Synthesis and Characterization of an Antimicrobial Honey-based Composite Bone Cement

Orthopaedic infections are a major cause of morbidity and mortality with increasing prevalence. Antibiotics are often added to bone cement or polymethyl methacrylate (PMMA) for local delivery to prevent or treat infection. However, PMMA is not inherently antimicrobial and has poor antibiotic elution kinetics. A biomaterial composite of PMMA and antimicrobial medical-grade honey (PMMA-H) could address some of these shortcomings of current bone cements to better inhibit bacteria.

(1) How does honey affect the architecture and static mechanical properties of bone cement? (2) How does honey affect antibiotic elution? (3) What is the in vitro antimicrobial activity of PMMA-H with and without antibiotics?

Three different formulations of bone cements were created by mixing a generic dental bone cement without honey (0 wt%), with a low amount of medical-grade manuka honey (15 wt%), or with a high amount of honey (30 wt%). Vancomycin or gentamicin were added to the three formulations at a typical cement loading dose (3 wt%) or no additional antibiotic for a total of nine formulations. We characterized the subsequent total porosity and percentage of open porosity of constructs synthesized from the nine formulations using microcomputed tomography. Both the compressive and bending yield strength and modulus were measured by mechanical testing. Constructs were placed in media, and the concentration of eluted vancomycin and gentamicin was measured over 168 hours via liquid chromatography-mass spectrometry using unloaded formulations as negative controls. To better understand the impact of the addition of honey alone as well as the effect of both honey and antibiotic, we evaluated the efficacy of all nine different formulations of PMMA, PMMA-H, and antibiotic-loaded combinations in vitro against three different common organisms by measuring the zones of inhibition produced during Kirby-Bauer testing, as well as the bacterial burden in biofilm on the constructs and in the environment during a Staphylococcus aureus high inoculum challenge.

The addition of honey increased the total porosity of the constructs from a mean ± SD of 1% ± 0.4% (0 wt% honey) to 13% ± 5% (mean difference 12% [95% confidence interval (CI) 4.0% to 20.8%]; p = 0.006) for 15 wt% honey and to 27% ± 6% (mean difference 26% [95% CI 18.3% to 35.1%]; p < 0.001) for 30 wt% honey. Honey-based formulations were mechanically weaker; for example, the mean compressive yield strength of unloaded PMMA (80.8 ± 9.9 MPa) decreased to 22.5 ± 6.1 MPa with 30 wt% honey (mean difference 58.3 MPa [95% CI -47.9 to -68.7]; p < 0.001). The addition of honey increased the elution of antibiotics; for example, 86% ± 28% of loaded gentamicin had been released from 30 wt% honey PMMA versus only 25% ± 5% of loaded gentamicin from PMMA without honey (mean difference 61% [95% CI 31.6% to 89.8%]; p < 0.001) over 168 hours. PMMA-H without antibiotics resulted in zones of inhibition against S. aureus and Staphylococcus epidermidis , and this inhibition was more potent with antibiotic loading. In a high inoculum challenge, PMMA with 30 wt% honey reduced environmental bacterial burden by > 20,000-fold compared with PMMA, with mean ± SD burdens of 6.4 ± 4.3 log colony-forming units (CFU)/mL and 12.8 ± 0.1 log CFU/mL, respectively (mean difference 6.4 CFU/mL [95% CI 1.77 to 11.03]; p = 0.007).

PMMA-H is a promising biomaterial that takes advantage of the antimicrobial mechanisms of honey to introduce new properties to bone cement with improved antibiotic elution and bacterial inhibition compared with conventional bone cement.

This proof-of-concept study demonstrates the enhanced antimicrobial activity of PMMA with honey in vitro and warrants further investigation in preclinical models of osteomyelitis.