The epidemiology of shoulder PJI has been thor­oughly investigated, with data collected from various sources including observational studies and meta-analyses. In a recent systematic meta­analysis using 105 articles, the pooled PJI inci­dence following TSA was 0.61% (0.34–0.93) over a follow-up period of 1.1years [32]. The authors also found that the PJI incidence was higher following inverse TSA compared with ana­tomical TSA.In an arthroplasty register analysis, the authors described a 1.4% (95% CI 1.2 to 1.6) ten-year cumulative revision rate due to infection [38]. The adjusted relative risk of revision surgery due to infection was about 2.5 times higher for inverse TSA compared to anatomical TSA.Shah and colleagues could demonstrate a 2.4% shoul­der PJI rate at a mean follow-up of 4.3years for primary inverse TSA [61]. There was also a ten­dency for increased infection rates in the revision surgery groups compared to the primary arthro­plasty groups, i.e., in line with other studies [67,

components, radiolucent lines, osteolysis, endos­tal scalloping, and new bone formation [11, 13,

Aspiration of synovial uid is mandatory if a dis­tinct uid collection is identied, however, “dry taps” are often observed in Cutibacterium acnes infections [6, 7]. Due to the lower virulence of Cutibacterium acnes, the threshold for an increased number of leukocytes in the context of a shoulder, PJI remains unknown and may be lower than accepted values for other PJIs. Consequently, the currently accepted leukocyte count thresholds of >1100–3000 cells/ml with a polymorphonu­clear neutrophils (PMN) differentiation of >80% for chronic hip and knee PJI are unlikely to be appropriate for the diagnosis of shoulder PJI, as the bacterial pathogens in the shoulder usually produce a less severe inammatory response [18].

The uniqueness of shoulder PJI lies in the under­lying processes, which are primarily due to the specic microorganisms that colonize the shoul­der. The predominant pathogens of surgical site infections (SSI) and shoulder PJI surgery are usu­ally coagulase-negative Staphylococcus species, Cutibacterium acnes and Staphylococcus aureus [24, 31, 56, 63]. Cutibacterium acnes, in particu­lar, has recently emerged as the predominant cause of PJI in the shoulder [18]. However, Cutibacterium acnes infection can be particularly challenging because of its slow and gradual pro­gression, and conrming positive cultures often necessitates longer incubation periods for sam­ples [10]. A potential disadvantage associated with the increased sensitivity of prolonged cul­tures for Cutibacterium acnes is the fact that this may also carry an increased risk of contamina­tion and false-positive results [16]. Unexpected positive cultures (UPCs) without clinical or radiographic signs of infections are a dilemma. However, the exact denition and management of UPCs in the context of shoulder arthroplasty revisions remains unclear, as does the role of Cutibacterium acnes [18]. Cutibacterium acnes is the predominant microorganism identied in most positive UPC cases, however, it remains uncertain whether the signicance of UPC caused by one bacterium differs from that of UPC with another bacterium. Providing semi-quantitative and quantitative reports of bacterial culture

when infection is conrmed within a 30-day win­dow of symptom onset [7, 20]. However, it is noteworthy that this strategy is associated with a reported failure rate of 42% to 63% and less favourable functional outcomes [7, 19]. Given these results, the surgeon must carefully weigh the potential for recurrent infection against the morbidity associated with implant removal.