Culturing Explants for Cutibacterium at Revision Shoulder Arthroplasty: An Analysis of Explant and Tissue Samples at Corresponding Anatomic Sites

Making the diagnosis of a Cutibacterium periprosthetic shoulder infection is difficult because the patient is typically not ill, the peripheral blood typically does not show WBC, C-reactive protein or sedimentation rate evidence of inflammation and because histology on frozen section is often unremarkable (see this link for an example). Thus the diagnosis depends on the results of cultures of multiple deep specimens obtained at revision arthroplasty, submitting these specimens for aerobic, anaerobic and broth cultures and observing the cultures for 2 to 3 weeks.

While submitting deep tissue samples tissue is commonplace, the value of culturing explanted components has not been well-described. Culturing explants may enhance detection of Cutibacterium in revised shoulders because these bacteria tend to become sequestered in biofilms on the surface of prosthetic components and to be less prevalent in samples of tissue or fluid.

This study sought to answer the following questions:

1) How does the culture positivity of explant cultures compare to that of deep tissue cultures?

2) How often are explant cultures positive when tissue cultures are not, and vice versa?

3) How does the bacterial density in explant cultures compare to that in tissue cultures?

The authors reviewed 106 anatomic arthroplasties revised over a 7-year period.

Explant (humeral head, humeral stem, glenoid) and tissue (collar membrane, humeral canal tissue, periglenoid tissue) specimens were sent for semiquantitative Cutibacterium culture.

Tissue samples were placed in a stomacher with saline, and the saline was streaked onto three different anaerobic and aerobic media and observed for 21 days.

Explanted components were vortexed with saline, and the saline was streaked in a similar fashion.

The authors compared culture positivity and bacterial density when cultures of an explant and tissue adjacent to the implant were both available.

They found that explants had positive cultures at a higher rate than the adjacent tissue specimens for most anatomic sites.

Of the shoulders that had Cutibacterium growth, a higher proportion of explants were culture positive when tissue samples were negative (23-43%) than vice versa (0-21%).

The load of Cutibacterium was higher in explants than in tissues.

Inclusion of explant samples almost doubled the number of revised shoulders meeting the author's criteria for treatment for Cutibacterium periprosthetic infection. Considering only the results of tissue samples, 16% of the shoulders met this threshold (≥2 positive cultures); however, with the inclusion of the results for explant cultures an additional 14% of cases –for a total of 30% - met the criteria for infection treatment.

They concluded that in this group of patients, culturing explants in addition to deep tissue samples increased the sensitivity for detecting Cutibacterium in revision shoulder arthroplasty.

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