Incidence and Time Course of Adjacent Segment Disease After Anterior Cervical Fusion: A Multimodal Assessment in 400 Patients.

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.