The diagnosis of periprosthetic infection in hip and knee replacement surgery is often difficult1. Although overt signs of infection are readily apparent and easily interpreted, the diagnosis of infection in the absence of such overt signs may be problematic2.
A number of indirect indicators of infection have been studied, including bone scintigraphy, systemic inflammatory markers, and synovial fluid analysis. None of these methods will yield a result that, in and of itself, is diagnostic of periprosthetic infection; however, taken together, multiple indicators may reasonably lead to an appropriate diagnostic conclusion3.
The authors have undertaken a complex research study in an attempt to determine the natural course of the synovial fluid white blood-cell (WBC) count as a function of time, from the date of the index operation up to the time of arthrocentesis and synovial fluid analysis.
The study involved 571 primary total knee arthroplasties that required arthrocentesis within the first two postoperative years; the times between surgery and aspiration were then segmented as outlined in the paper. The synovial fluid WBC count, the percentage of polymorphonuclear leukocytes (PMNs), and the total neutrophil count were determined. In the body of the paper, the authors clearly outline the rate of progression in these three parameters with time elapsed from surgery.
The interpretation of these results has to be approached with some caution for a number of reasons. First and foremost, the knees from which the samples were derived were all knees with a problem. The patients obviously exhibited signs and/or symptoms that led the treating surgeon to suspect periprosthetic infection. Even though the included synovial fluid analyses were restricted to those patients without evidence of periprosthetic infection on final evaluation, they still reflect the patient with an abnormal postoperative course.
Second, it is my practice not to aspirate a painful knee in the presence of normal systemic inflammatory markers. It is not clear from the data presented in this paper what percentage of patients had abnormal inflammatory markers prior to the decision to carry out arthrocentesis.
The value of this paper, however, is the clear demonstration that there will be a change in the WBC count and the PMN percentage during the first six postoperative weeks. The rate of change varies with time and it will therefore be important, when relying upon these results, to be aware of the time elapsed between the index surgery and arthrocentesis.
The authors also suggest that the total neutrophil count may be a more sensitive indicator of infection than either the synovial WBC count or the PMN percentage is, and they have appropriately suggested that further investigation of this particular laboratory value be conducted. I support their statement that “because these markers change at different rates over time, the use of specific thresholds for the synovial fluid WBC count and differential would probably represent an oversimplification of a complex phenomenon.” Their recommendations regarding criteria development seem appropriate, and I will look forward to further research conducted in this area in order to better define these important laboratory values.