Axis screw parallel to the sagittal plane versus traditional pedicle screw in the treatment of atlantoaxial fixation: a finite element study.

Background: This study aims to conduct a finite element analysis (FEA) to assess the bio-mechanical properties of C2 sagittal-parallel pedicle screw (PPS) in fixation for atlantoaxial instability, thereby providing a theoretical foundation for its clinical application.

Methods: A total of 5 intact C1-2 finite element models were established. Based on this, instability models were developed and two different fixation methods were applied for each model: C1 lateral mass screw (LMS) combined with C2 sagittal-parallel pedicle screw (C1LMS + C2PPS), and C1 lateral mass screw combined with C2 traditional pedicle screw (C1LMS + C2PS). Under a physiological load of 40 N, a pure moment of 1.5 Nm was used to simulate movements of the cervical spine in flexion, extension, lateral bending, and axial rotation. The von Mises stress of implants and the segment range of motion (ROM) were analyzed and compared statistically.

Results: The intact model was validated and showed good consistency with other studies in terms of range of motion (ROM). In flexion and extension, the C1LMS + C2PPS resulted in lower segment ROM (12.4% and 6.3% decrease) and stress concentration (15.9% decrease in flexion) compared to C1LMS + C2PS. However, in lateral bending and axial rotation, the C1LMS + C2PPS exhibited higher segment ROM (42.9% and 5.9% increase) and stress concentration (8.7% and 21.4% increase) compared to C1LMS + C2PS.

Conclusion: Both methods were safe and stable for the fixation of atlantoaxial instability. Compared to C1LMS + C2PS, the use of C1LMS + C2PPS may provide better stability and a lower risk of implant failure in flexion and extension. Clinically, it is feasible to utilize the C2 sagittal-parallel pedicle screw in fixing atlantoaxial instability.