mm, respectively;

P <.001). In addition, under the lateral flexion loading condition, the fixed spine vs control group showed higher construct stiffness (P <.001). Further, the traditional TPS vs the CBT method yielded significantly higher construct stiffness (P <.001).

Local stiffness in the TPS group was significantly higher than in the CBT group under rotation and lateral flexion conditions (P <.001). In addition, under the rotational and lateral flexion loading conditions, a higher relative rotation angle between the fracture fragments was seen in the TPS group (P <.001). The maximum von Mises stresses were 1714.4±129.8 MPa and 1208.7±107.3 MPa under compressive loading in the CBT and TPS groups, respectively (P <.001). In the present complete fusion spine models, the TPS vs CBT technique offered superior stability against axial and rotational loads.

The researchers concluded, “Compared with CBT, TPS was more effective in maintaining the stability of AS thoracolumbar fractures from a finite element analysis perspective.”

Reference

Tong Z, Xiao B, Yan K, Xing Y, Zhang Y. Biomechanical evaluation of the transcortical and transpedicular trajectories for pedicle screw insertion in thoracolumbar fracture fixation for ankylosing spondylitis. Front Surg. 2021;8:706597. doi:10.3389/fsurg.2021.706597