Background: Prophylactic systemic antibiotics significantly lower the risk of postoperative infection, and injection of antibiotics directly into the wound cavity has been found to be even more effective. In this study, we investigated the efficacy of direct injection of antibiotics into a wound cavity after wound closure, both alone and in combination with systemic administration of antibiotics. We hypothesized that a combination of preoperative systemic administration and postoperative local injection would be the most effective treatment.
Methods: Rats were divided into six treatment groups: no treatment, local gentamicin, systemic cefazolin, local cefazolin, systemic cefazolin plus local gentamicin, and systemic cefazolin plus local cefazolin. A wound cavity was opened along the femur, an implant was placed, and the wound was inoculated with 2.5 × 108 colony forming units of Staphylococcus aureus. Systemic antibiotics were injected subcutaneously thirty minutes before the initial incision. Local antibiotics were injected percutaneously into the wound cavity after closure. The rats were killed at forty-eight hours postoperatively, and quantitative cultures were performed.
Results: All groups that received antibiotics showed significantly lower bacterial counts than the no-treatment control group (p < 0.0003). Local gentamicin treatment decreased the number of colony-forming-unit isolates by approximately two orders of magnitude as compared with the number in the group treated with systemic cefazolin (p = 0.00005) and five orders of magnitude as compared with the number in the control group (p = 0.00003). The combination of systemic cefazolin and local gentamicin decreased the bacterial count by approximately seven orders of magnitude as compared with the count in the no-treatment control group and significantly decreased the count as compared with that in the group treated with local gentamicin alone (p = 0.00006).
Conclusions: As we hypothesized, the combination of systemic cefazolin and local gentamicin proved to be the most effective regimen. Local injection of gentamicin proved more effective than systemic administration of cefazolin but was not as effective as the combination of both antibiotics. The initially high concentrations of locally applied antibiotic and the utilization of two different classes of antibiotics may have contributed to the observed efficacy.
Clinical Relevance: If our findings are supported by those in clinical trials, the combination of local gentamicin and systemic cefazolin could prove valuable as a regimen for prophylaxis against surgical wound infection.