ROCHESTER, Minnesota—Shaking biofilms loose from resected joint prosthesis with ultrasound (US) and culturing the sonicate solution is both more sensitive and more specific than conventional multiple-sample tissue culture methods for identifying microorganisms infecting hip or knee prosthesis, and the method could be done by most hospital microbiology laboratories, Mayo Clinic researchers report in The New England Journal of Medicine.¹

"In this study, culture of samples obtained by sonication of prostheses was more sensitive than conventional periprosthetic-tissue culture for the microbiologic diagnosis of prosthetic hip and knee infection, especially in patients who had received antimicrobial therapy within 14 days before surgery," writes lead author Andrej Trampuz, MD, with the division of infections diseases, department of internal medicine, at the Mayo Clinic College of Medicine, Rochester.

Sonication and culture compared with standard tissue sampling and culture

The prospective trial compared culture of samples obtained by sonication of explanted hip and knee prostheses to dislodge adherent bacterial from the prosthesis with conventional culture of periprosthetic tissue samples in 331 patients with either prosthetic joint infection (N = 79) or aseptic failure (N = 252). The new method relies on the fact that infecting organisms typically form biofilms that attach to the surface of the prosthesis.

The researchers put the removed prostheses into 1-liter, straight-sided, wide-mouthed polypropylene jars for processing. They added 400 mL of Ringer's solution to each jar, then vortexed the jars for 30 seconds before sonication in a US bath for 5 minutes, followed by an additional 30 seconds of vortexing. Vortexing is thought to increase the concentration of air bubbles, which should increase the cavitation caused by US.

Multiple samples of tissue from the most inflamed areas were collected for microbiologic and histopathologic studies prior to vortexing and sonication. Both the tissue samples and the sonication fluid were cultured on aerobic blood agar, chocolate agar, and anaerobic blood agar, and into thioglycollate broth.

Sensitivity better with sonicate-fluid culture, with no loss in specificity

Comparison of the two methods showed:

• Greater sensitivity of sonicate-fluid culture (78.5% vs 60.8%, P <.001)
  
• Similar specificity of sonicate-fluid culture, tissue culture, and synovial-fluid culture (98.8%, 99.2%, and 98.1%)
  
• Greater number of organisms detected in sonicate-fluid culture than in tissue culture
  
• Greater sensitivity of sonicate-fluid culture in patients receiving antibiotics within 14 days before procedure

"Of the 62 patients with positive sonicate-fluid cultures, the infection would have been missed in seven had only aerobic culture been performed, and in three had only anaerobic culture been performed," Dr. Trampuz writes.

Senior author Robin Patel, MD, comments, "We found a wide variety of different types of bacteria. This is important to recognize, because it is ideal for doctors to know what type of infection they are dealing with. This determines what type of antibiotic to give, and in some cases, what type of surgery to perform."

In addition, the sonication technique yields high numbers of viable organisms (at least 50 colony-forming units per plate) which can then be studied for other factors such as antibiotic sensitivity.

Dr. Patel continued, "The problem with the conventional method is that you need multiple tissue specimens because the sensitivity of a single specimen is not good. Another issue is that bacterial normally found on the skin can be picked up on the tissue specimen as it is extracted and passes through the skin."

The time required for processing explanted orthopaedic components using sonication and culture was about twice as long as for processing tissue specimens, but only a single specimen was processed, in contrast to the multiple tissue specimens processed with standard tissue sampling. Sonication and culture are now standard in clinical care in the Mayo Clinic's infectious disease group that cares for patients with prosthetic joint infection.

In an accompanying editorial, Francis A. Waldvogel, MD, from the World Knowledge Dialogue Foundation in Geneva, Switzerland, notes, "Prosthetic-joint failure is often due to infection, and sonication of the prosthesis increases the bacterial yield, which can be further improved by new microbiologic techniques. The procedures already used for the diagnosis of catheter-related infections may soon become a standard approach for all infections suspected to be associated with prosthetic material. Ultrasound technology not only is diagnostic imaging—sound for sight—but also may improve microbiologic diagnosis of a vexing clinical problem: sound for bugs."²

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