Varying fusion criteria for anterior cervical discectomy and fusion: Is a 1-mm or 2-mm interspinous motion threshold associated with more favorable patient-reported outcomes?

BACKGROUND: Radiographic criteria for assessing cervical spinal fusion vary, with interspinous motion thresholds of ≤1 mm or ≤2 mm on flexion-extension radiographs commonly used. This lack of standardization complicates cross-study comparisons and may influence outcome reporting. Establishing a universally accepted, evidence-based fusion assessment standard is essential to ensure consistent evaluation, guide treatment decisions, and promote optimal outcomes. To our knowledge, this is the first review comparing 1-mm vs 2-mm thresholds and evaluating their impact on reported radiographic fusion rates and patient-reported outcome measures (PROMs) after anterior cervical discectomy and fusion. METHODS: PubMed and Embase were systematically searched for studies published between database inception and August 1, 2025 that reported cervical fusion outcomes using 1-mm and 2-mm thresholds on flexion-extension radiographs. Data extracted included sample sizes, radiographic fusion criteria, and PROM scores for modified Japanese Orthopedic Association, Neck Disability Index (NDI), arm visual analog scale (VAS), and neck VAS. Study quality was assessed using the Newcastle-Ottawa Scale. Random-effects meta-analysis was performed with subgroup and heterogeneity analyses. Outcomes reported in <3 studies were not meta-analyzed. RESULTS: We included 7 studies reporting NDI and neck VAS outcomes and 6 studies reporting arm VAS outcomes. Among patients in whom radiographic fusion was achieved (n = 357), mean NDI improvement was comparable between the 1-mm and 2-mm groups, with no significant difference (p = 0.85). Similarly, in patients classified radiographically as non-fused (n = 142), there was no significant difference in NDI improvement between 1-mm and 2-mm groups (p = 0.90). Neck VAS improvement in fused patients (n = 391) was comparable across thresholds (p = 0.85); in non-fused patients (n = 129), significance was approached between groups (1-mm:-3.72, 95% CI:-4.84 to -2.60; 2-mm:-2.17,95% CI:-3.28 to -1.05 l p = 0.05). Arm VAS improvement did not differ significantly across thresholds in fused (n = 370, p = 0.5043) or non-fused (n = 12, p = 0.1093) patients. Heterogeneity was high across studies. CONCLUSIONS: Using the 1-mm interspinous motion threshold, a higher proportion of patients were identified as non-fused, but PROMs did not significantly differ between 1-mm and 2-mm groups. Further studies are needed to clarify clinical impact and support standardizing fusion assessment criteria.