Journal of Musculoskeletal Trauma

Review Article

How do we know a fracture has healed? A narrative review of radiographic bone union definitions and assessment methods: Jeong-Hyun Koh, Seungyeob Sakong; Received April 2, 2026 Accepted April 20, 2026 Published online May 20, 2026; DOI: https://doi.org/10.12671/jmt.2026.00150 [Epub ahead of print]

Abstract PDF Supplementary Material: Bone union is the most commonly reported primary outcome in fracture treatment trials, yet no universally accepted radiographic definition exists. The widely taught criterion of “bridging callus on 3 of 4 cortices on anteroposterior and lateral radiographs” has no clearly identifiable primary source in the indexed literature. This narrative review traces the historical origins of radiographic bone union assessment, documents the heterogeneity of definitions used in clinical studies, and provides a comparative analysis of the standardized scoring systems developed to address this problem. A systematic PubMed search using six prespecified strategies, from database inception to March 2026, supplemented by hand-searching and citation tracking, identified 2,380 records. After screening, 359 articles on long-bone fractures were included. The “3 of 4 cortices” criterion appears most plausibly to derive from Panjabi’s 1985 finding that cortical continuity was the strongest radiographic predictor of fracture strength (r=0.80), but no traceable validation study was identified despite citation tracking through successive Cochrane reviews (CD008579, pub2‒pub4). In their 2008 study, Corrales and colleagues documented 11 different radiographic criteria across 123 studies, finding that ‘3 cortices’ was used in only 27%. Five standardized scoring systems (Radiographic Union Score for Tibial fractures [RUST], modified RUST [mRUST], Radiographic Union Score for Hip [RUSH], Radiographic Union Score for Humeral fractures [RUSHU], and Radiographic Humerus Union Measurement [RHUM]) have improved interobserver reliability within specific anatomical settings but remain fragmented by site and limited to secondary bone healing. A 2024 analysis by Bax and his team further illustrated that this inconsistency is not limited to fractures, documenting 13 different criteria and nine classification systems within the osteotomy literature. The most widely used radiographic union criterion likely emerged through clinical teaching rather than formal validation. A minimum reporting framework is proposed to improve standardization in future studies. Consensus definitions, cross-site validation, and more objective assessment strategies are needed to resolve this four-decade-old problem.

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