DEVELOPMENT OF AN IN VIVO POLYMICROBIAL FRACTURE-RELATED INFECTION MODEL AND EVALUATION OF THE EFFICACY OF A TOBRAMYCIN-LOADED HYDROGEL AS PROPHYLAXIS

Aim Fracture-related infections (FRI) are known to be a devastating complication after fracture fixation. Up to one-third of these infections are polymicrobial and associated with a higher risk for treatment failure, with Staphylococcus aureus (SA) and Pseudomonas aeruginosa (PA) amongst the most common pathogens. This study aims to investigate the in vitro interactions between PA and SA and to establish a preclinical rabbit model for polymicrobial FRI following plate osteosynthesis. We also assess the effectiveness of a tobramycin-loaded hydrogel as a preventive strategy against FRI in this in vivo model. Method In vitro competition assays were performed between clinical isolates of PA ( n =3) and SA ( n =5). Bacterial survival of either PA or SA was quantified after 24h of co-culture. A combination of 2 clinical strains, named PAV66 and JAR, was chosen for the in vivo experiments. A standardized osteotomy was created in the humerus of female New Zealand White rabbits, fixed by a LCP plate and inoculated with 10 3 CFU/animal of PA and 10 4 CFU/animal of SA, in a total of n=7 animals. The samples were collected and processed for bacteriological quantification after an observation period of 7 days, mimicking an acute infection scenario. In the treatment group ( n =3 rabbits), the tobramycin-loaded hydrogel was delivered via a catheter in the surgical situ after bacterial inoculation. The same observation period and bacteriological quantification process was applied. All animals received a standard dose of Cefuroxime as perioperative antibiotic prophylaxis. Results In vitro , all 15 competition assays showed that PA outcompeted SA after 24 hours of co-culture, resulting in a reduction of SA by 1 to 6 logs. In vivo , a similar trend was observed with a 1-log reduction for SA compared to PA (median value 10 5 CFU/animal for SA and 10 6 for PA). In the tobramycin-loaded hydrogel group, all collected samples (100%) were culture-negative, confirming successful prophylaxis treatment through the intraoperative application of the antibiotic-loaded hydrogel, compared to a 75% infection rate in the control group with the same inoculum. Conclusions We developed a polymicrobial infection model to create a clinically relevant approach in the context of FRI and further advance prophylaxis and treatment strategies. Additionally, our preliminary results demonstrated the effectiveness of the tobramycin-loaded hydrogel as a preventive measure against polymicrobial FRI.