Sex stratified associations between fracture site and recorded mFI-5 categories in patients with nonunion a cross sectional study

Background We examined whether associations between fracture site and recorded five-item modified frailty index (mFI-5) categories differed by sex among adults with established nonunion. Methods This retrospective cross-sectional study included 689 adults with nonunion at a single hospital during 2020–2025. Multinomial logistic regression included sex, fracture site and their interaction, with adjustment for age group, smoking and alcohol use. Sex-specific relative risk ratios (RRRs) and nominal 95% confidence intervals (CIs) were estimated for 1 versus 0 and ≥2 versus 0 deficits, using the truncal group as the site reference. A joint likelihood-ratio test assessed the sex × site interaction. Holm adjustment was applied to the eight primary site contrasts. Results The study included 491 men and 198 women. Overall, 261 patients (37.9%), 284 (41.2%) and 144 (20.9%) had recorded mFI-5 categories of 0, 1 and ≥2 deficits, respectively. The interaction test yielded χ 2 (4) = 10.976, with an unadjusted P value of 0.027. For ≥2 versus 0 deficits, RRRs for lower- and upper-extremity sites relative to the truncal group were 0.197 (95% CI 0.051–0.760) and 0.161 (0.035–0.737) in men, respectively. Corresponding estimates in women were 2.079 (0.532–8.118) and 2.997 (0.626–14.355). Conclusions Exploratory analyses suggested that fracture-site associations with recorded mFI-5 categories may vary by sex.