Background: The presence of bacteria forming biofilms or prior antimicrobial use has been shown to reduce the sensitivity of the standard technique (PT cultures) in patients with infection of orthopedic implants. Culturing fluid resulting from sonication (FRS) of prosthesis could improve the microbiologic diagnosis.

Objective: To analyze the diagnostic validity of culturing FRS of different orthopedic implants and PT culture

Methods: Between Jan 2007 and Apr 2008, patients undergoing knee or hip prosthesis removal, and those with ostheosyntesis or spinal instrumentation removal, were prospectively included (44 hip prosthesis, 63 knee prosthesis, 91 osteosynthesis and 14 spinal instrumentations). 5 PT specimens were collected for culture. Removed implants were sonicated during 5 min. (40Hz). Both, PT and FRS, were inoculated in aerobic agar (Chocolate Polyvitex), anaerobic agar (Schaedler + 5% blood) and in thioglycolat, for 7 days. Positive culture cut-off was defined as growing of > 5 CFU. Clinical diagnosis of prosthetic-joint infection was made as commonly accepted. Previous antimicrobial therapy was assessed. Diagnostic validity was calculated for both culturing methods. Sensitivity of methods was compared by Chi-square test (SPSS 15.0).

Results: 212 cases were included. Diagnostic of infection was made in 17 hip prosthesis (THA), 20 knee prosthesis (TKA), 24 osteosynthesis (OS) and 6 spinal fusions (SI).

Tissue culture was positive in 9 THA, 11 TKA 18 OS and 4 SI. Sonication culture was positive in 14 THA, 18TKA, 23 Os and 6 SI.

Tissue culture: Sensibility: THA53%, TKA 55%, OS 75% and SI 66%. Specificity: THA 96%, TKA 100%, OS 96%, SI 100%

Sonications: Sensibility: THA 82%, TKA 90%, OS 95% and SI 100%. Specificity: THA 96%, TKA 100%, OS 92%, SI 100%

Statistical differences favoring sonication were found in sensitivity in knee arthroplasty and osteosynthesis implants. 6 patients received antibiotics for > 7 days before implant was removed. Sonication culture was positive in 4 of them whereas only one standard culture yielded positive.

Conclusions: FRS cultures are more sensitive than PT cultures. Sensitivity of the method depends on which device is evaluated. Sonication also improves sensitivity of culture after preoperative antimicrobial therapy.