Propionibacterium acnes in shoulder surgery: true infection, contamination, or commensal of the deep tissue?

Background Propionibacterium acnes has been linked to chronic infections in shoulder surgery. Whether the bacterium is a contaminant or commensal of the deep tissue is unclear. We aimed to assess P. acnes in intraoperative samples of different tissue layers in patients undergoing first-time shoulder surgery. Methods In 118 consecutive patients (mean age, 59.2 years; 75 men, 43 women), intraoperative samples were correlated to preoperative subacromial injection, the type of surgical approach, and gender. One skin, one superficial, one deep tissue, and one test sample were cultured for each patient. Results The cultures were positive for P. acnes in 36.4% (n = 43) of cases. Subacromial injection was not associated with bacterial growth rates ( P = .88 for P. acnes ; P = .20 for bacteria other than P. acnes ; P = .85 for the anterolateral approach; P = .92 for the deltopectoral approach; P = .56 for men; P = .51 for women). Skin samples were positive for P. acnes in 8.5% (n = 10), superficial samples were positive in 7.6% (n = 9), deep samples were positive in 13.6% (n = 16), and both samples (superficial and deep) were positive in 15.3% (n = 18) of cases ( P < .0001). P. acnes was detected in the anterolateral approach in 27.1% (n = 32) of cases and in the deltopectoral approach in 9.3% (n = 11) of cases ( P = .01; relative risk, 1.93; 95% confidence interval, 1.08-3.43). Thirty-five of the P. acnes –positive patients were men (81.4%), and 8 patients were women (18.6%; P = .001; relative risk, 2.51; 95% confidence interval, 1.28-4.90). Discussion P. acnes was detected in more than one third of patients undergoing first-time shoulder surgery. Preoperative subacromial injection was not associated with bacterial growth. P. acnes was observed more frequently in the deep tissues than in the superficial tissues. The relative risk for obtaining a positive P. acnes culture was 2-fold greater for the anterolateral approach than for the deltopectoral approach, and the risk was 2.5-fold greater for men.