Influence of Cervical Level Fused on Subsidence of Cage and Allograft in Anterior Cervical Discectomy and Fusion

Objective: This study aims to evaluate the relationship between the cervical levels fused and the degree of subsidence following anterior cervical discectomy and fusion (ACDF) procedures. Background: Subsidence following ACDF may worsen clinical outcomes. Previous studies have linked lower cervical levels with higher rates of subsidence, but none have quantified the relative degree of subsidence between levels. Materials and Methods: Patients who underwent ACDF from 2016 to 2021 at a tertiary medical center were included in this study. Lateral cervical radiographs from the immediate postoperative period and the final follow-ups were used to calculate subsidence. Analysis of variance was used to examine the association between cervical levels fused and subsidence. Multivariable linear regression analysis controlled for age, sex, smoking status, osteopenia/osteoporosis, number of fused levels, cage-to-body ratio, and cage type while examining the relationship between the cervical level fused and subsidence. Results: This study includes 122 patients who underwent 227 levels fused. There were 16 (7.0%) C3–C4 fusions, 55 (24.2%) C4–C5 fusions, 97 (42.7%) C5–C6 fusions, and 59 (26.0%) C6–C7 fusions. There was a significant difference in the degree of anterior subsidence between cervical levels fused (P = 0.013) with a mean subsidence of 1.0 mm (SD: 1.6) for C3–C4, 1.1 mm (SD: 1.4) for C4–C5, 1.8 mm (SD: 1.5) for C5-C6, and 1.8 mm (SD: 1.6) for C6–C7 fusions. Relative to C6–C7 fusions, C4–C5 (P = 0.016), and C3–C4 (P = 0.014) fusions were associated with decreased anterior subsidence, whereas C5–C6 (P = 0.756) fusions were found to have similar degrees of anterior subsidence in the multivariable analysis. Conclusion: We found upper cervical levels experienced a smaller degree of anterior subsidence than lower levels, after controlling for demographic and implant characteristics. Surgeons can consider using larger cages at lower cervical levels to minimize these risks.