Journal of the American Academy of Orthopaedic Surgeons January 2008, Volume 16, Issue 1 Cato T. Laurencin, MD, PhD and Saadiq, F. El-Amin, MD, PhD We define xenotransplantation as including any procedure that involves the transplantation, implantation, or infusion into a human recipient of cells, tissues, or organs from a nonhuman animal source or of human body fluids, cells, tissues, or organs that have had ex vivo contact with nonhuman animal cells, tissues, or organs. The current FDA definition of xenotransplantation relates to procedures involving live, nonhuman materials. The proposed use of xenotransplanted tissues for treatment of a wide variety of human diseases is increasing. In orthopaedic surgery, a number of xeno-based products for treatment of musculoskeletal conditions have been cleared by the FDA. Commercially available products include those used as alternatives for bone, cartilage, and soft-tissue repair. Most xenografts are from bovine- or porcine-derived sources. Studies internationally have demonstrated a low relative risk of disease transmission, although there is concern regarding the potential for transmission into humans of agents not considered pathogenic or not detected in animals.