In vitro analysis of various antibiotic and cement combinations against S. epidermidis and S. lugdunensis for treatment of periprosthetic shoulder infection

Background: Periprosthetic joint infections (PJIs) are devastating potential complications after arthroplasty surgery with significant therapeutic challenges. Many authors advocate for two-stage revision arthroplasty for PJI of the shoulder which includes explanation with antibiotic spacer placement. The optimal antibiotic/cement combination for PJI in the shoulder is not known. The purpose of this study is to analyze various cement and antibiotic concentrations against Coagulase negative staphylococcus species, S. epidermidis and S. lugdunensis. Methods: Five strains of coagulase negative Staph were taken from clinically documented orthopedic infections Both Simplex and Palacos cement were used to create antibiotic laden beads and mixed with either ertapenem, vancomycin, gentamicin, or a combination of ertapenem + vancomycin or ertapenem + gentamicin. The bacteria were plated on lysogeny broth agar plates and 3 beads were added per plate. Samples were analyzed for zone of inhibition at 24 hours, 72 hours, and 1 week. After 1 week, beads were transferred to new plates with bacteria and the process was repeated for 6 weeks. Results: Results showed that ertapenem beads with both Simplex and Palacos cement showed the largest zones of inhibition for all samples. Vancomycin in Palacos cement and vancomycin in combination with ertapenem in Simplex and Palacos cement showed consistent zones of inhibition for the duration of the experiments. Ertapenem in combination with either vancomycin or gentamicin may allow for a powerful initial burst of killing followed by consistent antibiotic elution as opposed to gentamicin alone. Conclusion: While many premade spacers on the market are infused with gentamicin, our in vitro models demonstrate more efficacious bacterial eradication for antibiotics such as ertapenem and vancomycin specifically for certain low virulence organisms that are commonly found in PJI around the shoulder.