Antibiotic elution and compressive strength of gentamicin/vancomycin loaded bone cements are considerably influenced by immersion fluid volume.

The effect of doubling the immersion fluid (eluate) volume on antibiotic concentrations and on mechanical stability from vancomycin and gentamicin loaded bone cements was investigated in vitro. Antibiotic loaded bone cements containing premixed 1.34% gentamicin antibiotic concentration in the cement powder (wt), premixed 1.19% gentamicin wt and 4.76% vancomycin wt and premixed 1.17% wt gentamicin additionally manually blended with 4.68% wt vancomycin were tested. Six specimens per group were immersed in 4 ml and 8 ml for 6 weeks while the eluate was exchanged every 24 h. The antibiotic concentrations were repeatedly measured. Then the specimens were tested for compressive strength. Doubling the eluate volume significantly decreased gentamicin and vancomycin concentrations from 6 h and 24 h on, except for the gentamicin concentration of the additionally manually blended formulation after 3 weeks. The additionally manually blended vancomycin formulation produced significantly higher gentamicin concentrations in 8 ml compared to the other formulations. The reduction ratios of the vancomycin concentrations were significantly smaller than the reduction ratios of the gentamicin concentrations for the manually blended vancomycin formulation. Vancomycin containing formulations showed significantly lower compressive strengths than the vancomycin free formulation after immersion. Doubling the eluate volume lead to significant compressive strength reduction of the vancomycin containing formulations. Eluate volume change influences antibiotic elution dependent on the antibiotic combination and loading technique. The reducing effect is higher on vancomycin than on gentamicin elution. Compressive strength of gentamicin/vancomycin loaded bone cements after immersion is eluate volume dependent.