Evaluation of the International Consensus Meeting (ICM) diagnostic categories for shoulder periprosthetic joint infection: Cutibacterium acnes predominates even in definite infections

Consensus-derived diagnostic criteria for shoulder periprosthetic joint infection (PJI) were established at the Second International Consensus Meeting (ICM) on Musculoskeletal Infection in 2018. Data are still limited, however, on the clinical presentation and distribution of shoulder PJI cases with regard to ICM diagnostic categories. The purpose of this study was to use data from a multicenter prospective cohort of revision shoulder arthroplasties to evaluate the frequency of cases meeting criteria for each ICM PJI category and to examine the distribution of demographic data, microbiology, and positive criteria in each category. Data were prospectively collected on consecutive revision shoulder arthroplasty cases in the American Shoulder and Elbow Surgeons (ASES) Revision Shoulder Arthroplasty and PJI Multicenter Research Group. Standardized collection was performed for patient demographics, microbiology, and major diagnostic criteria for the Definite PJI category, and minor criteria for the Probable, Possible, and Unlikely PJI categories in the ICM classification. Variations in demographic data, microbiology, and frequency of major and minor criteria were then evaluated within each PJI category for associations and differences. A total of 490 cases were evaluated, including 71 (14%) Definite PJI, 52 (11%) Probable PJI, 87 (18%) Possible PJI, and 280 (57%) Unlikely PJI. Male sex was significantly more common in the Probable PJI group (80.8%) compared with the Definite (63.3%), Possible (59.8%), or Unlikely PJI (43.2%) (P < .001). The Probable PJI group was also more likely to be younger (P = .010), on fewer narcotics (P = .014), and of lower American Society of Anesthesiologists class (P = .004) than the Definite PJI group. Gross intra-articular purulence was the most common Definite PJI criteria (65%). Across Non-Definite PJI groups, the mean ICM PJI score and mean number of positive tissue cultures were significantly higher in the Probable PJI group (P < .001) and remained statistically significant even after adjusting for amount of testing performed. Cutibacterium acnes was the most commonly cultured bacteria in both Definite PJI (25%) and Non-Definite PJI (27%) groups. C acnes was the most common bacteria present in Non-Definite PJI as well as Definite PJI cases, suggesting that classification of bacteria as "virulent" and "low virulence" may need reconsideration. Significant differences were seen in demographic characteristics, presence of minor criteria, and growth of positive cultures across Non-Definite PJI groups, even when adjusted by amount of testing performed. These findings support the usefulness of these diagnostic categories and criteria. Data from this multicenter effort will help refine ICM PJI categories and determine how they guide treatment decision making.