What is the most accurate radiographic criterion to determine anterior cervical fusion?

•We compared the accuracy of four radiographic anterior cervical fusion criteria.•ExGBB criterion demonstrated the highest reliability and accuracy.•ISM criterion has comparable accuracy to the CT scan as initial diagnostic test.•Bridging bone and InGBB criteria were less accurate in case used cage We recommend ISM and ExGBB criteria to increase accuracy in case used cage. The accuracy of radiographic criteria for determining anterior cervical fusion remains controversial, and inconsistency in the literature makes a comparison of published clinical results problematic. The descriptions of bridging bone are still lacking and subjective, and the interpretation of images can be influenced by the type of graft or cage used. To assess and validate the diagnostic accuracies of four radiographic fusion criteria using the results of surgical exploration. Retrospective, radiographic, and comparative study. This study included patients who required anterior or posterior exploration of a previous anterior cervical arthrodesis level(s) ranging from C3–C4 to C7–T1 for suspected pseudarthrosis or adjacent-segment pathologies. They underwent radiologic examinations to determine the four fusion criteria. We included patients whose images were taken at least 1 year after the index surgery, and 82 patients with 151 cervical segments were enrolled. The inter- and intra-rater reliabilities and validity that correlated with the results of surgical exploration for the four fusion criteria were assessed using data (fusion or not) that were collected by two raters. The four published radiographic fusion criteria were interspinous motion (ISM) < 1 mm and superjacent ISM ≥ 4 mm, seen on dynamic radiographs; conventional bridging bone, as seen on computed tomography (CT) scans; and extra-graft bridging bone (ExGBB) and intragraft bridging bone (InGBB), observed on multi-axial reconstructed CT scans. The criteria were evaluated by two raters (spine surgeons with 5 and 7 years of experience). The raters evaluated each criterion twice at two different time points, 3 to 4 weeks apart. First, ISM and conventional bridging bone on CT scans were evaluated, followed by ExGBB and InGBB, with a time interval of 4 months. This Research was supported by the Chung-Ang University Research Grants (less than 5,000 US dollars) in 2016. The inter- and intra-rater reliability values of the ExGBB (0.887–0.933) criteria were the highest, followed by those for the ISM (0.860–0.906), bridging bone (0.755–