Abstract
Background
Previous data from our institution show that more than half of all prosthetic joint infections are due to S. aureus. A significant proportion of these bacteria may have an endogenous source. Detecting and treating asymptomatic S. aureus nasal carriers preoperatively has been shown to reduce the risk of infection.
Material and Methods
This is an ongoing prospective study that started in March/2009 and involves primary total knee or hip arthroplasties candidates. So far preoperative nasal swab cultures were performed in 211(61%) out of 347 patients operated until April/2010. Carriers are identified and randomly chosen for preoperative treatment consisting of nasal mupirocin twice a day and daily cloro-hexidine baths in the 5 days that precede surgery. Antibiotic prophylaxis is cefazolin 24hours adding a single vancomycin dose in MRSA carriers.
Results
Around 30% (64/211) of patients presented Saureus nasal carriage and 6 of them (9.4%) were methicilin-resistant. Infection rate was lower in the non-carriers: 1.36% (2/147). The group of 33 untreated carriers so far has the higher prevalence of infection (6.06%; Odds ratio=4.677; p=0.098). In the 31 treated carriers there was only one infection to date (3.23%; Odds ratio=2.371; p=0.463). The 136 patients control group in which no preoperative nasal culture was made registered 4 infections (2.94%; Odds ratio=2.197; p=0.356).
Five out of the nine infections involved S. aureus. We further isolated 3 S. epidermidis, 1 enterococcus and 2 gram-negatives.
Discussion
It seems that two S. aureus infections were due to endogenous contamination as the infecting bacteria possesses the exact same phenotype as the colonizing preoperative one (one untreated and one treated carrier). In other two S. aureus cases the seeding appears exogenous (a preoperative negative culture and a infecting bacteria different than the preoperative screening). One infection was registered in a non-studied patient. Accordingly, exogenous contamination seems responsible for at least 6 out of 9 infections.
Conclusion
These preliminary results suggest that S. aureus colonization may be an endogenous risk factor for prosthetic joint infection that can be influenced. However exogenous contamination is still a major risk factor in our institution.
