Resurrection of an Old Technology (Sonication): Is Prosthetic Joint Failure Due to Infection?

Abstract & Commentary

By Ellen Jo Baron, PhD, Professor of Pathology and Medicine, Stanford University; Medical School Director, Clinical Microbiology Laboratory, Stanford University Medical Center, is Associate Editor for Infectious Disease Alert.

Dr. Baron reports no financial relationships relevant to this field of study.

The case is typical. the patient had a hip replacement eight years ago and, over the past six months, the hip has begun to hurt, walking is impaired, and the patient is becoming less mobile. The hip must be replaced. But is it infected? If so, the patient will face a period of time without a mobile joint so that antibiotics can do their job before the hip hardware is replaced. If the joint is only inflamed but not infected, the patient will be walking again soon on his/her new hip.

The culture results are among the key indicators that the orthopedic surgeon will use to make the difficult decision. How well do those cultures reflect the situation in the joint? Everyone knows that the most likely organisms to cause such an indolent infection many years after the original surgery are coagulase-negative staphylococci in the knee or hip; Propionibacterium acnes if the joint is a shoulder. But these are the same species most likely to be introduced during the removal procedure and, thus, lead to false-positive culture results. On the other hand, most of the time, cultures of the tissue surrounding the infected prosthesis will yield "no growth" even when the histopathology clearly indicates an infectious process. Microbiologists have tried to clarify the issue by requesting that surgeons send at least 5-6 separate tissue samples obtained from different sites within the surgical site. If three or more of these samples yield the same species (usually coagulase-negative staphylococci), the organism is deemed to be the etiological agent of the infection.1 Until now, that protocol, which was labor-intensive and very expensive, was the only option. And still, cultures often fail to reveal an etiological agent.

Working with Robin Patel, workers at the Mayo Clinic have revisited an old method: sonication.1 Based on the knowledge that prosthetic joint infections represent yet another biofilm-based infectious process, joining cystic fibrosis and catheter-related bloodstream infections, not to mention two other well-known biofilm models of infection, they proposed that high-energy soundwaves might disrupt the cement-like intercellular glycocalyx matrix of the biofilm that allows the infecting organisms to adhere tightly to the joint material. In fact, sonication worked to dislodge organisms on the prosthesis itself and free them into the liquid culture medium so that they could be detected in culture, whereas cultures obtained at the time of surgery from tissue surrounding the joint showed no growth.2,3

A group of scientists from Norway, studying how to improve the process, used metal discs seeded with bacteria to form biofilms to confirm that sonication did, indeed, release organisms that could not be detected in cultures even if the disks were scraped with scalpels.4

A second group from Trondheim has recently compared sonication conditions to try to optimize the process, finding seven minutes on high energy at 22° C to reduce organism death results in the best overall recovery.5 This group has also shown that even if the container holding the device extends above the surface of the liquid in the sonication bath, organisms are effectively removed from the device.

Commentary

The laboratory process is still not perfect. Laboratories need to validate their ultrasound device with some preliminary studies; the exact number of cfu/mL that correlates with infection is yet to be defined. The entire volume of liquid in which the joint is sonicated probably needs to be concentrated by centrifugation, which can be laborious when the maximum amount one can place into a single centrifuge tube is 50 mL, necessitating pooling multiple sediments. However, these studies have signaled a true breakthrough in our ability to assist the orthopedic surgeons with the difficult process of differentiating prosthetic device failure caused by infection from aseptic failure. In fact, I have rescued an old ultrasound device out of "dead storage," and plan to start validating some version of prosthetic joint device sonication and culture for our institution.

Reference

  1. Atkins BL, et al. Prospective evaluation of criteria for microbiological diagnosis of prosthetic-joint infection at revision arthroplasty. The OSIRIS Collaborative Study Group. J Clin Microbiology. 1998;36:2932-2939.
  2. Trampuz A, et al. Sonication of removed hip and knee prostheses for diagnosis of infection. N Engl J Med. 2007;357:654-663.
  3. Piper KE, et al. Microbiologic diagnosis of prosthetic shoulder infection by use of implant sonication. J Clin Microbiology. 2009;47:1878-1884.
  4. Bjerkan G, et al. Sonication is superior to scraping for retrieval of bacteria in biofilm on titanium and steel surfaces in vitro. Acta Orthop. 2009;80:245-250.
  5. Monsen T, et al. In vitro effect of ultrasound on bacteria and suggested protocol for sonication and diagnosis of prosthetic infections. J Clin Microbiology. 2009;47:2496-2501.