Samer S. Hasan,Eric T. Ricchetti

Propionibacterium acnes represents the most common pathogen in periprosthetic joint infections of the shoulder, accounting for up to 70% of positive culture results at the time of revision surgery. This diagnosis can be elusive for several reasons. P. acnes is a commensal organism of low virulence that populates the dermal sebaceous glands about the shoulder. Its clinical presentation is often insidious and delayed and is frequently characterized clinically by persistent shoulder pain and a paucity of cutaneous, systemic, or radiographic signs. Furthermore, the preoperative work-up has been characterized historically by poor sensitivity. Acute-phase serologic markers such as the erythrocyte sedimentation rate and C-reactive protein level are often normal in P. acnes infections and the organism can be difficult to culture, requiring long-duration culture holds in multiple culture media for optimal culture yield. The usefulness of intraoperative frozen sections for aiding in the diagnosis of periprosthetic joint infections of the shoulder varies from one pathology laboratory to another, but has been inconsistent at best.

Because of the low virulence of P. acnes and the associated diagnostic delay and uncertainty, surgeons are often confronted with difficult treatment dilemmas at the time of revision surgery, such as determining whether or not a revision should be carried out in a staged fashion until a periprosthetic joint infection can be effectively ruled out. In addition, positive cultures at the time of revision surgery may represent a true periprosthetic joint infection but may also arise unexpectedly as a contaminant during specimen procurement and culture incubation.

Mook et al. set out to determine the rate of positive culture growth from deep-tissue specimens obtained from patients undergoing open surgery of the glenohumeral joint as well as the demographic correlates of the culture results. One of the study strengths is the availability of a control group comprising cultures of sterile gauze sponge specimens. The authors found that male sex strongly correlated with positive P. acnes cultures, which is in keeping with previous studies. The authors also found that two or more previous intra-articular corticosteroid injections correlated with positive P. acnes cultures, which is compelling, given the frequency with which these are performed prior to the consideration of prosthetic shoulder arthroplasty. However, the authors also acknowledged that post hoc analysis revealed that their study was underpowered to test this association.