Diabetes mellitus is an independent risk factor for surgical site infection after orthopedic Class I incision surgery: a retrospective cohort study

Surgical site infection (SSI) is one of the most devastating complications following orthopedic Class I incision surgery. Diabetes mellitus (DM) has been implicated as an important potential risk factor, but previous studies have yielded conflicting results, largely due to the widespread use of traditional logistic regression for rare event analysis and inadequate control for confounding by surgical procedure type. This retrospective cohort study included 5,845 consecutive patients who underwent orthopedic Class I incision surgery at a tertiary hospital in Nanjing between January 2021 and December 2025. Patients were stratified into DM and non-DM groups. Stratified analysis and Firth penalized maximum likelihood logistic regression were used to control for confounding factors. Four hierarchically adjusted models were constructed to evaluate the stability of the effect of DM, and the pathogenic bacteria spectrum of SSI in our institution was characterized. The overall incidence of SSI was 0.43%. The SSI rate was significantly higher in the DM group than in the non-DM group (1.09% vs. 0.37%, relative risk ( RR ) = 2.93, 95% confidence interval (95% CI ): 1.11–7.78, p = 0.042). After adjusting for demographic characteristics, perioperative factors and surgical procedure category across four hierarchically structured Firth penalized logistic regression models, DM remained a stable independent risk factor for SSI, with odds ratio (OR) values ranging from 3.17 to 3.51 (all p < 0.05). In the fully adjusted model, DM was associated with a 3.35-fold increased risk of SSI ( OR = 3.35, 95% CI : 1.11–8.76, p = 0.034). Among the 24 isolated pathogenic strains, gram-positive cocci and gram-negative bacilli each accounted for 45.8%. DM is an independent and stable risk factor for SSI after orthopedic Class I incision surgery, and its effect remains significant even after adequate adjustment for surgical procedure type. Gram-positive cocci and gram-negative bacilli are equally prevalent among SSI pathogens in our region, and empirical antimicrobial prophylaxis should provide coverage for both categories of pathogens.