The prognostic accuracy of three classification systems used for periprosthetic joint infection : a comparative study of the McPherson, PJI-TNM, and JS-BACH classifications

The aim of this study was to evaluate and compare the prognostic value of three classification systems for periprosthetic joint infection (PJI) - McPherson, PJI-TNM, and JS-BACH - by analyzing their association with three key clinical outcomes: recurrence of infection after the initial treatment, final PJI status, and PJI-related mortality. This was a retrospective cohort study using a prospectively maintained institutional database. All patients who underwent surgery for PJI of the hip or knee between January 2011 and December 2022 were included, provided that they had a minimum of two years of follow-up, unless they had earlier recurrence of infection or had died. Each case was classified according to the three systems and grouped into three levels of severity (A: less severe; B: intermediate; C: most severe) to allow direct comparison. Associations between the variables of the classification systems and outcomes were examined using chi-squared and Fisher's exact tests. Discriminative ability was assessed using receiver operating characteristic (ROC) curve analysis and the area under the curve was calculated for each outcome. Of the 186 patients who were included, recurrence after initial treatment occurred in 50 (26.9%) and PJI-related mortality in 19 patients (10.2%). Among the sub-categories of the classifications, host-related variables consistently correlated with most outcomes across all systems. Local extremity grade/soft-tissues also correlated with recurrence after initial treatment throughout. The JS-BACH classification was significantly associated with all three outcomes: recurrence (p = 0.002), final PJI status (p = 0.008), and mortality (p < 0.001). It also showed the best discriminative ability on ROC analysis. PJI-TNM was significantly associated with the final status (p = 0.004) and mortality (p = 0.002), but not with recurrence of infection. The McPherson classification was only significantly associated with the final PJI status (p = 0.039). Patients classified as 'limited options' in JS-BACH had a 28.6% mortality rate, while none in the 'uncomplicated' group died or required suppressive antibiotic treatment. The JS-BACH classification system showed the best prognostic performance, correlating with all the outcomes which were evaluated and offering a balanced combination of simplicity, clinical use, and predictive value. The host status remained a critical predictor regardless of which classification system was used.