Post-Operative Changes in Cervical Screw Orientation: a Descriptive Analysis of 77 Semiconstrained, Rotational Plates

Introduction: Rotational dynamic plates are commonly utilized in anterior cervical fusion procedures, with screw insertion trajectories determined at the surgeon's discretion. However, the changes in screw-plate angle over time and their relationship with the initial angle of screw insertion remain unexplored. This study aims to (1) describe the longitudinal post-operative changes in cervical screw angles during the early post-operative period, and (2) investigate the implications of these changes on early radiographic outcomes.

Methods: We conducted a retrospective review of 77 semi-constrained rotational dynamic plates implanted by a single surgeon over a 33-month period for 1 and 2-level anterior cervical discectomy and fusion (ACDF) procedures. Radiographic follow-up was obtained at baseline, 1 week, 6 weeks, and 3 months post-surgery. Measurements of the screw-plate angle, segmental lordosis, C2-C7 sagittal vertical axis, and graft subsidence were performed on each radiograph.

Results: The mean upper and lower screw insertion angles were 113.8° ± 6.2° and 106.1° ± 5.7°, respectively. The lower screw angle (LSA) demonstrated a significant negative correlation with the 3-month change in screw-plate angle (β -0.524, 95% CI -0.971, -0.239, p = 0.002), while the upper screw angle (USA) did not (β -0.160, 95% CI -0.422, -0.168, p = 0.388). The change in segmental lordosis was significantly associated with the 3-month change in LSA (r = 0.345, p = 0.016), but not with the change in USA. No association was found between the 3-month angle change and graft subsidence for either screw.

Conclusion : Screw-plate angles in ACDF procedures with rotational dynamic plates exhibit significant variation during the early post-operative period. Obtuse screw insertion trajectories correlate with a decrease in screw-plate angle over time. Although changes in screw angle correspond with alterations in segmental lordosis, they are not associated with the magnitude of graft subsidence.