Different Center of Rotation Trajectories after Cervical Hybrid Surgery: A Minimum 7-Year Retrospective Comparison of Bryan and Mobi-C Prostheses

Study Design Retrospective Study. Objectives To compare long-term adjacent-segment kinematic changes and radiographic adjacent segment degeneration (rASD) after two-level cervical hybrid surgery using Bryan or Mobi-C prostheses. Methods We retrospectively reviewed 78 patients who underwent contiguous two-level anterior cervical hybrid surgery between 2010 and 2018, including 38 treated with Bryan and 40 with Mobi-C prostheses. All patients had more than 7 years of follow-up. Flexion-extension center of rotation (FE-COR) trajectories at the superior adjacent segment were measured on dynamic radiographs. Long-term rASD was evaluated using the Walraevens score. Results Clinical outcomes improved significantly in both groups (P < 0.001), with no significant between-group differences. Segmental range of motion remained stable and comparable between groups (P > 0.05). However, FE-COR trajectories differed. At 1 year, the Mobi-C group showed anterior-inferior FE-COR displacement, with FE-COR_Y decreasing to 44.92% ± 12.55%, whereas the Bryan group maintained a relatively stable FE-COR_Y of 59.01% ± 10.66% (P < 0.001). At final follow-up, rASD occurred in 12.5% of Mobi-C patients and 23.7% of Bryan patients, although this difference was not statistically significant (P = 0.198). Conclusion Conventional ROM measurements alone cannot fully capture segmental biomechanics. Early anterior-inferior FE-COR displacement in the Mobi-C group was observed and was associated with a numerically lower, but statistically non-significant, rASD rate. These findings suggest that FE-COR may be a sensitive radiographic parameter for describing postoperative micro-kinematic patterns, but its causal relationship with long-term adjacent segment degeneration requires further biomechanical and MRI-based validation.