Friday, February 9, 2018

Periprosthetic infection simplified and the POOI

The clearest definition of a bacterial infection is "bacteria causing harm".

Consider three scenarios:
(1) The normal gut is not infected: there is no harm (diarrhea) even though bacteria (E. Coli) are present.
(2) Shell fish allergy is not an infection: there is harm (diarrhea) but it is not related to the presence of bacteria.
(3) Pseudomembranous enterocolitis is an infection: there is harm (diarrhea) related to the presence of bacteria (Clostridium difficile)

The importance of defining "infection" is that in such cases treatment needs to be directed at the bacteria. In considering a failed arthroplasty, we define an infection in the same way: "bacteria causing harm" and for the same reason: treatment needs to be directed at the bacteria.

The issue with treating patient with a failed arthroplasty is that before the revision surgery we usually do not know whether bacteria are causing the harm (need for revision). Thus our treatment must be based on our estimation of the POOI (preoperative odds of infection); the POOI is never zero and never 100% - it is always intermediate.

At revision surgery for a failed athroplasty we have two types of decisions that must be based largely on the POOI: (1) the need for prosthesis exchange and (2) the type of antibiotic therapy. The higher the POOI the more aggressive we are likely to be with each of these decisions. However, even when the POOI is relatively low, we often elect a complete prosthesis exchange and vigorous initial antibiotic therapy because we want to avoid the situation where intraoperative cultures come back positive after an inadequate surgery and inadequate initial antibiotic management.

Our estimation of the POOI is influenced by patient characteristics (sex, age, general health), history (prior surgeries, injections, progression of symptoms), physical findings (draining sinus, erythema), lab tests (elevated serum ESR or CRP, abnormal joint fluid cell counts or cytokines), and imaging (prosthetic loosening, bone destruction).

At surgery we have the opportunity to determine if bacteria are associated with the arthroplasty failure by carefully taking multiple specimens and submitting them for a culturing protocol appropriate for the bacteria most likely to be present (specifically Priopionibacterium for the shoulder). With these culture results in hand, we can modify the antibiotic regimen implemented at the time of surgery.