OTA 1997 Posters - Scientific Basis for Fracture Care

*Gentamicin and Vancomycin Incorporation into Carbonated Apatite Cement: Effect on Antibiotic Elution and Cement Attributes

Robert D. Poser, DVM, Stuart B. Goodman, MD, PhD, James Nachiondo, David C. Delaney, Duran N. Yetkinler, MD, PhD, Brent R. Constantz, PhD

Norian Corporation, Cupertino, California, USA

Purpose: The purpose of this study was to determine the release kinetics and activity of gentamicin and vancomycin antibiotics following incorporation into a nonexothermic carbonated apatite cement. Secondarily, effects of antibiotic incorporation on the material and compositional attributes of the cement were examined.

Materials and Methods: Two antibiotics commonly used in orthopaedics, gentamicin sulfate and vancomycin hydrochloride, were evaluated individually. Both antibiotics were evaluated at a concentration of 1% by weight which corresponds to 222 mg of gentamicin or vancomycin added to 22.2 grams (10 cc) of cement. The antibiotic powder was added to the solution component of the cement prior to mixing. The cement (Norian SRS), a carbonated apatite susceptible to the native bone remodeling process, hardens via nonexothermic reaction (1,2).

Release kinetics were examined through 44 days at 14 time intervals. Antibiotic impregnated cement was placed in 50 mls of phosphate buffered saline (PBS), pH=7.4 and maintained at 37°C. The antibiotic concentration in the eluates was analyzed using a fluorescence immunoassay.

Activity of the antibiotic in the eluate from the kinetic study specimens was also tested at 1, 8, and 16 days. Eluates were pipetted on monolayer cultures of E. coli and Staph. aureus and allowed to dry in a laminar flow hood. Positive controls consisted of aliquots of PBS, pH=7.4 containing 200µg/ml gentamicin sulfate. Negative controls were PBS alone. Zones of growth inhibition surrounding the eluate samples were compared with positive and negative controls to determine if the antibiotic was still active. Vancomycin and gentamicin activity in the hardened SRS specimens was determined by placing the antibiotic containing SRS specimens on confluent monolayers of both Staph. aureus and E Coli. Zones of bacterial growth inhibition due to antibiotic susceptibility were compared with standardized disks containing antibiotic (10µg gentamicin or 30µg vancomycin, Difco). Control SRS cement samples with no antibiotic served as negative controls. The material properties of SRS incorporated with antibiotics were evaluated by a variety of adapted ASTM test methods. Early strength attainment, injectability, and ultimate compressive strength were examined and compared with control cement specimens containing no antibiotic. Early strength attainment was evaluated by indentation testing at 15 minutes post mix. Injectability was evaluated by measuring the load in pounds necessary to inject 5 cc of cement from a 10 cc syringe through a 12 gauge needle. Ultimate compressive strength was evaluated by allowing the cement to harden in 37°C bovine serum for 24 hours. Samples were tested until fracture in uniaxial compression at a rate of 0.1 inches/minute. Crystallographic and compositional attributes of the cement were examined by X-ray diffraction (XRD) and carbon coulometry to determine relative crystallinity and carbonate content respectively.

Results: No detectable differences in crystalline structure, carbonate content, injectability, rate of strength strength attainment, and ultimate compressive strength of the cement was observed. Release kinetics for 1.0% gentamicin showed an average release rate of 80 µg/minute over the first 15 minutes (total released =1500µg). Sustained release of gentamicin was observed over the remaining 44 days at an average rate of 0.5 µg/minute (total released = 25mg). These data suggest that the solution adjacent to the SRS contained gentamicin concentrations (average 50 µg/ml) well above the minimum inhibitory concentration of Staph. aureus (0.5-2.0 µg/ml). Growth inhibition zones greater than positive control specimens were observed with all eluate samples tested at all time intervals demonstrating each contained active antibiotic.

Discussion: The basis for nonexothermic, apatite cement's strength and susceptibility to the native remodeling process is their crystal structure and chemical composition. These attributes may be adversely affected by the addition of therapeutic agents. This study demonstrates that this cement's mechanical, crystallographic, and compositional attributes can be maintained while eluting active antibiotic, suggesting surgical handling, mechanical integrity, and in vivo response to the cement should be similar to that previously reported for the cement without antibiotics (1,2). Antibiotic incorporation into a remodelable cement raises the possibility that one-step surgical debridement of open fractures or osteomyelytic lesions may be possible, thereby eliminating the conventional two-step procedure used with PMMA antibiotic impregnated beads. Further, incorporation of different antibiotics or other therapeutic agents is attractive for both prophylactic and therapeutic purposes, especially for heat labile agents which are denatured in the exothermic reaction of conventional bone cement (PMMA).

Conclusions: These in vitro studies suggest that therapeutic doses of antibiotic may be administered locally using this nonexothermic, remodelable, carbonated apatite cement while maintaining it's structural integrity. Gentamicin was shown to remain active at clinically relevant concentrations after release. The cement has the potential to be integrated and remodeled into host bone while delivering effective dosages of antibiotic, and therefore may not require secondary surgery for removal. In vivo pharmacokinetic studies of both heat stable and labile antibiotics are planned.

References:

1. B.R. Constantz et al.: Science 267:1796-99, 1995.

2. E. Frankenburg, S. Goldstein et al.: Trans. ORS 224: 1996