Biomechanical Comparison of Three Internal Fixation Techniques for Subaxial Cervical Spine Fractures in Ankylosing Spondylitis: A Finite Element Analysis

<title>Abstract</title> <p>To compare the biomechanical performance of anterior long-segment fixation, posterior long-segment fixation, and combined anterior-posterior fixation for subaxial cervical spine fractures in ankylosing spondylitis (ASCF) using finite element analysis, providing quantitative evidence for clinical decision-making. A C2-T1 finite element model was constructed from CT data of a typical ASCF patient (male, 56 years). The fracture line crossed the C5-C6 disc space and obliquely involved C6. Three constructs were developed: anterior C4-C7 plate, posterior C4-C7 pedicle screw-rod, and combined (anterior C5-C6 short plate+posterior C4-C7 screw-rod). A 50 N axial preload was applied to C2, followed by a 2.0 N·m pure moment to simulate six physiological motions. Overall maximum displacement, von Mises stress in bone and implants, and equivalent strain were compared among the three constructs. Anterior fixation showed the smallest displacement, highest stiffness, and lowest implant stress under all conditions. Posterior fixation had the largest displacement, while combined fixation was intermediate. Under flexion, peak screw stress in posterior fixation (256.98 MPa) was much higher than in anterior (59.82 MPa) and combined (138.70 MPa) fixation, indicating the highest failure risk. Combined fixation consistently exhibited the highest equivalent strain, suggesting greater bone microdamage potential. For all three constructs, stress/strain concentrations localized at the adjacent lower vertebra (T1). Anterior long-segment fixation provides superior immediate stability and implant safety. Posterior fixation carries a high failure risk under flexion. Combined fixation offers balanced mechanics for complex fractures but warrants caution regarding bone microdamage. Clinical choice must integrate stability, safety, surgical morbidity, and long‑term osseointegration.</p>