Preoperative Skin-Surface Cultures Can Help to Predict the Presence of Propionibacterium in Shoulder Arthroplasty Wounds

Abstract

Background: Propionibacterium species are commonly cultured from specimens harvested at the time of revision shoulder arthroplasty. These bacteria reside in normal sebaceous glands, out of reach of surgical skin preparation. The arthroplasty incision transects these structures, which allows Propionibacterium to inoculate the wound and to potentially lead to the formation of a biofilm on the inserted implant. To help identify patients who are at increased risk for wound inoculation, we investigated whether preoperative cultures of the specimens from the unprepared skin surface were predictive of the results of intraoperative cultures of dermal wound-edge specimens obtained immediately after incision of the surgically prepared skin.

Methods: Sixty-six patients (mean age, 66.1 ± 9.4 years [range, 37 to 82 years]; 73% male) undergoing primary shoulder arthroplasty had preoperative cultures of the unprepared skin surface and intraoperative cultures of the freshly incised dermis using special culture swabs. For the first 50 patients, a control swab was opened to air during the same time that the dermal specimen was obtained. The results for female and male patients were characterized as the Specimen Propionibacterium Value (SpPV). We then determined the degree to which the results of cultures of the skin surface specimens were predictive of the results of culture of the dermal specimens.

Results: The skin-surface SpPV was ≥1 in 3 (17%) of the 18 female patients and 34 (71%) of the 48 male patients (p < 0.001). The dermal SpPV was ≥1 in 0 (0%) of the 18 female patients and 19 (40%) of the 48 male patients (p < 0.001). None of the control samples had an SpPV of ≥1. The predictive characteristics of a skin-surface SpPV of ≥1 for a dermal SpPV of ≥1 were as follows: sensitivity, 1.00 (95% confidence interval [CI], 0.82 to 1.00); specificity, 0.62 (95% CI, 0.46 to 0.75); positive predictive value, 0.51 (95% CI, 0.34 to 0.68); and negative predictive value, 1.00 (95% CI, 0.88 to 1.00).

Conclusions: A preoperative culture of the unprepared skin surface can help to predict whether the freshly incised dermal edge is likely to be positive for Propionibacterium. This test may help to identify patients who may merit more aggressive topical and systemic antibiotic prophylaxis.

Clinical relevance: This study shows that surgeons have the opportunity to use preoperative skin cultures to determine the likelihood that the shoulder arthroplasty wound will be culture-positive for Propionibacterium.

Figures

Fig. 1-A

Results of the culture of a specimen from the unprepared skin surface. Combining the culture positivity for the 2 different species of Propionibacterium yielded an SpPV of 2. The SpCV value is 1. The Corynebacterium result was not considered in our analysis. Note that, despite the presence of these bacteria, the Gram smear was negative. C and S w/Gram, Ortho = culture and sensitivity with Gram stain as per orthopaedic protocol.

Fig. 1-B

Results of the culture of a specimen from the freshly incised dermis. The SpPV was 3, and the SpCV was 0.

Fig. 1-C

Results of the culture of the control swab. The SpPV was 0, and the SpCV was 0.

Fig. 2

ROC curve for predicting a fresh dermal-wound SpPV of ≥1 from the result of the culture of a specimen from the unprepared skin surface. An ROC curve is a graphical plot that illustrates the performance of a binary classifier system (in this case, whether the dermal wound SpPV is ≥1) as the discrimination threshold (in this case, the skin-surface SpPV) is varied. In this plot, the values next to each dot are the skin-surface SpPV, with the specificity and sensitivity shown in parentheses. Thus, for a threshold of skin-surface SpPV of 4, the specificity for a dermal wound SpPV of ≥1 is 1 and the sensitivity is 0.21. The area under the curve (0.93) is high (a perfect predictor would have an area under the curve of 1).