Comparison of the effects of four atlantoaxial fusions on adjacent segments

Atlantoaxial instability (AAI) necessitates surgical stabilization to prevent neurological compromise. This finite element study compares the biomechanical performance of four atlantoaxial fixation constructs under physiological loading: standalone atlantoaxial cage (AAC), C1-C2 pedicle screws (C1-C2 PS), C1 pedicle screws with transarticular screws (C1 PS + TA), and a combined 360° construct (C1-C2 PS + AAC). A validated C0-C7 FE model with simulated AAI was subjected to flexion, extension, lateral bending, and axial rotation. Results demonstrated that the 360° construct provided the highest stability, with the lowest range of motion in all directions. It also significantly reduced peak von Mises stress on screws (by up to 91.7%) and cages (by up to 57.2%) compared to standalone constructs, while minimizing vertebral stress (over 90% reduction vs. C1-C2 PS). Adjacent disc stress remained largely unaffected by fixation type, with consistent stress concentrations at the C2-C3 level. The C1-C2 PS + AAC configuration offers superior biomechanical stability, enhanced load sharing, and reduced implant and bone stress, supporting its potential as an optimal strategy for complex AAI fixation.