Assessing the TNM Classification for Periprosthetic Joint Infections of the Knee: Predictive Validity for Functional and Subjective Outcomes

<b>Background:</b> Periprosthetic joint infection (PJI) following knee arthroplasty can significantly compromise patient mobility and quality of life. The newly proposed TNM classification system, adapted from oncology, categorizes PJI severity but has not yet been correlated with both subjective and objective outcomes post PJI treatment. <b>Objective:</b> This study evaluates the applicability of the TNM classification system for predicting outcomes in knee PJI revision surgeries. <b>Methods:</b> We conducted a retrospective analysis of 108 patients who underwent revision surgeries for knee PJI at our institution from January 2012 to January 2023. We assessed the correlation between the TNM classification and postoperative outcomes using the Knee Society Score (KSS) function and knee score, as well as the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). <b>Results:</b> The TNM classification demonstrated that higher 'T' stages were significantly associated with worse functional and subjective outcomes. The 'N' classification had limited predictive value, likely due to treatment adjustments based on pathogen type. The 'M' classification correlated with functional outcomes but not with subjective scores, suggesting that patients with more severe preoperative comorbidities might adjust their expectations. <b>Conclusions:</b> While the TNM classification shows potential, its current form as a prognostic tool in PJI management is limited. Enhancing the 'T' component, coupled with the integration of a validated morbidity score such as the CCI could improve its prognostic value. Despite its shortcomings, the TNM system may still provide valuable prognostic insights for both patients and surgeons in tackling complex PJI.