Some shoulder arthroplasties are unexpectedly painful or stiff; some of these turn out to have periprosthetic infections (PJI). The most common organism causing shoulder PJI is Cutibacterium - an organism that typically does not cause systemic signs of infection or elevation of laboratory markers indicating inflammation. Attempted aspiration of fluid for culture and cell count from these joints is often unhelpful in diagnosing or excluding the diagnosis of PJI. The most reliable method for diagnosing shoulder PJI is obtaining five tissue or explant specimens for Cutibacterium-specific culturing. Obtaining these specimens requires either arthroscopic or open surgical biopsy.

The authors of Diagnostic Performance of Advanced Metal Artifact Reduction MRI for Periprosthetic Shoulder Infection sought to determine the reliability, repeatability, and diagnostic performance of advanced metal artifact reduction MRI (MARS-MRI) in diagnosing PJI.

89 patients suspected of having PJI underwent standardized clinical, radiographic, and laboratory evaluations and advanced MARS-MRI and had at least 1-year clinical follow-up.

When applying the International Consensus Meeting 2018 criteria, 22 of the 89 participants were deemed as being infected.

The inter-observer rreliability and intra-observer repeatability were good (k = 0.61 to 0.80) to excellent (k > 0.80) for advanced MARS-MRI findings, including lymphadenopathy

joint effusion

rotator cuff muscle edema

synovitis, extra-articular fluid collection, a sinus tract, and periprosthetic bone resorption.

Lymphadenopathy, complex joint effusion, and edematous synovitis had sensitivities of >85%, specificities of >90%, odds ratios of >3.6, and AUC values of >0.90 for diagnosing PJI. The presence of all 3 findings together yielded a PJI probability of >99%.

Comment: This study suggests that MARS-MRI may be a useful tool for evaluating shoulders suspected of having a periprosthetic infection. It is of note, however, that in the PJI group many of the preoperative observations strongly suggested infection as shown below

In a painful shoulder with elevated ESR, CRP, fluid aspirates positive for culture or elevated WBC count, and prosthetic loosening, surgical revision would seem indicated without needing to proceed with MARS-MRI. It is reasonable to ask what the sensitivity and specificity of MARS-MRI is for PJI in patients without these findings previously shown to be strongly suggestive of infection (see for example Prognostic factors for bacterial cultures positive for Propionibacterium acnes and other organisms in a large series of revision shoulder arthroplasties performed for stiffness, pain, or loosening).

Only 42 of the 89 patients had revision surgery; operative findings and intraopertive culture results were not available for the other 47. It is of interest that 14 of the 42 operated patients had intra operative cultures that were positive for Cutibacterium; while 10 of these were placed in the "infected" group, 4 were placed in the "non-infected" group. Many surgeons would find that shoulders with positive deep cultures would merit treatment for PJI.

Some cases of PJI have clinically obvious clinical presentation clearly leading to treatment of the infection; others have a stealth presentation in which cases the decision to treat infection is difficult. As emphasized by the authors of Characterizing the Propionibacterium Load in Revision Shoulder Arthroplasty the real challenge lies in diagnosing the cases of PJI having a stealth presentation. Further clinical research is necessary to determine whether MARS-MRI is of value in evaluating painful shoulder arthroplasties for which the diagnosis of PJI is not otherwise obvious.

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