Systemic versus free antibiotic delivery in preventing acute exogenous implant related infection in a rat model.

We studied systemic ceftriaxone, and free/local tobramycin and doxycycline in a controlled rat model representing a generic acute exogenous joint infection. We hypothesized that evidence of infection (quantitative colony forming units (CFU), qualitative SEM, histopathology) (1a) would be reduced with local vs. systemic antibiotic, (1b) any antibiotic would be superior to control (2) there would be a difference among antibiotics, and (3) antibiotic would not be detectable in serum at 4-week euthanasia. Study groups included infected and non-infected (1) control (no treatment), (2) systemic ceftriaxone (daily), (3) local tobramycin, and (4) local doxycycline (10 rats/group; power =0.8). With IACUC approval, a reliable acute exogenous joint infection was created by slowly injecting 50-μl, 10x4 CFU Staphylococcus aureus, into the distal femoral medullary canal. Antibiotic formulation was introduced locally to the femoral canal and joint space. After 4-weeks, serum, pin, bone, and synovium were obtained. CFU/ml of bone and synovium were quantified using macrotiter method. SEM imaged biofilm on surface of pin, histopathology identified tissue response, liquid chromatography/mass spectrometry quantified plasma antibiotic. Results: (1) Groups receiving any antibiotic reported lower CFU/ml in synovium compared with no treatment. (2) In the synovium, free/local tobramycin reduced CFU/ml to a greater extent than free/local doxycycline (p<0.05). (3) Antibiotic in plasma after local application was nondetectable in all groups after 4 weeks. SEM revealed no difference in biofilm on pin among all groups. This article is protected by copyright. All rights reserved.