CORR Insights®: Are Sonication Cultures of Antibiotic Cement Spacers Useful During Second-stage Reimplantation Surgery for Prosthetic Joint Infection?

Where Are We Now?

We lack effective tools to diagnose prosthetic joint infection (PJI) when antibiotic spacers are in a patient’s hip or knee. Recent studies suggest that widely used tests for infection, including the erythrocyte sedimentation rate, c-reactive protein, white blood cell count, neutrophil percentage, and cultures cannot tell us reliably whether an infection has been eradicated when a spacer block remains in situ [4, 5, 8].

One of the major problems in the arthroplasty field is the absence of a “gold standard” in assessing the status of a joint awaiting reimplantation. The Musculoskeletal Infection Society criteria likely are ineffective in identifying infected spacer blocks [2, 3], leaving us without a way to measure the performance of a diagnostic test for infection. Furthermore, there is still little agreement on classifying the diagnostic outcomes of a reimplantation, by which we can evaluate the performance any diagnostic test. While a certain proportion of patients will have infections that persist after reimplantation, this high-risk host population should also be expected to have a high risk of new infections with the same or different organisms after reimplantation [1].

In the current study, Olsen and colleagues [6] report on the sonication and culturing of explanted spacer blocks as a potential method of improving the culture yield of persistently infected spacer blocks. It is important to note that this group has previously reported on, and is enthusiastic about, the performance of routine sonication in the setting of revision arthroplasty [7].

The authors retrospectively identified patients having a reimplantation, excluding those who went on to have a second spacer block due to obvious persistence of infection. They compared the results of spacer block sonication and culture with the patients’ outcomes to evaluate the performance and/or utility of routine spacer block sonication during reimplantation [6]. Although it is easy to criticize this study for having too few total patients, or too few reinfections, studying a large number of spacer blocks is very, very difficult. Still, my interpretation of their results is that the routine sonication and culture of spacer blocks at the time of reimplantation is not helpful. Routine sonication rarely was positive (two of 41 spacers), did not help predict which reimplanted patients are still infected, and so does not really help identify patients whose infections will be cured (because none of the infections were identified by testing).

However, the study is important; it advances knowledge by demonstrating that at one major center that favors sonication, routine sonication of spacer blocks at the time of reimplantation was fruitless.

Where Do We Need To Go?

Several important questions remain unanswered. For example, how do we differentiate between persistent and new infections? Is a culture-negative infection after reimplantation a persistent or a new infection? If a patient underwent spacer block treatment for a Staphylococcus aureus infection, and then develops an Staphylococcus epidermidis infection 8 months post reimplantation, is this a new infection or persistence of an old multiorganism infection?

Although classifying any infection after reimplantation as persistent infection seems like a good strategy, this ignores the fact that even patients cured by the spacer block and reimplantation will have a high risk of new infection due to their host status (likely 12% or higher [1]). Unfortunately, failing to differentiate between recurrent and new infection makes it impossible to identify a good test for infection, as this will inappropriately flip a large percentage of true-negative test results to false-negatives. Therefore, until these questions can be answered, it will remain methodologically impossible to measure the performance of any test for infection in the setting of a spacer block, as we will underestimate test sensitivity considerably.

How Do We Get There?

It appears that studying diagnostic tests in the setting of spacer blocks may not yield satisfactory or reliable results in the foreseeable future, due to the unavoidable methodologic concerns. This realization makes it clear that to move forward, our field must identify methods and strategies to improve our detection and identification of infecting organisms. Technologies such as microbial-antigen detection and sequencing technologies may one day bring us to the point where we can differentiate between a new and persistent infection, which would then provide a valid methodology by which we can study the diagnostic performance of tests at the time of a second stage reimplantation. Until that point, it is prudent to continue standard serum and synovial fluid tests in patients with spacer blocks, keeping in mind that only when the laboratory values are overwhelmingly diagnostic of infection can we use them for decision-making.