Table 2. Adjacent segment disease.

Outcomes	Strength of evidence	Conclusions/comments
1. ASD – symptomatic		No statistical differences between treatment groups noted across three CoE II RCTs and one cohort (CoE III) study. Given the low rates of ASD across studies, one might question whether studies were sufficiently powered to assess this outcome
2. ASD – radiographic		No statistical differences between treatment groups noted across one CoE II RCT and four CoE III cohort studies. The relationship between radiographic and symptomatic ASD is not clear. Radiographic ASD may be an intermediate outcome, and no directional relationship between radiographic and symptomatic ASD was evaluated.
3. Timing of ASD		No statistical differences between treatment groups noted across two cohort studies.
4. Range of motion at adjacent segments		No statistical differences between treatment groups noted across two RCT analyses