Microbiologic Diagnosis of Prosthetic Shoulder Infection by Use of Implant Sonication

We recently described a sonication technique for the diagnosis of prosthetic knee and hip infections. We compared periprosthetic tissue culture to implant sonication followed by sonicate fluid culture for the diagnosis of prosthetic shoulder infection. One hundred thirty-six patients undergoing arthroplasty revision or resection were studied; 33 had definite prosthetic shoulder infections and 2 had probable prosthetic shoulder infections. Sonicate fluid culture was more sensitive than periprosthetic tissue culture for the detection of definite prosthetic shoulder infection (66.7 and 54.5%, respectively; P = 0.046). The specificities were similar (98.0% and 95.1%, respectively; P = 0.26). Propionibacterium acnes was the commonest species detected among culture-positive definite prosthetic shoulder infection cases by periprosthetic tissue culture (38.9%) and sonicate fluid culture (40.9%). All subjects from whom P. acnes was isolated from sonicate fluid were male. We conclude that sonicate fluid culture is useful for the diagnosis of prosthetic shoulder infection.The frequency of shoulder replacement surgery is increasing (1). The incidence of prosthetic shoulder infection varies from 0.4 to 15.4% (6, 7). When an infection is present, the infection requires unique medical and surgical management, rendering an accurate diagnosis critical. However, since patients with prosthetic shoulder infection often present with stiffness and/or pain alone (7), the achievement of an accurate diagnosis is challenging.Periprosthetic tissue has been the specimen cultured for the microbiologic diagnosis of prosthetic shoulder infection. Specificity is an issue, as microorganisms (e.g., Propionibacterium and Staphylococcus spp.) can be contaminants, and the number of microorganisms in tissue is small. As a result, it has been suggested that multiple samples be obtained; for prosthetic hips and knees, it is recommended that five or six periprosthetic tissue specimens be cultured (2). No such data are available for shoulder implants.We recently clinically validated a sonication technique that is used to sample biofilm bacteria on the surface of removed hip and knee implants placed in solid containers. We demonstrated that the culture of samples obtained by sonication of the implant was more sensitive than the culture of periprosthetic tissue for the diagnosis of prosthetic hip and knee infections (22). The poor sensitivity of the latter likely relates to the presence of bacteria in biofilms on the prosthesis surface, a site not well sampled when periprosthetic tissue samples for culture are obtained. No data on the accuracy of sonication for the diagnosis of prosthetic shoulder infection are available.The proportion of patients with shoulder infections due to Propionibacterium acnes is significantly greater than the proportion of patients with lower limb infections due to P. acnes (12). Sperling et al. reported that Propionibacterium spp. account for 16% of prosthetic shoulder infections (16). Franta et al. reported that among 31/282 patients (11%) with unsatisfactory shoulder arthroplasties, positive intraoperative cultures were found in 23 at the time of revision surgery, with the most common organisms isolated being coagulase-negative Staphylococcus spp., followed by P. acnes (11). Cheung et al. reported the results of reimplantation of glenoid components following removal and allogeneic bone grafting in seven patients; specimens from two patients demonstrated the growth of P. acnes (5). These two patients had continuing pain and radiographic evidence of glenoid component loosening and subsequently underwent repeat revision surgery, whereas the remaining patients did well and did not require repeat revision surgery (5), suggesting a role for P. acnes in pain and component loosening. Accordingly, the accurate detection of a Propionibacterium spp. is paramount in the diagnosis of prosthetic shoulder infection.The purpose of the present study was to compare implant sonication to periprosthetic tissue culture for the diagnosis of prosthetic shoulder infection. We also evaluated immunofluorescence microscopy and PCR analysis of sonicate fluid to detect prosthetic shoulder infection caused by the two most frequently associated microorganisms. Finally, we compared patient characteristics associated with Propionibacterium prosthetic shoulder infection versus those associated with non-Propionibacterium prosthetic shoulder infection.(This work was presented in part at the 16th Annual European Congress of Clinical Microbiology and Infectious Diseases, April 2006, Nice, France, and the 47th Interscience Conference on Antimicrobial Agents and Chemotherapy, September 2007, Chicago, IL.)