How Contaminated Is the Surgical Field in Reverse Total Shoulder Arthroplasty? A Preliminary Quantitative Intraoperative Microbiological Study

Background/Objectives: Bacterial contamination of the surgical field during shoulder arthroplasty may contribute to periprosthetic joint infection (PJI), yet data on intraoperative bacterial load and its clinical correlates remain limited. This study aimed to evaluate culture positivity and bacterial load in specimens collected at the end of reverse total shoulder arthroplasty (RTSA) and to explore their association with patient-related factors. Methods: Fifty-five patients undergoing elective RTSA were consecutively enrolled. At the end of surgery, three specimens per patient (two prosthetic swabs and one periprosthetic tissue sample) were collected for qualitative and quantitative microbiological analysis. Associations between bacterial load and clinical variables were assessed using mixed-effects linear regression models, while time to culture positivity was analysed using mixed-effects Cox regression models. Results: Among 165 specimens, Cutibacterium acnes was isolated in 42.4% and coagulase-negative staphylococci in 29.1%. C. acnes showed significantly higher bacterial loads (1.38 × 103 CFU/mL) compared with aerobic bacteria (6.54 × 101 CFU/mL). Higher C. acnes load was associated with male sex, older age, higher body mass index, smoking, and cuff tear arthropathy, whereas massive rotator cuff tear and longer time to positivity were inversely associated. Aerobic bacterial load was primarily associated with longer surgical duration. Time to positivity was shorter for aerobes than for C. acnes. Conclusions: Intraoperative bacterial contamination during RTSA is frequent and characterized by marked differences in bacterial load and growth kinetics. Quantitative assessment of bacterial burden may improve the interpretation of unexpected positive cultures.