The Radiographic Union Score for Hip (RUSH) is an outcome instrument designed to describe radiographic healing of femoral neck fractures. The ability to identify fractures that have not healed is important for defining non-union in clinical trials and predicting patients that likely require additional surgery to promote fracture healing. We sought to determine a RUSH threshold score that defines nonunion at 6-months post-injury. Our secondary objective was to determine if this threshold was associated with increased risk for non-union surgery. A sample of 248 patients with adequate six-month hip radiographs and complete two-year clinical follow-up were analysed from a multi-national hip fracture trial (FAITH). All patients had a femoral neck fracture and were treated with either multiple cancellous screws or a sliding hip screw. Two reviewers independently determined the RUSH score based on the six-month post-injury radiographs, and agreement was assessed using the Interclass Correlation Coefficient (ICC). Fracture healing was determined by two independent methods: 1) prospectively by the treating surgeon using clinical and radiographic assessments, and 2) retrospectively by a Central Adjudication Committee using radiographs alone. Receiver Operator Curve analysis was used to define a RUSH threshold score that was specific for fracture nonunion. RUSH score inter-rater agreement was high (ICC: 0.81, 95% CI 0.76 to 0.85). The mean six-month RUSH score for all included patients was 24.4 (SD 3.4). A threshold score of <18 was associated with a greater than 98% specificity for nonunion. Furthermore, patients with a six-month RUSH score below 18 were more the seven-times more likely to require revision surgery for nonunion (Relative Risk: 7.25, 95% CI 2.62 to 20.00). The six-month RUSH score can effectively be used to communicate when a femoral neck fracture has not healed. The validity of our conclusions was further supported by the increased risk of nonunion surgery for patients below the RUSH threshold. We believe our findings can standardise a definition of nonunion for clinical trials and recommend the use of the RUSH and its <18-point threshold when describing femoral neck nonunion.
