The Effects of cTDR and Fusion Surgery on Adjacent Level Strain

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 use an established measure of apparent strain to evaluate compensatory adjacent-level mechanics following ACDF and cTDR. Summary of Background Data: The mechanism of adjacent segment disease remains incompletely understood. One theory proposes that fusion eliminates motion at the index level, increasing stress at adjacent segments and accelerating degeneration. Apparent strain, which correlates with stress, provides a quantitative method to assess these biomechanical changes. Methods: Flexion–extension radiographs were obtained from an IDE trial including Mobi-C and ACDF cohorts. Subjects with >5 degrees of motion at adjacent levels and available C2–C7 range of motion (ROM) were included. Apparent strain was calculated at the anterior and posterior aspects of the disc space at levels immediately above and below the index procedure. Measurements were performed at 6 postoperative timepoints: 6, 12, 24, 48, 60, and 84 months. Results: Adjacent-level maximum strain increased with greater C2–C7 ROM in both cohorts. In subjects with C2–C7 ROM >65 degrees, the ACDF group demonstrated a marked rise in average adjacent-level maximum strain, exceeding the normative 90% maximum strain threshold. In contrast, the Mobi-C cohort maintained average adjacent-level maximum strain below 90% across all ROM ranges. Conclusions: ACDF was associated with more frequent excessive adjacent-level strain than cTDR, particularly in patients with preserved or high cervical ROM. Apparent strain derived from dynamic radiographs may serve as an early indicator of abnormal adjacent-segment biomechanics.