[In vitro assessment of vancomycin released from bone grafts]

Infections of the musculoskeletal system present a serious problem in orthopaedic and trauma medicine because, typically, they are often recurrent and associated with the development of resistance to antibiotics. The aim of this study was to ascertain whether the local concentration of vancomycin released from cancellous bone grafts exceeded the minimum inhibitory concentration (MIC) for vancomycin-resistant Staphylococcus aureus (VRSA ≥ 16 mg/L) during a 16-day in vitro experiment.

Morselised grafts of spongy bone were selected as ideal local carriers of antibiotic. They were impregnated with vancomycin (Edicin®). Its concentration was assessed by Agilent 1200 high performance liquid chromatography coupled with a diode array detector (Agilent Technologies, USA). Morselised bone was impregnated with vancomycin at 0.1 g antibiotic per 10 g bone, and 20 samples each weighing 1 g were prepared. They were placed in test tubes with phosphate buffer at pH = 7.4 and maintained in a thermostat at 37°C. During the 16-day period, buffer samples were taken at intervals and examined for vancomycin concentration by the above-described method.

During the whole experimental period, the level of released vancomycin was high above the MIC for VRSA. The maxi- mum average concentration was obtained between day 2 and day 4 and it reached 507.68 mg/L. At this interval the vancomycin level was stable, because there was no significant difference (p >.0.005) between the values of the 2nd and the 4th day. Then a gradual decrease in antibiotic levels was detected, with an average concentration of 332.29 mg/L recorded at 16 days.

Recently, the occurrence of methicilin-resistant Staphylococcus aureus (MRSA) infections has been increasing as well as the risk of VRSA infections, and therefore our experiment was set up to assess the releasing properties of bone grafts impregnated with vancomycin The levels of released vancomycin were much higher than the MIC for VRSA for the whole period of measurement. This finding is different from the results of an in vitro study by Witso et al., in which the vancomycin level dropped below the MIC after 2 weeks. The decrease in vancomycin levels following its maximum values was greater than it had been expected although the samples were diluted only minimally..There are several explanations for this finding. However, from the clinical point of view it is important that, for a sufficiently long period, vancomycin is maintained at a level exceeding the MIC for VRSA.

In an in vitro experiment under conditions simulating a human internal environment, the elution of antibiotic from vancomycin-impregnated cancellous bone grafts was investigated. The local vancomycin concentrations much exceeded the MIC for VRSA for more than 2 weeks. The highest levels, i.e. the total vancomycin amount, were recorded at 2 to 4 days after carrier application. Based on the experimental results, vancomycin-loaded bone grafts can be recommended for local treatment of the musculoskeletal system infected with antibiotic-sensitive staphylococci, MRSA strains or possibly also for VRSA infections.