¹Univ. Klinik für Orthopädie und Unfallchirurgie, AKH Wien, Wien, Österreich

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Objectives and questions: Periprosthetic joint infections (PJI) are a leading cause of revision surgery after total hip and knee arthroplasty (THA & TKA). While white blood cell count (WBC) and polymorphonuclear neutrophil percentage (PMN%) in synovial fluid are key diagnostic markers, their variability caused by multiple influencing factors complicates diagnosis. The aim of this study is to investigate the influence of the number and type of prior revision surgeries, the time elapsed since the last surgery, and the specific joint on the diagnostic value and thresholds of synovial fluid WBC and the percentage of PMN% in diagnosing PJI following THA and TKA revisions and re-revisions.

Material and methods: In this prospective single-centre register analysis, data from 205 revised THAs and TKAs were collected, of which 100 have undergone at least one revision surgery. Diagnosis was based on the EBJIS criteria, and we identified 163 septic and 42 aseptic cases. Data on cell count and PMN% were collected and analysed for their diagnostic accuracy and optimal cutoffs.

Results: There were no significant differences in cell counts between patients after primary implantation compared to those with prior revisions for septic (p=0.40) and aseptic cases (p=0.83). The overall diagnostic accuracy was high for rerevision cases, with a sensitivity of 0.86, specificity of 0.89, AUC of 0.91, and optimal cutoff value at 2439.50G/L. As for rerevised hip joints, optimal cutoffs were higher compared to knee joints (4125.5G/L vs. 2626.5G/L, hip AUC = 0.86, knee AUC=0.93, p=0.44), and PMN% differed significantly between both groups (hip PMN% AUC=0.74, knee PMN% AUC=0.91, p=0.03) with optimal cutoffs at 87.5% and 85.5% respectively. Further, the AUCs for cell count and PMN% differed significantly depending on the type of previous surgery in rerevision (p=0.01 and 0.02).

Discussion and conclusions: Cell count and PMN% remain reliable for diagnosing PJI in patients with prior revisions, with minor threshold variations from EBJIS criteria. While the type of revision and joint involved affect accuracy, their diagnostic value remains consistently high, regardless of the time since or number of previous surgeries. In patients with previous septic revisions, a lower cutoff of %PMN to diagnose PJI was identified.