Risk factors for nonunion following surgically managed, traumatic, diaphyseal fractures: a systematic review and meta-analysis

center dot Background: There are several studies on nonunion, but there are no systematic overviews of the current evidence of risk factors for nonunion. The aim of this study was to systematically review risk factors for nonunion following surgically managed, traumatic, diaphyseal fractures. center dot Methods: Medline, Embase, Scopus, and Cochrane were searched using a search string developed with aid from a scientific librarian. The studies were screened independently by two authors using Covidence. We solely included studies with at least ten nonunions. Eligible study data were extracted, and the studies were critically appraised. We performed random-effects meta-analyses for those risk factors included in five or more studies. PROSPERO registration number: CRD42021235213. center dot Results: Of 11,738 records screened, 30 were eligible, and these included 38,465 patients. Twenty-five studies were eligible for meta-analyses. Nonunion was associated with smoking (odds ratio (OR): 1.7, 95% CI: 1.2-2.4), open fractures (OR: 2.6, 95% CI: 1.8-3.9), diabetes (OR: 1.6, 95% CI: 1.3-2.0), infection (OR: 7.0, 95% CI: 3.2-15.0), obesity (OR: 1.5, 95% CI: 1.1-1.9), increasing Gustilo classification (OR: 2.2, 95% CI: 1.4-3.7), and AO classification (OR: 2.4, 95% CI: 1.5-3.7). The studies were generally assessed to be of poor quality, mainly because of the possible risk of bias due to confounding, unclear outcome measurements, and missing data. center dot Conclusion: Establishing compelling evidence is challenging because the current studies are observational and at risk of bias. We conclude that several risk factors are associated with nonunion following surgically managed, traumatic, diaphyseal fractures and should be included as confounders in future studies.