Consensus-based radiographic threshold for identifying vertical periodontal bone loss: an international observer study.

Radiographic assessment of periodontal bone loss is essential for diagnosis and staging of periodontitis, yet the distinction between horizontal and vertical bone loss remains largely based on subjective visual interpretation without a defined quantitative reference. This study aimed to identify a consensus-based angular threshold used by clinicians to classify interproximal bone loss as vertical on dental radiographs and to evaluate factors influencing this interpretation. An international panel of periodontists evaluated digitally modified radiographs in which interproximal bone levels were simulated at angles ranging from 0° to 35° relative to the line connecting adjacent cementoenamel junctions. Seventy complete responses from specialists in 22 countries were analyzed. The likelihood of vertical classification increased with angular deviation, with a marked transition in interpretation between approximately 20° and 30°. A 35° angle achieved the predefined ≥ 70% agreement threshold across seven of eight sites, representing the most consistent consensus threshold. Mandibular sites were more frequently classified as vertical, while angled cementoenamel junction alignment increased false-positive rates, and clinician demographics had no significant effect. Mixed-model analysis demonstrated significant clinician-level variability, confirming observer-dependent interpretation. These findings, which have educational implications, provide a reproducible reference for radiographic interpretation of vertical bone loss.