Rate of P Acnes in arthritic shoulders undergoing primary total shoulder replacement surgery using a strict specimen collection technique

Objectives

The primary objective of this Study was to determine the rate of P Acnes contamination in shoulder arthroplasty surgery. We hypothesised that Levy et al’s reported 42% prevalence was most likely the result of contamination. A secondary objective of the study was to contribute to the literature in the area and in turn potentially inform future treatment protocols for shoulder replacement surgery including changes to antibiotic prophylactic treatments and/or surgical preparation procedures.

Design

This was a prospective observational study that enrolled 40 patients over a 6 month recruitment period.

Study Procedure

At the time of surgery, as soon as the glenohumeral joint is opened, 5 x swabs were taken: 4 of the surgeon’s instruments (outside knife, inside knife, forceps and glove) and 1 of the skin at the incision. The 4 x instrument swabs and 1 x skin swab were sent for microbiology. Specimens were processed by the laboratory within 1 hour. Testing comprised being put into culture for an extended period of time (14 days). Results of cultures were assessed by the surgeon in consultation with an infectious disease specialist. If 3 or more of the 5 swabs tested positive for P Acnes in the presence of acute clinical findings of infection, the patient was considered infected by P Acnes and received treatment and follow-up. If not the results were registered and the patient had a normal follow up.

Ethics and Governance

Approved by North Shore Private Hospital Human Research Ethics Committee to be undertaken at North Shore Private Hospital. HREC reference NSPHEC 2014-003. Also approved by St Vincent’s HREC, reference HREC/14/SVH/108.

Results

Forty patients (25 females, 15 males) were included. Sixteen had at least one positive specimen for P acnes – five of 25 females (20%) and 11 of 15 males (62.5%). The most common site of growth was the skin edge (12 positive samples); followed by forceps (7), tip of surgeon’s glove (7), outside knife (4) and inside knife (3). There was a total of 22/75 positive swabs in males compared to 11/25 in females. Males also had 6 fold higher odds of having skin contamination and 11 fold higher odds of having any contamination.

Our findings suggest that P Acnes contamination of the surgical field is likely to come from exposed wound edge and the surgeon’s repeated manipulation of these with gloves and instruments. Our findings were also consistent with literature regarding increased rates of P acnes bacterial load and intraoperative growth in males compared to females.