Mechanical properties and elution characteristics of polymethylmethacrylate bone cement impregnated with antibiotics for various surface area and volume constructs

Richard E Duey, Alexander C M Chong, David A McQueen, James L Womack, Zheng Song, Tristan A Steinberger, Paul H Wooley
Iowa Orthopaedic Journal 2012, 32: 104-15

BACKGROUND: Numerous studies have examined the elution characteristics and the effects of antibiotics from bone cement. this study seeks to determine the effect that surface area and volume have on the elution characteristics and bioavailability of tobramycin and vancomycin when mixed in polymethylmethacralate (PMMA) bone cement in various combinations. It also investigates the mechanical properties of antibiotic-impregnated bone cement and its relationship to surface area and volume.

METHODS: Three antibiotic-bone cement combinations were used, and these consisted of PMMA mixed with tobramycin and vancomycin or tobramycin alone. Four groups of specimens (different surface area and volume) were made. the elution characteristics of the different specimens were examined using the minimum inhibitory concentration (MIc) method at different time intervals. the bacteria used during testing were methicillin-sensitive staphylococcus aureus (MssA). the ultimate compressive strength (Ucs) of the specimens was also determined at various time intervals.

RESULTS: the bactericidal activity of a tobramycin/vancomycin combination against MssA was not significantly greater than tobramycin alone. tobramycin was more effective than vancomycin against MssA (average: 168%, p<0.05). the inhibitory capabilities of tobramycin and vancomycin individually were not found to be additive. combination 2 (1.0 g tobramycin/1.0 g vancomycin) had a higher antibiotic elution mass and rate for all sample sizes compared to the other two combinations (average: 170%, p<0.05). surface area and volume did not have a significant effect on the elution rate of the antibiotics. the Ucs of all samples tested was greater than 70 MPa at all three testing intervals.

DISCUSSION: Mixing tobramycin and vancomycin did not have a synergistic effect against the bacteria as expected. Increasing the concentration of antibiotics in bone cement increases both elution mass and elution rate over time. Although the Ucs of the antibiotic-impregnated bone cement was affected by antibiotic elution and sample geometry, all testing results fell within previously accepted standards.

CLINICAL RELEVANCE: This study advanced our overall understanding of the elution characteristics and biomechanics of PMMA bone cement impregnated with tobramycin and vancomycin.

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