Periprosthetic infections - why is joint aspiration insensitive, how much better is tissue biopsy?

Better diagnostic value of mini-open and arthroscopically obtained tissue cultures compared to sterile punctures to identify periprosthetic shoulder infections: a retrospective cohort study

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.

As can be seen from the table below, while the aspirate cultures were specific (91%), they were insensitive (20%). By contrast arthroscopic and mini-open cultures were just as specific (86%) specific, but three times as sensitive (60%).

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. 

As in considering all tests for infection, the surgeon needs to decide how each of the possible outcomes will influence treatment. So in the case of a fluid aspirate, a negative culture result would have no effect on treatment, whereas a negative result from six tissue biopsies suggest that a periprosthetic infection is less likely. However, as shown clearly in this study, neither a negative arthroscopic or mini open set of samples can exclude the possibility of infection - in fact one third of the shoulders with negative preoperative tissue biopsies were found to have a periprosthetic infection at revision surgery.

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Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link)

Shoulder arthritis - x-ray appearance (see this link)

The smooth and move for irreparable cuff tears (see this link)

The total shoulder arthroplasty (see this link).

The ream and run technique is shown in this link.

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).

Shoulder rehabilitation exercises (see this link).

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Can joint aspiration rule out an infection?

The accuracy of joint aspiration for the diagnosis of shoulder infections

These authors performed 106 aspirations with fluoroscopic guidance between 0 and 179 days before a revision surgery. Native joints and post–fracture repair, post-arthroscopy, and post-arthroplasty shoulders were included.

They were able to sample intra-articular liquid directly in 60 cases and after saline solution injection in 43 cases, whereas 3 cases remained unsuccessful.

According to intraoperative samples, 24 shoulders were infected but only 10 of 24 (42%) yielded pathogens in the aspirate. 

Among the 43 saline solution–enforced irrigations, none revealed bacteria but 8 (17%) had confirmed infection based on intraoperative samples. 

The most common pathogens were Cutibacterium acnes (50%) and Staphylococcus aureus (33%).

Altogether, aspiration missed 16 of 24 confirmed infections (67%): 8 in the successful aspiration group and 8 after saline solution irrigation.

The missed microorganisms were  all gram positived C acnes (n . 10), S aureus (n . 5), and Staphylococcus warneri (n . 1) and were equally distributed between successful aspirations and dry tap cases.

Comment: As these authors point out, presurgical shoulder joint aspiration is not reliable in excluding shoulder joint infection. Nevertheless, a positive aspiration finding can guide clinical decision making. As we try to explain the utility of an aspiration, we compare it to a needle biopsy: a positive result can be used to diagnose cancer, but a negative biopsy cannot exclude the diagnosis.

The results of this study do not support the use of irrigation as a means of gathering information from shoulders without joint fluid.

Finally, aspiration may be of use in cases where infection is strongly suspected in hopes of identifying the organism so that specific antibiotic therapy can be instituted immediately after surgery.

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