Intuitively, the introduction of artificial discs into spinal surgery offered the promise of reducing the incidence of adjacent segment (AS) reoperation compared with fusion. Several early clinical studies reported nonstatistically significant differences in AS disease between total disc replacement and fusion. Given the relatively low rate of AS reoperation (∼1%-2% per year) following fusion, any appropriately powered study designed to demonstrate a statistically significant difference compared with arthroplasty would require thousands of patients and/or long-term follow-up (>5 years). Therefore, these differences only become apparent with large study size or meta-analyses and longer follow-up.