Adjacent Segment Kinematics After Cervical Total Disc Replacement and Fusion

Study Design: Retrospective motion analysis of prospectively collected IDE imaging data from subjects treated with anterior cervical discectomy and fusion (ACDF) or cervical total disc replacement (cTDR). Objective: To quantify adjacent segment motion from preoperative assessment through 7 years postoperatively and evaluate how fusion versus motion preservation at the index level influences adjacent-level kinematics. Summary of Background Data: Fusion eliminates motion at the treated segment, often leading to compensatory increases in adjacent-level motion. In contrast, cTDR preserves motion at the index level, but its long-term effect on adjacent segment mechanics remains unclear. Methods: Flexion-extension radiographs were obtained from an IDE trial comparing Mobi-C and ACDF. Adjacent levels demonstrating >5 degrees of motion in subjects with analyzed C2–C7 range of motion (ROM) were included. Sagittal plane rotation between maximal flexion and extension at adjacent levels and overall C2–C7 ROM were measured using an automated, FDA-cleared spinal motion analysis system. Assessments were performed preoperatively and through 7 years postoperatively. Results: Superior adjacent-level rotation remained within ±3 degrees in both cohorts; however, the ACDF group demonstrated a significantly greater increase at 24 months ( P =0.048). Inferior adjacent-level rotation increased postoperatively in the ACDF cohort and gradually declined at later follow-up, with significantly greater motion than cTDR at 6 months ( P =0.0001) and 12 months ( P =0.0005). The contribution of the superior adjacent level to total C2–C7 flexion-extension motion was significantly higher in the ACDF cohort at all time points ( P <0.0001), as were inferior adjacent-level contributions ( P <0.0001). Conclusions: Over 7 years, cTDR maintained stable adjacent segment motion, whereas ACDF led to compensatory increases in adjacent-level motion and greater adjacent segment contributions to overall cervical mobility.