Preparation and release characteristics of tobramycin-impregnated polymethylmethacrylate beads

Preparation of tobramycin-impregnated polymethylmethacrylate (PMMA) bone cement beads and release of tobramycin from the beads in vitro and after implantation in a patient are described. Tobramycin sulfate powder 1.2 g was mixed with Palacos PMMA bone cement 40 g in a custom-made mold to produce 25 beads containing 3.26 mg tobramycin (as the sulfate salt) per bead. Chains of the beads, strung on stainless-steel suture, were sterilized with ethylene oxide. Three single beads were each placed in multiple-electrolyte solution (pH 7.4); the solution was removed and replaced with fresh solution every 24 hours for 28 days. The tobramycin content of each day's solution was determined by fluorescence polarization immunoassay. After day 28, solution was removed weekly for assay until day 84. Tobramycin concentrations were measured in drainage from the surgical wound after six chains of tobramycin-PMMA bone cement beads were implanted in the right acetabulum and femur of a patient whose hip prosthesis had been removed because of infection. Tobramycin concentrations in the dissolution medium averaged 34.3 micrograms/mL initially, and 7.5 micrograms/mL on day 2, gradually decreasing to 0.6 microgram/mL on day 28. Release of tobramycin followed a predictable pattern, and variation among samples was small. Over 12 weeks, less than 20% of the theoretically available tobramycin from a single bead was released. Tobramycin concentration in wound drainage was 90.0 micrograms/mL during the first 24 hours after surgery, while serum tobramycin concentrations were less than 0.5 microgram/mL. Extemporaneously prepared beads of bone cement are effective for delivering high concentrations of tobramycin to an infection site.(ABSTRACT TRUNCATED AT 250 WORDS)