Background: The diagnosis of implant-associated infections is difficult due to organisms attached to surfaces as biofilms. We hypothesize that diagnosis can be improved by removing biofilm microorganisms from implant surface by sonication, followed by Gram stain, culture and calorimetric detection in sonication fluid.

Methods: We prospectively included adult patients from May 2005 until December 2006 from whom an orthopedic implant (joint prosthesis or internal fixation device) was removed for any reason. Removed implants were vortexed and sonicated in solid containers 5 min at 40 kHz in 100 to 400 ml Ringer’s solution. The resulting sonicate was plated and incubated on aerobic and anaerobic blood agar and aliquots were in parallel incubated at 37°C for 3 days in an isothermal calorimeter TAM III (TA Instruments, New Castle, DE). Gram stain was performed on sonicates centrifuged at 5000 g for 10 min. Definitive infection diagnosis was of the implant was defined if purulence surrounding the implant, or growth of the same microorganism in ≥2 synovial fluid or intraoperative tissue specimens, or acute inflammation in histopathology, or a sinus tract was present. Sonicate culture was defined positive if > 10 cfu (colony forming units) grew/ml sonicate. Calorimetry was defined positive if heat flow rate increased ≥10 μW above baseline (detection limit ~0.3 μW).

Results: 846 implants (367 joint prostheses and 479 internal fixation devices) were studied, of which 171 (20%) were infected and 675 (80%) were aseptic cases. The sensitivity of intraoperative tissue cultures was 74%, of sonicate culture 89%, of sonicate Gram stain 51%, of sonicate calorimetry 96%. The specificity of all specimens was ≥95%.

Conclusion: Sonicate culture and calorimetry were more sensitive than intraoperative tissue cultures for diagnosing implant infections. With Gram stain of centrifuged sonicate, infection was diagnosed in > 50% cases. Sonicate culture and calorimetry may replace the current approach using multiple intraoperative periprosthetic tissue specimens, whenever the implant or part of it is removed.