These authors sought to compare the sensitivity and specificity of sterile shoulder needle aspiration in comparison to arthroscopic and mini-open obtained cultures for detecting periprosthetic shoulder infections, using tissue cultures of revision surgery as the reference standard.
Between 2012 and 2018 joint aspiration was used to evaluate possibly infected shoulders (i.e. those with unexplained pain and stiffness after arthroplasty). Sterile punctures were performed under fluoroscopic control. Only patients in which fluid could be aspirated were included in the study (i.e. dry taps were excluded).
In 2018 arthroscopic and mini-open culture acquisition were used when a periprosthetic joint infection was suspected because of unexplained shoulder pain and/or stiffness. The choice of mini-open or arthroscopic tissue cultures depended on the type of the prosthesis in situ: arthroscopic tissue cultures were commonly obtained in case of a hemi or total shoulder prosthesis, whereas most mini-open biopsies were obtained in case of a reverse total shoulder prosthesis. Six separate sterile instruments were used to obtain the six samples for arthroscopic and mini open biopsies.
During revision surgery, prophylactic antibiotics were withheld until cultures were obtained. Six tissue cultures were taken from different locations at the interface with the humerus and glenoid component using six separate sterile rongeurs were used.
Cultures were observed for 14 days. A low-grade infection was diagnosed when at least two cultures with the same low virulence organism were positive.
The most commonly recovered organism was Cutibacterium followed by coagulase-negative staphylococcus.
Comment: The bottom line of this important study is that a substantially positive culture on joint aspiration is a helpful indication of a periprosthetic infection, but a negative aspirate culture does not prove the absence of infection ("absence of evidence is not evidence of absence"). One might ask, why are tissue samples more likely to be culture positive in cases of periprosthetic infections than a fluid sample? There are at least two possible explanations. First, the aspirate is only one sample, whereas these surgeons thoughtfully obtained 6 separate tissue samples at arthroscopic, mini-open and revision surgery - thus the odds of a positive culture are greater with a higher number of samples. Secondly, a positive fluid aspirate depends on the bacteria being in planktonic form (i.e. suspended in the joint fluid); however, Cutibacterium in particular has a tendency to become embedded in tissue and in biofilms on implants rather than being freely swimming around in joint fluid.
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