Biomechanical comparison of three C2 subfacetal screw trajectories: a finite element analysis with reference to established posterior atlantoaxial fixation techniques

C2 subfacetal screw fixation has emerged as a crucial alternative for atlantoaxial instability when a high-riding vertebral artery (HRVA) precludes the use of conventional C2 pedicle screws. However, a comprehensive finite element analysis focusing on the biomechanical variances among different subfacetal trajectory variations remains absent. To systematically evaluate and compare the biomechanical characteristics (range of motion, implant stress, and pullout strength) of three distinct C2 subfacetal screw trajectories (Types I–III), utilizing established posterior C2 fixation techniques (pedicle, pars, and translaminar screws) as reference benchmarks. A previously validated C0-7 finite element model was modified to simulate atlantoaxial instability by removal of major stabilizing ligaments. C1 received bilateral pedicle screws. On the C2 left side, one of six screws (pedicle, pars, translaminar, or three subfacetal types) was placed; the right side received a standard pedicle screw. C1-2 range of motion (ROM) and implant stress were evaluated under 50 N/1.5 Nm loading. Pullout strength was assessed separately. Subfacetal screws provided stability comparable to the pedicle screw in lateral bending and axial rotation, with a slight increase in flexion-extension ROM. Regarding pullout strength, subfacetal screws generally outperformed pars and translaminar screws, with Type III being the highest overall and the pars screw the lowest. However, subfacetal screws exhibited higher von Mises stress, concentrated at the screw-rod junction for Type III and at the proximal threads for Types I/II. Generally, C2 subfacetal screws provide primary stability and pullout strength comparable to conventional reference constructs, despite higher overall implant stress. Individually, Type II exhibits the most favorable stress distribution; Type I experiences slightly higher stress; whereas the theoretical Type III yields the maximum pullout resistance but concurrently suffers from the most severe stress concentration. Therefore, subfacetal screws are viable salvage options for HRVA, requiring case-by-case selection.