Multicenter reliability study of the universal long bone nonunion classification

A universal classification providing a clinically relevant and anatomically comprehensive framework for long bone nonunions has been recently introduced. This study aimed to evaluate its inter- and intra-observer reliability and compare its performance with the widely used Weber-Cech classification.

This multicenter, three-stage validation study included 191 cases meeting the FDA definition of nonunion. Four expert raters participated. In Stage 1, cases were classified using existing systems: AO/OTA for anatomical location and Weber-Cech for biological type. In Stage 2, 133 eligible cases were independently classified using the Universal Long Bone Nonunion Classification (ULBNC) in two rounds, two weeks apart. Stage 3 involved refinement of classification criteria based on feedback and statistical analysis, followed by re-assessment of 90 cases. Inter-observer reliability was assessed using free-marginal Fleiss' kappa; intra-observer reliability using Cohen's kappa with linear weighting.

Substantial to almost perfect inter-observer agreement was observed for type classification (κ = 0.85), with the highest reliability in diaphyseal nonunions (κ = 0.90). Incorporating pathological mobility significantly improved agreement compared to Weber-Cech (κ = 0.38, p < 0.05). Periarticular nonunions showed substantial agreement (κ = 0.72). Group-level agreement improved from moderate (κ = 0.42-0.57) to substantial (κ = 0.79-0.82) after refining criteria. Subgroup agreement was excellent (κ = 0.89-1.00). Intra-observer reliability ranged from substantial to almost perfect across all levels.

ULBNC is a reliable and reproducible classification system for long bone nonunions. Incorporation of clinical features-such as pathological mobility, alignment, and correction strategy-enhances its clinical utility and supports standardization in treatment and research.