Incidence and Time Course of Adjacent Segment Disease After Anterior Cervical Fusion

Study Design. Retrospective cohort study. Objective. To evaluate the incidence and time course of adjacent segment disease (ASD) after anterior cervical fusion (ACF) using radiographic, MRI, clinical, and reoperation endpoints. Summary of Background Data. ASD is a recognized concern after ACF, but reported rates vary by definition. Radiographic degeneration, MRI-confirmed neural compression, symptoms, and reoperation may represent different stages, yet these endpoints are rarely assessed together in one cohort. Methods. We reviewed 400 patients who underwent ACF for degenerative cervical spine disease from January 2008 to June 2015 and had ≥1 year of follow-up. ASD was assessed using four endpoints: radiographic ASD on plain radiographs (Xp-ASD), MRI-confirmed ASD (MRI-ASD), clinical ASD, and reoperation for ASD. Kaplan–Meier analyses were used to estimate ASD-free survival for each endpoint. Secondarily, Xp-ASD–free survival was compared between 1-level and ≥2-level fusion. Results. Mean age was 59.8 years, and mean follow-up was 3.1 years. Xp-ASD occurred in 141 patients (35.2%) at a mean of 2.2 years after surgery. MRI-ASD was identified in 31 patients (7.8%) at a mean of 3.3 years. Clinical ASD developed in 12 patients (3.0%), and reoperation for ASD was performed in 9 patients (2.3%). The annualized event rates were 11.4%, 2.5%, 0.97%, and 0.74%, respectively. ≥2-level fusion was associated with a higher risk of Xp-ASD than 1-level fusion (hazard ratio, 1.44; 95% confidence interval, 1.03–2.03), whereas MRI-ASD, clinical ASD, and reoperation did not differ between groups. Conclusion. After ACF, Xp-ASD was common during medium-term follow-up, but MRI-confirmed compression, clinical ASD, and reoperation were infrequently observed. The results support a staged course of ASD, with a time lag between radiographic degeneration and clinically meaningful disease. Level of Evidence : 3.