Periprosthetic knee infection is a serious complication of total knee replacement, connected to the formation of microbial biofilm on the surface of the implant. The most common bacteria causing periprosthetic infections are: Staphylococcus aureus, coagulase negative staphylococci and Gram negative bacteria. The purpose of this study is to present a rare case of periprosthetic knee infection caused by Ralstonia pickettii, which ultimately resulted in knee arthrodesis.

A 70 years old patient was referred to our clinic because of persistent pain, swelling and fistula in his left knee, 18 months after total knee arthroplasty. The patient had received oral ciprofloxacin and rifampicin during the last three months. The cultures of the fistula and knee aspiration were negative. A 2-stages revision of the total knee arthroplasty was decided. The patient underwent surgical removal of the prosthesis, cement and suspicious tissues and a spacer was placed. Samples from periprosthetic tissues were sent for conventional culture and all metal components were sonicated.

Intraoperative periprosthetic tissue cultures were negative. The culture of the sonicated fluid was positive for Ralstonia pickettii, a finding which was confirmed by the method of 16s rDNA. Ralstonia pickettii is a gram negative bacterium, capable of forming biofilm on metal surfaces. Postoperatively, the patient received intravenous antibiotic therapy with ciprofloxacin and cefepime, for 6 weeks, according to the results of the antibiogram. Due to fistula relapse after two surgical debridements, large bone defects and degeneration of the patellar tendon, knee arthodesis was performed.

In literature, there is only one reference of periprosthetic knee infection because of Ralstonia pickettii. Although quite rare, the periprosthetic knee infection caused by Ralstonia pickettii can be extremely durable with disastrous results for the knee joint.

In spite of its incidence decreasing to 1% nowadays, prosthesis-related infections remain a research, diagnostic, therapeutic and cost-related problem. Early diagnosis, selection of an appropriate surgical strategy, accurate identification of the responsible microorganisms and construction of an appropriate antibiotic regimen are essential elements of any management strategy. Our study aim was firstly to compare the diagnostic accuracy of conventional periprosthetic tissue culture and culture of fluid derived from vortexing and bath sonication of the explanted hardware and secondly to investigate the role of possible metabolic factors affecting the sensitivity of the sonication method.

We investigated 70 patients undergoing revision hip or knee arthroplasty because of loosening of the prostheses, at our institution, between October 2011 and November 2013. Patients’ medical history and demographic characteristics were recorded. We compared the culture of samples obtained by sonication of explanted hip and knee prostheses with conventional culture of periprosthetic tissue for the microbiological diagnosis of prosthetic-joint infection.

Infectious Diseases Society of America (IDSA) Guidelines were used for the definition of prosthetic-joint infection. Thirty-two patients had septic loosening and 38 aseptic loosening (48 hip prostheses and 22 knee prostheses). The sensitivity of sonication fluid culture was 81.25% and the sensitivity of conventional tissue cultures was 56.25% (p-value = 0.043). The sensitivity of the sonication method was statistically higher in obese, diabetic patients, with age above 60, in uncemented arthroplasties and in arthroplasties because of primary osteoarthritis (p-values < 0.05).

The sonication method represents a reliable test for the diagnosis of prosthetic – joint infections with a greater sensitivity than the conventional periprosthetic tissue cultures, especially in obese, diabetic patients, with age above 60, in uncemented arthroplasties and in arthroplasties because of primary osteoarthritis.