In some cases the diagnosis of a deep periprosthetic joint infection will be straightforward, with purulent drainage from the incision or dehiscence of the wound making the diagnosis clear. In many other cases, however, it is less obvious if the patient has a superficial wound cellulitis that can be managed nonoperatively, if deep PJI exists, or if there is no infection present at all. Normal postoperative pain and inflammation around the surgical site make normal cues to diagnosis less helpful.

In these cases, we have found the serum C-reactive protein (CRP) to be an excellent screening test to determine if an aspiration of the knee is warranted [2]. Specifically, in a study of 146 knees that were evaluated at two centers for deep PJI in the first 6 weeks postoperatively, the serum CRP was found to have excellent overall accuracy at an optimal cutoff value of 95 mg/L (normal, <10 mg/L). Hence, if there is any concern regarding infection in the early postoperative period, we always obtain a serum CRP. If the value is above or anywhere near 100 mg/L (100 being easier to remember than 95), an aspiration of the knee is performed. In general, the aspiration should be performed through an area that is clear of erythema, if possible.

Fluid obtained at the time of aspiration should be sent for a synovial fluid white blood cell (WBC) count and differential to determine the percentage of polymorphonuclear cells, as well as for culture. Prior work from our center has shown the utility of these tests for the diagnosis of CHRONIC PJI with optimal cutoff values of 3,000 WBC/μ[mu]L and 80 % for the differential [3]. However, in the acute postoperative phase, defined as within the first 6 weeks after surgery, the work referenced above [2] suggests optimal synovial WBC count and differential values of approximately 10,000 WBC/μ[mu]L and 90 %, respectively. More specifically, the data suggested an even higher cutoff value of 27,800 WBC/μ[mu]L to optimize specificity (good rule in test) and 10,700 WBC/μ[mu]L to optimize sensitivity (good rule out test). In practice I prefer to use the lower threshold number of 10,000 WBC/μ[mu]L. Thus, synovial fluid white blood cell counts below 10,000 WBC/μ[mu]L are considered as not infected, while values above are deemed infected, particularly if the differential is greater than 90 % polymorphonuclear cells. Of note, the synovial fluid white blood cell count and differential values actually change over time [4]. However, in my own experience, the majority of patients where there is concern over infection typically present within the first 6 weeks after their surgery. In cases, where the values are equivocal or if the clinician is just not sure, cultures can be observed and a final decision made based upon their result.

To avoid confusion on diagnosis for any infection, it is imperative that antibiotics be withheld until an aspiration of the knee joint is performed [5]. The administration of antibiotics prior to a thorough evaluation for infection, while tempting, can greatly confuse the diagnosis. Not only will the deep culture results potentially be compromised, but also because the effect of antibiotic treatment on the synovial fluid WBC count and differential as well as serum markers of PJI is not well understood. Furthermore, antibiotic administration prior to obtaining synovial fluid for culture is the strongest risk factor for culture negative PJI [6], which typically leads to suboptimal antibiotic treatment as the offending pathogen can no longer be identified or its antibiotic sensitivities determined. This inability to identify the infecting organism or determine antibiotic sensitivities leads to compromises having to be made in order to cover the most likely organism as opposed to directly targeting a known infecting organism. Finally, if wound drainage is present, it should NOT be cultured as these culture results can often confuse treatment decisions [7].

Management of deep periprosthetic joint infections should be operative. The most commonly utilized strategy for acute postoperative treatment in North America is irrigation and debridement with an exchange of the modular polyethylene bearing surface. However, reported results in the literature are quite variable, with many contemporary series suggesting failure rates of more than 50 %. Several series have also suggested that infections with staphylococcus aureus [8], especially if resistant to methicillin, do particularly poorly [9]. Hence, in some situations the surgeon might consider removing the implants if the identity of the infecting organism is known to be methicillin-resistant staphylococcus aureus prior to embarking upon operative intervention.

If an irrigation and debridement is chosen for management, it should not be relegated to a junior member of the team, as surgical technique may be important to optimizing results and good judgment is required to ensure an adequate and thorough debridement. First, the skin should be meticulously mechanically debrided prior to prepping and draping to remove any sutures, staples, skin glue, and scabbing. The skin edges must be carefully handled to optimize the chances of uneventful wound healing, including raising small but full thickness skin flaps prior to the arthrotomy. Next, the debridement should include a stepwise and thorough anterior synovectomy and exchange of the modular polyethylene liner. Liner trials must be available to ensure the correct size is inserted. In many cases the optimal size liner is somewhat thicker than the original, while in some other cases the surgeon may decide a thinner insert is better.

Next, a posterior synovectomy is performed, but this is admittedly challenging in most situations given the limited exposure posteriorly with implants in place. Then the wound is cleansed with pulsatile lavage, and in our center also soaked with a dilute betadine lavage [10] in an attempt to further decrease bacterial counts in the wound. This is followed by an attempt at mechanical debridement of the metallic surfaces. We typically perform the mechanical debridement with a brush used to cleanse the femoral canal in total hip arthroplasty.