The Use of Endothelial Progenitor Cells (Epcs) and Local Antibiotics for the Treatment of Infected Nonunions in a Small Animal Model

The purpose of this study was to assess the efficacy of Endothelial Progenitor Cell-based therapy (EPC) in the presence and absence of local antibiotics (ABX) in the treatment of infected nonunions in a small animal model. We hypothesized that EPC therapy would promote bone healing and eradicate infection, and that the addition of local antibiotics would further facilitate these outcomes. A 5-mm segmental defect was created and stabilized with mini-plate fixation in the right femur of Fischer 344 rats. A control solution (Experiment 1: non-contaminated arm) or an inoculum of Staphylococcus epidermidis (Experiment 2: contaminated arm) was delivered at the defect site on a gelatin scaffold. Fourteen days later, a second surgery was performed and one of the following treatments was applied in both the non-contaminated and contaminated arms, creating a total of seven groups: 1) control (no treatment), 2) EPC, 3) EPC + ABX (local vancomycin and rifampin), and 4) ABX (only in the contaminated arm). Biweekly radiographs were obtained to monitor bone healing and infection. All animals were euthanized 10 weeks after the second surgery. Soft tissue and fixation material samples were collected and processed for microbiological analysis. The radiographs were scored and assessed blindly for union status, defect filling, and infection. Bone healing was also assessed with micro-CT and biomechanics following specimen harvest. Our primary outcome for Experiment 2 was the rate of non-infected union. Experiment 1: Non-contaminated: All animals in the EPC and EPC+ABX groups demonstrated significantly improved healing relative to controls with regards to union rates. Similarly radiographic scores, micro-CT (bone volume (BV) and bone volume fraction (BV/TV)), and biomechanics (torque and stiffness) all demonstrated significantly improved outcomes in the EPC and EPC+ABX compared to the control group. There were no significant differences between the EPC and EPC+ABX groups (Table 1). Experiment 2: Contaminated: Animals that received EPC+ABX or ABX alone demonstrated high rates of infection eradication compared to the EPC only and control groups (Table 2). The EPC groups (EPC only and EPC+ABX) demonstrated improved bone healing outcomes relative to control and ABX alone groups. A post-hoc analysis test comparing rates of non-infected union showed that the EPC+ABX group had a significantly higher rate of non-infected union compared to ABX alone and control group (p=0.003). These results demonstrate several important findings. First, the use of local ABX (vancomycin and rifampin) alone or in combination with EPCs is highly effective at eradicating infection in a small animal model of infected nonunion. Second, combining local ABX with EPCs does not negatively impact the fracture-healing efficacy of EPCs. Finally, the combined treatment of infected nonunions with EPCs and local ABX significantly improved both infection and healing rates. These results support the continued investigation of EPCs and local ABX as a potential single-stage treatment for infected nonunions. For any figures or tables, please contact the authors directly.