Cervical sagittal alignment after multilevel ACDF: correction goes along with loss of compensation

Background: Cervical spondylosis can cause loss of lordosis or even kyphosis, altering sagittal alignment and triggering compensatory mechanisms to maintain horizontal gaze. Despite the widespread use of multilevel anterior cervical discectomy and fusion (ACDF), its impact on cervical sagittal alignment and compensatory mechanisms remains unclear. This study investigates degenerative changes in cervical sagittal alignment and compensatory mechanisms before and after multilevel ACDF. Methods: A retrospective analysis was conducted on 290 patients who underwent multilevel ACDF (2–3 levels) for cervical spondylosis. Preoperative and postoperative sagittal parameters including C0–C2 lordosis, C2–C7 lordosis, surgical (fused-level) lordosis, unfused-level lordosis, T1 slope (T1S), T1S minus cervical lordosis (T1S–CL), and cervical sagittal vertical axis (c–SVA) were measured using a previously described deep learning model. Demographic data were collected at the same time points. Changes from baseline to follow-up were calculated. Correlations between changes in surgical lordosis and sagittal parameters were assessed via linear regression, Pearson’s correlation, and multiple logistic regression analysis. Results: Mean preoperative C2–C7 lordosis was 11.4° ± 13.7°, increasing by 6.5° in two-level, and 8.6° in three-level ACDF patients. Statistically significant improvements were observed in C2–C7 lordosis, C0–C2 lordosis, and T1S–CL mismatch (all p < 0.01). Changes in surgical lordosis correlated negatively with changes in unfused segment lordosis (R = − 0.26, p < 0.01). The strongest negative correlation was observed between changes in C2–C7 and changes in C0–C2 lordosis (R = − 0.60, p < 0.01), highlighting subaxial realignment’s impact on upper cervical compensation. A moderate positive correlation emerged between changes in surgical lordosis and changes in T1S (R = 0.29, p < 0.01). Subgroup analyses by fusion area confirmed the consistency of these relationships across different ACDF configurations. Conclusions: Multilevel ACDF leads to a gain of surgical lordosis and reduces compensatory mechanisms. By recognizing and accounting for preoperative compensatory mechanisms, surgeons can optimize surgical planning to achieve a more stable and biomechanically favorable alignment. We propose a three-step algorithm to analyze cervical sagittal alignment and take into consideration the presence of compensatory mechanisms of the upper cervical, the unfused subaxial and when available the global spine. Level of evidence: IV.