A comparison of prophylactic antibiotic regimens against airborne orthopaedic wound contamination

Deep infection remains a major complication of joint replacement surgery despite advances in theatre design, surgical technique and antibiotic prophylaxis. Major randomized controlled trials to determine the most effective antibiotic prophylaxis are difficult to construct and interpret. In a conventional theatre, most orthopaedic intra-operative wound contamination arrives by the airborne route. This paper describes a unique method used to compare antibiotics against airborne bacteria. Seven antibiotics were incorporated into blood agar at concentrations equivalent to serum levels. Plates were then exposed to airborne theatre bacteria using a multiple synchronous collection technique. After incubation, the percentage kill was calculated for each antibiotic. At concentrations equivalent to serum level 1h post i.v. dose, all the antibiotics proved highly effective, with kill rates > 95%. Imipenem and co-amoxiclav significantly outperformed the other antibiotics with kill rates of 99.6% and 99.4%, respectively. At trough levels, the antibiotics achieved kill rates from 61% to 97.6%. Future randomized controlled trials comparing large numbers of antibiotics in the setting of an already low infection rate are inappropriate. This technique for comparing antibiotic prophylaxis in quick, inexpensive and repeatable. The superiority of imipenem is not unexpected, but of more interest is the effectiveness of co-amoxiclav over the presently favoured cefuroxime.