Low-frequency, low-power ultrasound applied to failed orthopedic implants prior to culture improves the identification of infection, according to results of a prospective study at the Mayo Clinic College of Medicine in Rochester, Minnesota.
The standard way of diagnosing prosthetic joint infection is to culture samples of periprosthetic tissue, "but this method is neither sensitive nor specific," Dr. Robin Patel and colleagues report. The issue is relevant, they say in the
New England Journal of Medicine for August 16, because aseptic failure of a prosthetic joint is treated differently than joint infection.
As a 'new diagnostic approach,' Dr. Patel's team set up a system for sonicating explanted hip and knee prostheses to dislodge adherent bacteria. The specimens are placed in sterilized, one-liter jars along with 400 mL of Ringer's solution. The container is set in an ultrasound bath for five minutes, preceded and followed by 30 seconds of vortexing, used to "increase the concentration of air bubbles and thus enhance the cavitation effect of subsequent sonication."
Dr. Patel and associates compared culture results obtained from homogenized periprosthetic tissue and from sonicate fluid. Specimens from 207 knee prosthesis and 124 hip orthoses were incubated in culture medium both aerobically and anaerobically.
Standardized nonmicrobiological criteria were used to determine the presence of infection: visible purulence, histopathologic identification of acute inflammation, or a sinus tract communicating with the prosthesis. In this manner, 79 patients were diagnosed with prosthetic joint infection, and 252 joints were considered to be aseptic failures.
Results showed that the sensitivity of sonicate-fluid cultures was superior to that of tissue culture (78.5 versus 60.8 per cent, p < 0.001). Specificity was > 98 per cent in both.
The strength of the new approach was further validated by the identification of 14 patients with prosthesis infection, as evidenced by positive sonicate-fluid cultures alone.
"Our results emphasize the importance of performing both aerobic and anaerobic sonicate-fluid culture," the group points out: seven infections would have been missed if only aerobic conditions had been used, as would three if only anaerobic cultures had been used.
Seventeen joints deemed infected by nonmicrobiological criteria were culture-negative in both sonicate fluid and periprosthetic tissue specimens, supporting the investigators' conclusion that "a multimodal approach ...including clinical, microbiologic and tissue histopathological findings," is justified for diagnosing failed prosthetic joints.
Prosthetic joint infections remain a significant problem in orthopedic surgery, Dr. Francis A. Waldvogel notes in a related editorial. "The isolation of the offending organism is difficult and, therefore, treatment strategy often is speculative," he adds. "Improved diagnostic tools clearly are warranted."
Dr. Waldvogel, from the World Knowledge Dialogue Foundation in Geneva, commends the Mayo Clinic research team for evaluating another means of increasing bacterial yield.
He expects even greater improvements in diagnosing failed prosthetic joints will be achieved as newer microbiological techniques, including immunofluorescence and confocal laser scanning microscopy, along with immunological and molecular techniques, gain wider acceptance.
Source: Reuters
