Risk Factors for Nonunion in Conservatively Treated Humeral Shaft Fractures: A Multivariable Analysis

Background and Objectives: Nonunion remains an important clinical concern after nonoperative treatment of humeral shaft fractures. Although several patient- and fracture-related variables have been proposed as determinants of healing, their independent contributions remain uncertain. This study aimed to identify patient- and fracture-related factors independently associated with nonunion after nonoperative treatment of humeral shaft fractures. Materials and Methods: This single-center retrospective cohort study included adults with humeral shaft fractures treated nonoperatively between January 2015 and January 2025. Fractures were classified using the AO/OTA system. Patients with periprosthetic, pathological, pediatric, intra-articular fractures, previous nonunion, or insufficient follow-up were excluded. Treatment consisted of functional bracing after initial immobilization. Nonunion was defined as absence of clinical and radiographic union at or beyond 9 months after injury, with no radiographic progression over at least 3 months, persistent fracture-site pain, and absent bridging callus. Demographic characteristics, comorbidities, fracture-related variables, and radiographic parameters were recorded. Multivariable logistic regression was used to evaluate adjusted associations with nonunion, with Firth’s penalized regression performed as a sensitivity analysis. Results: Among 242 patients, union occurred in 207 (85.5%), while 35 (14.5%) developed nonunion. In the adjusted model, older age (OR = 1.76 per 10-year increase; 95% CI: 1.28–2.40), smoking (OR = 14.33; 95% CI: 4.16–49.32), and proximal-third fracture location (OR = 3.77; 95% CI: 1.38–10.31) were associated with nonunion. Firth’s penalized regression yielded similar findings. The association with diabetes mellitus was uncertain (OR = 2.63; 95% CI: 0.94–7.30). Conclusions: Older age, smoking, and proximal-third fracture location were associated with nonunion after nonoperative treatment of humeral shaft fractures. These exploratory associations may inform patient counseling and follow-up, but the limited number of events and wide confidence intervals warrant cautious interpretation.