Introduction: A major complication associated with external fixation is pin tract infection¹. This can occur in as many as 17–57%² of cases and in severe cases leads to premature removal of the fixator. An antimicrobial coated (AMC) sleeve has been designed to be placed over external fixation pin and wires that delivers an antibiotic, gentamicin, directly into the pin tract. The function of the sleeve is to inhibit bacterial colonisation of the pins and wires, the first step in the development of a clinical infection. This study reports the in vitro testing carried out to establish the effectiveness of the AMC sleeve.

Methods: The prevalence of gentamicin susceptibility amongst bacteria typically associated with pin tract infections was determined by comparing minimum inhibitory concentrations (MIC) of clinical isolates from the SENTRY Antimicrobial Surveillance Programme (1997–2002) to the NCCLS susceptibility breakpoint of < 4 μg/ml. The amount of gentamicin released over time from AMC sleeves into phosphate buffered saline (PBS) was measured using a microbiological zone of inhibition assay against S. epidermidis (NCTC 8853). Three 5 cm long sleeves, fitted over 6 mm diameter pins, were agitated in 5 ml of PBS eluant at 37°C. The eluant was replaced and tested at 2, 24, 48, 72 hours, then weekly until 26 weeks. Concentrations of gentamicin in the pin tract were calculated from these values using an estimated pin-tract volume. The ability of the sleeves to kill bacteria was measured by inoculating single 5 cm long, 5 mm diameter sleeves on pins with 1.5 ml of bacterial suspensions containing approx. 1 x 10⁸ cfu/ml. Surviving numbers of bacteria were counted after contact with the sleeves for 0.5, 1, 2 & 4 hours at 37°C. Effectiveness against clinical isolates of E. coli, S. aureus, S. epidermidis & Ps. aeruginosa was measured.

Results: The SENTRY database showed that of the 1456 individual surgical wound isolates gathered and evaluated, 1210 (83.1%) were found to be susceptible to gentamicin. Estimated concentrations of gentamicin in the pin tract reached 43.3 μg/ml at the end of the first week and exceeded the susceptibility threshold of 4 μg/ml over the next 19 weeks. The sleeves were able to reduce inoculum cell numbers of all organisms tested by 5 logs (99.9999% reduction) in ≤4h.

Discussion and Conclusion: Surveillance data confirms that gentamicin provides high level efficacy against pathogens commonly associated with pin tract infections. The AMC sleeves release gentamicin directly into the pin tract at concentrations above the susceptibility threshold for most clinically-encountered bacteria. These sleeves are also able to reduce significantly bacterial cell numbers when directly in contact with them. Therefore, this study demonstrates that the sleeves will inhibit bacterial colonisation of external fixation pins and emphasises their contribution to reducing the effects of pin tract infection.