Xenogeneic Amnion Inhibits Healing in Equine Distal Limb Wounds: A Pilot Study

Objective: To assess the safety and efficacy of lyophilized, milled, human amnion on wound healing in an equine distal limb wound model in a blinded controlled study. Animals: Four clinically normal adult horses (3 Thoroughbred and 1 Paint, age 6-19 years) obtained via donation. Institutional Animal Care and Use Committee approval was obtained for this study. Procedures: One forelimb of each horse was randomly assigned to the treatment group, and the contralateral limb was assigned as the control. Full-thickness skin wounds were created on each metacarpus. Treatment limb wounds were treated with lyophilized, milled, human-derived amnion material delivered on triple antibiotic ointment on non-adherent dressing. Control wounds were treated with triple antibiotic ointment on non-adherent dressing. Distal limb bandages were applied. Digital photographs of the wounds every 2-4 days throughout a 98-day study period were blinded and analyzed for wound area and epithelialization. Histopathology was performed on samples from days 7, 21, 35, and 84. Results: All treatment limbs exhibited an inflammatory response characterized by focal edema and discharge from the wounds, and this was not seen in control limbs. One horse developed unilateral cellulitis in the treatment limb, and not the control limb that resolved with additional treatment. Wounds were completely epithelialized in control limbs sooner than treatment limbs in each horse (p = 0.011). Mean wound size (junction of neoepithelium and wound margin) was smaller in control wounds than treatment wounds at all time points, with a significant difference on days 23 and 39, with trends toward significance on days 18 and 35. Overall histologic scores were better in control wounds than in amnion-treated wounds at all time points, only reaching significance on day 84 (p = 0.039). Conclusions and Clinical Relevance: In this pilot study, lyophilized human amnion did not improve wound healing compared to control, and may have hindered wound healing. We terminated the study after one cohort of 4 horses due to these results. Because wounds treated with amnion in this pilot study exhibited an inflammatory response that resulted in delayed time to wound closure, human lyophilized milled amnion is not recommended for use in equine wound management. Xenogeneic amnion may not be appropriate for wound healing in equines, but further study is needed to strengthen these conclusions.