Impact and Modification of the New PJI-TNM Classification for Periprosthetic Joint Infections

Abstract

The comprehensive "PJI-TNM classification" for the description of periprosthetic joint infections (PJI) was introduced in 2020. Its structure is based on the well-known oncological TNM classification to appreciate the complexity, severity, and diversity of PJIs. The main goal of this study is to implement the new PJI-TNM classification into the clinical setting to determine its therapeutic and prognostic value and suggest modifications to further improve the classification for clinical routine use. A retrospective cohort study was conducted at our institution between 2017 and 2020. A total of 80 consecutive patients treated with a two-stage revision for periprosthetic knee joint infection were included. We retrospectively assessed correlations between patients' preoperative PJI-TNM classification and their therapy and outcome and identified several statistically significant correlations for both classifications, the original and our modified version. We have demonstrated that both classifications provide reliable predictions already at the time of diagnosis regarding the invasiveness of surgery (duration of surgery, blood and bone loss during surgery), likelihood of reimplantation, and patient mortality during the first 12 months after diagnosis. Orthopedic surgeons can use the classification system preoperatively as an objective and comprehensive tool for therapeutic decisions and patient information (informed consent). In the future, comparisons between different treatment options for truly similar preoperative baseline situations can be obtained for the first time. Clinicians and researchers should be familiar with the new PJI-TNM classification and start implementing it into their routine practice. Our adjusted and simplified version ("PJI-pTNM") might be a more convenient alternative for the clinical setting.

Keywords: PJI classification; TNM; antibiotic-loaded bone cement spacer; periprosthetic joint infection; revision arthroplasty; septic surgery; total knee arthroplasty; two-stage revision.

Figure 1

Type of infected prosthesis at time of diagnosis. The “p-status” distinguishes between: p0 = standard implant, p1 = revision implant, and p2 = megaprosthesis.

Figure 2

Preoperative periarticular soft tissue condition. The new “T-status” distinguishes between: T0 = good soft tissue condition, T1 = bad soft tissue condition, and T2 = periarticular fistula or significant soft tissue defect requiring flap/plastic surgery.

Figure 3

The modified PJI-pTNM classification. Type of infected prosthesis “p-status” is added to the TNM-backbone, making it “pTNM”. An “x” in front of the p-status indicates if a loosened implant is assumed or confirmed. Morbidity is assessed based on the ASA classification. The number behind each letter indicates uniformly the degree of severity on a scale from 0 to 2, with 2 being most severe. If the current PJI involves a previously infected joint, the situation is considered as “reinfection” and an “r” is put in front of pTNM.

Figure 4

Distribution of subtypes according to the modified PJI-pTNM classification in our patient cohort.

Figure 5

This patient was diagnosed with a chronic PJI of a megaprosthesis (“p2”) with periarticular soft tissues in a good clinical condition (“T0”). The patient underwent two-stage revision surgery with successful reimplantation of a new megaprosthesis at second-stage surgery.

Figure 6

This patient was diagnosed with a chronic PJI of his total femur prosthesis (“p2”) and underwent first-stage surgery with implantation of a custom-made double-level spacer (hip and knee spacer of the same leg). Because of the patient’s high perioperative risk (“M2”) and low physical demand, a shared decision was made to not perform second-stage surgery.