Can the oncology classification system be used for prosthetic joint infection?

A major problem in prosthetic joint infection (PJI) is the lack of a generally used classification system although several different systems have been proposed in the literature.^1,2 The most frequently used classification of PJIs with early, delayed, and chronic infection proposed by Schafroth et al³ only focusses on the time of onset of infection symptoms. However, PJIs are multifactorial complications after arthroplasty and involve different parameters, such as the host, the implant with the surrounding soft tissue and bone, as well as the causative microorganisms, which are of relevance for the treatment strategy and final outcome for the patient.^4,5 Therefore, these parameters should be taken into account in a classification concept for PJIs. McPherson et al⁶ came up with a proposal for a staging system for PJIs that includes three categories: infection type (acute versus chronic), systemic host grade, and local extremity grade with significant correlation between the staging system and outcome parameters, particularly the relation of complication rates with worsening medical condition and a worsening local wound. This classification does not address the causative agent and the underlying implant, which both have an impact on the treatment decision process and outcome of PJIs. In conclusion, there is a strong need for an improvement in the classification of PJIs and the current editorial is intended to present a new idea for a new classification system for PJIs.

In general, new classification systems should allow professionals to derive treatment guidelines and prognosis for the disease. Another important aspect is that they should not be too complicated as this could prevent their use in clinical practice. On the other side, the use of established principles of other classification systems could help to enhance acceptance of a new classification.

One of the most widely and most successfully used classification systems in medicine is the TNM classification for malignant tumours in oncology. The basics of this system were developed in the late 1940s and early 1950s by Pierre Denoix and its eighth version was recently published in 2017,⁷ which is used globally for the majority of malignant tumours. The principle of the TNM system is based on details of the primary tumour (T), regional lymph nodes (N), and distant metastasis (M). For each item, further details regarding the size and/or extent of the local tumour (T0 to T4), involvement of regional lymph nodes (N0 to N3), and differentiation on distant metastasis (M0 to M1) are used to guide treatment. Furthermore, the TNM system provides information for survival and helps to compare treatment outcomes from a clinical and scientific perspective. All these aspects are also of relevance for PJIs and the frequently used sentence “treat the infection like a tumour” describes certain similarities between tumours and PJIs, in particular the need for resection of affected tissue. These aspects have prompted the authors of this editorial to present the idea for a TNM classification for PJIs.

The above-mentioned crucial parameters for PJIs: type of implant including surrounding soft tissue conditions, the causative microorganism, and the host were taken into account and transferred to the three significant letters T, N, and M, which are the keystones of the TNM classification system in oncology and which are also used for the PJI-TNM classification. ‘T’ represents the local situation of the tissue and the indwelling implant, ‘N’ stands for the causative non-human bacterial and/or fungal organisms, and ‘M’ for the morbidity of the patient for this new PJI-TNM classification. As with TNM in oncology, each item is further specified by numbers with increasing severity of the respective item, from 0 to 2 for T and N and from 0 to 3 for M. This is detailed in the accompanying infographic in this issue of Bone & Joint Research.⁸

Regarding the infected joint of the PJI, the name of the affected joint is put in front of the TNM letters in order to clearly state the affected body region, such as hip, knee, and shoulder. If it is a recurrence of infection, the letter ‘r’ is put additionally in front of the affected joint in order to emphasize reinfection.