Preoperative topical 5% benzoyl peroxide reduces intraoperative deep cutibacterium acnes contamination in shoulder arthroscopy: a retrospective cohort study

Background Intraoperative deep contamination with Cutibacterium acnes (C. acnes) from hair follicles and sebaceous glands remains a microbiological risk in shoulder arthroscopy, and routine chlorhexidine-alcohol skin preparation has limited efficacy against this reservoir.Objective To evaluate the association between preoperative topical 5% benzoyl peroxide use and intraoperative deep C. acnes contamination, and to explore skin colonization load.Methods A retrospective cohort analysis was performed on patients undergoing shoulder arthroscopy. Deep specimens (synovial fluid, synovium, inner portal cannula wall) were obtained for anaerobic culture. The primary outcome was the patient-level deep contamination rate. Relative risks (RR), adjusted odds ratios (aOR), and incidence rate ratios (IRR) were estimated using multivariable logistic regression, Poisson regression, and non-parametric tests. Sensitivity analyses used a strict positivity definition.Results The cohort included 162 patients (81 with BPO pretreatment, 81 with standard preparation alone). The BPO group had a lower deep contamination rate (16.05% vs. 34.57%; RR = 0.46, 95%CI 0.26-0.83, p = 0.007), with an adjusted aOR = 0.38 (95%CI 0.18-0.79). The number of positive deep specimens per patient was lower (IRR = 0.44, p = 0.004). Among positive cases, the time to culture positivity was delayed in the BPO group (p = 0.031). Preoperative skin colonization load was lower in the BPO group (p = 0.001). A higher skin score was independently associated with increased risk of deep contamination (aOR = 1.57 per 1-grade increase, 95%CI 1.19-2.07, p = 0.002). Sensitivity analyses showed consistent results. No significant between-group differences were found in short-term infection-related clinical events.Conclusion In this retrospective analysis, preoperative topical 5% benzoyl peroxide was associated with a significant reduction in intraoperative deep C. acnes contamination and bacterial burden during shoulder arthroscopy, supporting the microbiological rationale for a source decolonization strategy.Clinical trial registration Not applicable.