Take-Home Points
- Despite standardization of diagnostic criteria by the MSIS for the diagnosis of PJI, some low-grade inflections create a diagnostic challenge for clinicians.
- P acnes infection following TJA can be present despite patients having normal serum inflammatory marker levels and synovial fluid aspirations.
- Patients with a PJI with low virulence organisms can present with painful, arthrofibrotic joints that do not appear to be clinically infected.
- Biopsy for pathology and culture can aid in the diagnosis of suspected PJI in patients who fail to meet MSIS criteria.
- If detected and accurately diagnosed, PJI with P acnes can be successfully eradicated with IV antibiotics and 2-stage revision arthroplasty with a good functional outcome.
Total joint arthroplasty (TJA) is a routinely performed, highly efficacious procedure for patients with degenerative osteoarthritis.1,2 In the United States in 2003, more than 450,000 total knee arthroplasties (TKAs) were performed, and this number is projected to increase by more than 673% by 2030, as America’s population continues to age.3 With the increase in primary TJAs has come an increase in revision TJAs. The most common cause of revision TJA is infection (25.2%), which has a rate of 1% to 4% after primary TJA.1,4 Despite advancements in implant technology, preoperative preventive strategies, perioperative techniques, and postoperative management, a recent meta-analysis of patient follow-up data revealed that 15% to 20% of patients remained dissatisfied after TJA, despite having technically well-placed implants.5,6
Recent studies have suggested that prosthetic joint infection (PJI) may be underreported because of the difficulty in diagnosis, which may be one of the reasons why patients remain dissatisfied after TJA.7 As a result, new efforts have been made to develop uniform criteria for PJI diagnosis.8 In 2011, the Musculoskeletal Infection Society (MSIS) developed a new definition for the PJI diagnosis, based on clinical and laboratory criteria, in order to increase diagnostic accuracy. However, MSIS acknowledged that PJI may be present even if these criteria are not met, particularly in the case of low-grade infections, as patients may not present with clinical signs of infection and may have normal inflammatory markers and joint aspirates. The biofilm-forming bacteria Propionibacterium acnes and Staphylococcus epidermidis are 2 such low-virulence organisms—once commonly considered contaminants but now recognized as potential pathogens for postoperative joint infections.9 In a review performed at a major orthopedic hospital, Bjerke-Kroll and colleagues10 found that the rate of PJI with P acnes has been increasing linearly over the past 14 years. According to reports in the literature,11-13P acnes has been isolated in 2% to 4% of all cases of PJI, and Zappe and colleagues13 found a P acnes PJI rate of 6% in a retrospective analysis performed at their institution. Given the high rate of P acnes colonization of the axilla, this organism is now increasingly recognized as a cause of infection after shoulder surgery, as found in a case series of 10 patients with P acnes PJI after total shoulder arthroplasty (TSA).14 However, there is still limited data on the role of P acnes in lower extremity PJI.
Although patients with P acnes PJI can present with overt signs of infection, more often they lack systemic or local signs of infection, making the diagnosis difficult.15 Surgeons may not consider PJI as a cause of TJA failure in patients who do not meet diagnostic criteria.7 In a case series of patients with P acnes PJI after TSA, Millett and colleagues14 concluded that erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level are not always reliable indicators of infection with low-virulence organisms. Eighty percent of patients in their study had normal ESR and CRP level before surgery. Zappe and colleagues13 reported on P acnes PJI diagnoses in 4 total hip arthroplasties (THAs), 3 TKAs, and 1 TSA. Of the 8 patients, 6 (75%) had borderline elevated CRP levels, and 4 (50%) had normal synovial fluid analysis and cultures from joint aspirations. In a study using electron microscopy and fluorescence in situ hybridization (FISH) labeling, Stoodley and colleagues16 found, in 8 polyethylene liners removed from culture-negative THA patients for aseptic loosening, extensive biofilm colonization with S epidermidis.
Reports of PJI cases misdiagnosed as aseptic loosening also suggest that screening and diagnostic tools are not sensitive enough to detect all infections and that PJI likely is underdiagnosed. In a prospective cohort study, Portillo and colleagues17 categorized patients who were undergoing revision surgery after TJA by cause of failure: aseptic loosening, mechanical failure, or PJI based on current MSIS guidelines. Intraoperative cultures were taken during the revisions. P acnes was isolated in 2 (3%) of the 63 cases classified as PJI and in 12 (19%) of the 63 classified as aseptic loosening. Tsukayama and colleagues18 reported an 11% rate of positive intraoperative cultures for P acnes during revision surgery in cases that the operating surgeon considered aseptic, based on white blood cell (WBC) count, ESR, and CRP level. Rasouli and colleagues19 used an Ibis biosensor to perform polymerase chain reaction (PCR) on synovial fluid from 44 patients who underwent aseptic revision of TKA failures. The authors detected a pathogen in 17 (38%) of the 44 presumed aseptic patients and concluded some aseptic loosening cases are actually chronic low-grade organism PJIs not diagnosed according to current PJI criteria.
In this article, we present the case of a patient with a stiff, painful knee after TKA and with ESR, CRP level, and synovial fluid analysis within normal limits. Open biopsy for cultures showed P acnes PJI, which was successfully treated with 2-stage revision. The patient provided written informed consent for print and electronic publication of this case report.