Risk Factors for Nonunion After Intramedullary Nailing in Humeral Shaft Fractures: A Retrospective Study

Abstract

Humeral shaft fractures are frequently treated with intramedullary nailing (IMN), yet nonunion remains a significant complication. Although clinical predictors of nonunion have been described, radiographic factors such as fracture morphology and interfragmentary gap are less consistently reported. This study aimed to evaluate clinical and radiographic predictors of nonunion after IMN for humeral shaft fractures, with particular focus on the interaction between fracture classification and interfragmentary gap. We retrospectively reviewed 132 patients treated with IMN for acute humeral shaft fractures between 2018 and 2024. All patients had at least 12 months of follow-up. Demographic, surgical, and radiographic variables were analyzed. Fractures were classified according to AO/OTA Fracture and Dislocation Classification (AO/OTA) criteria, with interfragmentary gap and alignment measured on standardized postoperative radiographs. Nonunion was defined as the absence of union at 6 months. Univariable and multivariable logistic regression were performed, and receiver-operating characteristic (ROC) analysis identified optimal gap cutoffs. Among 132 patients, 120 (90.9%) achieved union and 12 (9.1%) developed nonunion. Wedge fractures (Type B) had significantly higher nonunion rates than simple fractures (18.2% vs. 6.1%, P = 0.037). Larger gaps increased nonunion risk, with wedge fractures and gap >2 mm showing the highest rate (20.0%, P = 0.028). In multivariable analysis, wedge fractures with a gap >2 mm were independently associated with nonunion (adjusted odds ratio [OR] 9.46, 95% confidence interval [CI] 1.02–87.63, P = 0.048). Alignment showed a borderline association (OR 1.19 per degree, 95% CI 0.995–1.418, P = 0.057). ROC analysis yielded an area under the curve of 0.662; the optimal cutoff was >3.3 mm (Youden’s Index 0.297), but >2 mm was considered the most clinically relevant threshold. Nonunion after IMN in humeral shaft fractures is associated with both fracture morphology and interfragmentary gap. Wedge fractures combined with a gap >2 mm significantly increase nonunion risk, highlighting the importance of minimizing fracture gap during fixation.