CORR Insights®: Neutrophil Extracellular Trap-related Biomarkers Are Increased in the Synovial Fluid of Patients With Periprosthetic Joint Infections

Where Are We Now?

An adage in orthopaedics is that “no problem is so bad that it cannot be made worse with surgery.” Certainly, many patients living with degenerative conditions of the hip and knee have been immeasurably helped by the development of total joint arthroplasty (TJA), and yet, for the minority of patients who experience a prosthetic joint infection (PJI), they may conclude that treatment is less beneficial than when they first met their surgeons. The management of PJI thus continues to be among the most important problems in TJA. Although diagnosis and treatment options continue to improve, the exact mechanism by which joint contamination becomes an infection is still poorly understood. In addition, a high proportion of patients continue to have pain after arthroplasty; many of them may be harboring undiagnosed infections. With that in mind, patients with painful joint replacements often undergo a battery of diagnostic tests to try to determine a cause, but answering the seemingly simple question about infection remains a challenge.

Groups like the Musculoskeletal Infection Society are dedicated to improving the treatments of patients with PJI. The most-recent Musculoskeletal Infection Society criteria for PJI attempted to improve on existing objective definitions of hip and knee infection and included synovial fluid markers such as alpha defensin [6]. These criteria remain the closest thing we have to a gold standard but require multiple different points of evaluation—there is no single test that can act with perfect sensitivity and specificity to confirm a diagnosis of PJI. Moreover, many of these tests require time and major expense to process. This means that a diagnosis, even when the data are conclusive, can be inconvenient, time consuming, or both. Every orthopaedic joint surgeon has had a version of this conversation: “I’m sorry you’re still in pain, Ms. Smith. Let us get these tests sent off and we will be in touch once we know a bit more about what is going on.” And the patient leaves, waiting an unclear amount of time to hear either good or terrible news.

And this quandary is where this article by de Sandes Kimura et al. [3] in this issue of Clinical Orthopaedics and Related Research® may offer some key steps forward. Here, the authors help shed light on a physiologic process—neutrophil-mediated extracellular trap (NETosis, a process by which neutrophils release cell-free DNA laden with granule enzymes that serve to entrap and kill infectious organisms)—that looks quite likely to improve our ability to diagnose PJI. Their data suggest a high degree of both specificity and sensitivity for PJI, particularly with the assay for synovial cell-free DNA, suggesting this process may be the most closely linked assayable process we have discovered to recognize infections associated with an arthroplasty. If these data can be replicated elsewhere and turned into a commercially available (especially at the point-of-care) assay, a new standard for diagnosing PJI would be upon us.

Where Do We Need To Go?

For unfortunate patients who end up on this path, there are three pivotal developments that must happen in the management of PJI: preventing it, streamlining its diagnosis, and improving its treatment.

We still diagnose infection based on our best proxy for infection—neutrophil activation. To my knowledge, other than cultures or polymerase chain reaction, all available tests for infection really measure neutrophil activation. We still do not know the extent to which these two are equivalent—there are instances, for example, where neutrophil activation markers are elevated without infection. There are also certainly instances where the opposite is true, which is why a direct test for infection is so important.

The perfect test for PJI would be office-based and time-efficient (if not quick) to allow a real-time discussion with patients, and highly accurate, with a near-zero rate of either false positives or false negatives. The paradigm for such a test, of course, is the urine pregnancy test, which can be obtained easily and has near-perfect sensitivity and specificity. We need to develop such a test for PJI. The ideal analyte, however, has remained evasive—alpha defensin, though much improved compared to white blood cell count, still falls short [7]. It may be that a marker of NETosis as described here [3] advances us considerably.

Looking beyond diagnosis, as we come to better understand the normal versus pathologic response to bacterial contamination that leads some patients to develop clinical infections, we should also improve our ability to prevent and treat the process as well. For instance, might we learn that the NETosis process can be augmented or enhanced with topical, intramedullary, or intravenous adjuvants, alone or in combination with an antibiotic, that may lessen the need for a two-stage revision procedure? Ideally, we will reach a state where we can successfully prevent PJI, and the days of two-stage exchanges will seem barbaric in light of improved treatment options.

Despite decades of research into degenerative joint conditions and PJI, there are still substantial gaps in our understanding. For instance, the joint in the setting of infection has a clear characteristic—signs or the presence of a high number of activated neutrophils in response to the presence of virulent organisms. That is about the extent of our agreed knowledge, however. The exact concentration of offending agent or host response cell, what cytokines or markers they secrete, and the presence of other signalling compounds or mediators remains surprisingly unsettled. Some have even hypothesized that bacterial contamination, or at least the microbiome, may be a part of all degenerative joint conditions [1]. One notable orthopaedic surgeon has even provocatively said, “all arthroplasties are infected all the time,” suggesting there is a spectrum of contamination [9]. So, to get to the point where we are directly testing for infection, we need a better understanding of what infection really is—rather than bacterial presence (whether planktonic or only visible through a DNA signature) or neutrophil activation.

How Do We Get There?

Continued work, whether via animal model, in vitro, or through genetic-assay-based models and other techniques, will be essential to help orthopaedists better understand the mechanisms by which the body attempts to respond to contamination of the joint, whether prosthetic or native. How do these environments differ in early contamination versus later when infection is well established? What leads to the establishment of biofilm, and perhaps worse, to osteomyelitis? Why do some individuals experience more of these types of problems while some seem relatively protected? As these questions are answered, our ability to prevent, diagnose, and treat PJI will certainly be improved.

In the meantime, emphasis must be placed on creating a reliable point-of-care test for PJI as outlined above. The alpha defensin lateral flow test (CD Diagnostics) has been such an attempt, but unfortunately has underperformed the conventional ELISA send-off test, with considerably lower sensitivity and accuracy (though with similar specificity) [4]. To do that, however, we need a better understanding of what truly differentiates infection from other inflammatory states in the knee.

Thus, as with all the best research, this study [3] answers some questions but suggests many more, and points us in a direction of new inquiry. Ideally, we can get a clearer picture of what NETosis entails, and its role in PJI—it may very well be a key differentiator of true infection rather than a mere marker for neutrophil activation or presence.

Read This Next

• Get a better understanding of the abnormal and confusing milieu of the infected total joint, and why pure neutrophil activation is not a clear link to mean “active infection” in this clinical science study examining the use of alpha defensin after joint explantation [8].

• A policy paper offers a nice summary of the “newer” modalities being used to make diagnoses in the infectious disease space [2].

• The microbiological profile of an infected arthroplasty can be difficult to understand, even with advanced techniques like next-generation sequencing [5], according to this review article summarizing current methods to understand the antibiotic susceptibilities of infectious pathogens.