Saturday, November 12, 2016

What is the significance of positive cultures at re-implantation in a two-stage treatment of a prosthetic joint infection?

Positive Culture During Reimplantation Increases the Risk of Subsequent Failure in Two-Stage Exchange Arthroplasty

These authors retrospectively reviewed the data of 259 patients who met the Musculoskeletal Infection Society criteria for periprosthetic joint infection (PJI) and who underwent both stages of 2-stage exchange arthroplasty from 1999 to 2013.

Most spacers contained 3 g of vancomycin and 3 g of tobramycin. At the time of reimplantation, between 3 and 6 samples were obtained for culture.
Among these patients were 267 PJIs (186 knees and 81 hips); 33 (12.4%) had ≥1 positive culture result at re-implantation.

The microorganism isolated at re-implantation was frequently different from that isolated at the time of the initial infection. Furthermore, the organism isolated at the time of subsequent infection was also frequently different from that of the initial infection and the re-implantation.

Treatment failure was defined at a minimum one year follow-up as: (1) failed infection eradication, characterized by a sinus tract, drainage, pain, or infection recurrence caused by the same organism strain; (2) subsequent surgical intervention for infection after reimplantation surgery; or (3) PJI-related mortality.

 The failure rate was
21% for those with negative cultures at re-implantation
50% for those with 1 positive culture at re-implantation and
35% for those with ≥ 2 positive cultures at re-implantation

Comment: This study points out how confusing is the current state of affairs is for diagnosing and managing cases of 'periprosthetic joint infections'.  In this study, when spacers were used at the first stage of a two-stage procedure, cultures were positive at the time of the second stage in over 10% of the cases. The rate of cases meeting a definition of 'failure' was over 20%, even if the cultures were negative at the time of the second stage. Although many authors dismiss a single positive culture as a 'contaminant', one positive culture at the time of re-implantation was associated with the highest rate of failure (50%).  When there was recurrent infection, it was often with different organisms than those cultured at the prior surgery.

It is obvious that we've got a lot to learn. Consistency in culturing practices will do a lot to clarify the situation. We know that the numbers of positive cultures and the types of organisms cultured depend on the number and types of specimens submitted for culture, the media on which they are cultured, and the time these cultures are observed. We also know that recurrence of infection is difficult to define because not infrequently these recurrences are clinically subtle and often become apparent at longer than expected intervals after surgery.

Stay tuned!

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