Michael Khazzam, M.D.
The most common bacterial infection to occur after revision shoulder arthroplasty surgery can be diagnosed more accurately by considering how quickly samples of the microbe grow in hospital labs and the level of bacteria that grows, a study partly performed at UT Southwestern shows.
The findings, reported in the Journal of Shoulder and Elbow Surgery, could lead to better postoperative care for thousands of patients who require follow-up surgery every year for shoulder replacements, known as shoulder arthroplasty revisions.
“Knowing which bacterial cultures are truly positive and which ones are not guides a surgeon on what treatments a patient should receive,” said study co-author Michael Khazzam, M.D., Associate Professor of Orthopaedic Surgery and Chief of Shoulder Surgery at UT Southwestern. “Our study suggests that clinicians should consider using a shorter time to positivity and strength of positivity in determining whether a tissue culture sample is a true positive in the setting of shoulder arthroplasty revisions.”
A postoperative radiograph shows a revision reverse total shoulder arthroplasty..
Cutibacterium acnes infections after shoulder arthroplasty revisions can be tricky to diagnose, Dr. Khazzam explained. Although some patients show obvious signs of infection, such as draining wounds at the surgical site, others have more subtle signs, such as pain or stiffness. This bacterial species can also be difficult to grow in the lab, requiring specialized media and prolonged growing times – which can result in confusion over whether positive or negative diagnoses are genuine.
To improve interpretation of C. acnes diagnoses, Dr. Khazzam and colleagues from 11 U.S. hospitals each were sent 12 blind samples to test for the presence of the bacterium. Ten were certain positives, with five samples apiece isolated from two patients who underwent shoulder arthroscopy revisions and diluted to different bacterial concentrations. Two were certain negatives.
The researchers found that all of the positive samples at the four highest concentrations tested positive in the lab, and about 91% of the lowest concentration also tested positive. In addition, about 14% of the negative samples tested positive for C. acnes.
When the researchers looked for factors to distinguish true positives from false positives, they found that the mean time to a positive result for all the known positive samples was about four days, while the time for a positive result from the known negative samples was about twice as long. In addition, the strength of positivity – a result associated with the number of bacteria growing in the lab – was significantly higher for the true positive samples than for the false positives derived from known negative samples.
Dr. Khazzam and his colleagues suggest that surgeons keep these factors in mind when deciding whether to treat patients for suspected C. acnes infections after shoulder arthroplasty revisions.
“The shoulder service at UTSW performs some of the highest volume of primary and revision shoulder arthroplasty in the country,” Dr. Khazzam said. “The results of this multicenter collaboration begin the framework of developing an evidence-based decision-making protocol for the treatment of these oftentimes difficult shoulder problems.”