Adjacent Segment Kinematics After Cervical Total Disc Replacement and Fusion: A Comparative Motion Analysis.

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.