Vacuum-Mixing Significantly Changes Antibiotic Elution Characteristics of Commercially Available Antibiotic-Impregnated Bone Cements

Background: Evidence-based medicine indicates the use of antibiotic-impregnated polymethylmethacrylate bone cement during hip and knee replacement reduces the rate of prosthetic joint infection. In the United States, so-called off-label use of antibiotic-impregnated polymethylmethacrylate for primary joint replacement is increasing and multiple antibiotic-containing polymethylmethacrylate products are commercially available. However, there are sparse published data comparing the antibiotic elution characteristics of these bone cement products and the effect that vacuum-mixing has on antibiotic elution from these products. This study compares the antibiotic elution characteristics of six commercially available antibiotic polymethylmethacrylate formulations mixed under atmospheric pressure and vacuum conditions. Methods: The antibiotic-impregnated polymethylmethacrylate products were mixed with use of a commonly employed intraoperative technique at atmospheric pressure and clinically relevant vacuum conditions. A standard Kirby-Bauer bioassay technique was subsequently used to quantify antibiotic elution from the products. An international infectious disease database was mined to determine antibiotic susceptibility of common bacteria causing prosthetic joint infection and to define the gentamicin concentration above which optimal antibiotic efficacy begins for these organisms. Statistical analyses incorporating the above susceptibility data were performed to compare antibiotic elution (1) among products mixed at atmospheric pressure, (2) among vacuum-mixed products, and (3) between atmospheric and vacuum-mixing for each individual product. Results: Comparisons of antibiotic-loaded polymethylmethacrylate products mixed at atmospheric pressure indicated that significant antibiotic elution differences exist among the products. Comparisons of vacuum-mixed antibiotic-loaded polymethylmethacrylate products indicated that significant antibiotic elution differences exist among the products. When mixing under atmospheric pressure was compared with vacuum-mixing for each individual antibiotic polymethylmethacrylate product, vacuum-mixing significantly increased the clinically relevant cumulative antibiotic elution from three products but significantly decreased antibiotic elution from three other products. Conclusions: The method by which antibiotic-containing polymethylmethacrylate products are prepared significantly affects their antibiotic elution characteristics. The effect of vacuum-mixing on antibiotic elution is product-specific.