Ultrasound Improves Infection Diagnosis in Hip and Knee Prostheses

ROCHESTER, Minn., Aug. 16 -- For microbiologic diagnosis of an infected hip or knee prosthesis, low-energy ultrasound of the device itself may be the preferred approach.
In a study of 331 such patients, the sensitivity of sonicate-fluid culture proved superior to that of tissue culture, especially in patients previously treated with antibiotics, Robin Patel, M.D., of the Mayo Clinic here, and colleagues reported in the Aug. 16 issue of the New England Journal of Medicine.
Culturing samples of periprosthetic tissue, the standard method for the microbiologic diagnosis of prosthetic joint infections, is neither sensitive nor specific when the pathogens are harbored by the device itself, the researchers wrote.
In these infections, microorganisms are encased in a biofilm on the surface of the prosthesis, which suggested that using ultrasound to dislodge adherent bacteria would improve diagnosis. Sonication is kept at a low level to preserve microbial viability.
In a prospective trial of a more practical way to do this by the Mayo clinicians, comparing conventional culture with cultures of samples obtained by sonication, the researchers studied 207 patients with total knee prostheses and 124 with hip prostheses undergoing removal of the prosthesis for aseptic failure or presumed infection.
Of the patients, enrolled from Aug. 12, 2003 to Dec. 13, 2005, 252 had apparent aseptic failure (median age 70), and 79 had prosthetic-joint infection (median age 68).
Using standardized nonmicrobiologic criteria to define prosthetic joint infection, the researchers found the sensitivities of periprosthetic tissue cultures and sonicate-fluid cultures were 60.8% and 78.5% (P<0.001), respectively. The specificities were 99.2% and 98.8%, respectively.
Fourteen cases of prosthetic-joint infection were detected by sonicate-fluid culture but not by prosthetic-tissue culture, the researchers reported.
Among patients receiving antimicrobial therapy within 14 days of surgery, the sensitivities of periprosthetic tissue and sonicate-fluid culture were 45.0% and 75.0% (P<0.001), respectively.
The optimal antimicrobial-free period required before arthroplasty to obtain meaningful culture results remains to be determined, the researchers said.
However, among patients receiving antimicrobial therapy within 14 days of surgery, sonicate-fluid cultures were more sensitive than tissue cultures. The researchers suggested that this may be because planktonic bacteria present in tissue are more susceptible to anti-infective agents than are biofilm bacteria.
The study had several limitations, including a lack of a gold-standard definition of prosthetic joint infection.
Processing explanted orthopedic components for culture takes about twice as long as processing tissue specimens. Thus only a single specimen (components of the explanted prosthesis) was submitted for study, compared with the multiple tissue specimens typically processed, the investigators said.
Also, they said, this study was not designed to detect mycobacterial or fungal infections. If these are suspected, tissue cultures should be done.
Despite improved sensitivity, sonicate-fluid culture failed to identify a substantial number of potentially infected cases among the aseptic-failure patients. This finding indicates either that bacteria are not involved in the pathogenesis of aseptic failure or that sonicate-fluid culture is inadequate to detect the microorganisms associated with aseptic failure if any are present.
In an accompanying editorial, Francis A. Waldvogel, M.D., of the University of Geneva in Switzerland, wrote that the procedure has already been used in the diagnosis of catheter-related infection and in selected cases of orthopedic-implant infections.
Dr. Waldvogel listed three study limitations. First, he said, the diagnosis of aseptic failure was established by conventional criteria, but the sonication method may have been suboptimal in efficiency, he said, and the prostheses may have been misclassified.
Second, the researchers used conventional microbiologic techniques, but newer techniques have been used to detect bacteria in sonicate fluid from prosthetic hips, and the results have been impressive.
Third, clinicians are often confronted with the question of possible infection before, and not during, surgical intervention. Unfortunately no test is currently available to answer this important question.
Despite these limitations, Dr. Waldvogel wrote, the procedures already used for the diagnosis of catheter-related infections may soon become a standard approach for all infections suspected of being 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," he concluded.
This study was supported by grants from the Mayo Foundation, the Orthopaedic Research and Education Foundation, the Swiss National Science Foundation, and the Roche Research Foundation (to Andrej Trampuz, M.D.).
Additional source: New England Journal of MedicineSource reference: Trampuz A, et al "Sonication of Removed Hip and Knee Prostheses for Diagnosis of Infection" N Engl J Med 2007; 357:654-663. Additional source: New England Journal of MedicineSource reference: Waldvogel FA "Ultrasound -- Now Also for Microbiologists?" N Engl J Med 2007; 357: 705-706.