Limb replacement (prosthesis) in animals is a well developed science and becoming more common. Organ replacement is rarer and much more difficult. The first pacemaker surgery on a dog was performed in 1968. About 300 pacemakers are implanted in dogs each year. Successful kidney transplants in cats have been around since the mid-1980s. Canine programs have been less successful due to the problems of immunosuppression. The way donor animals are treated remains a source of concern. Kidney transplantation Kidney transplantation has so far only been performed with any degree of success on cats and dogs, most usually cats as they are particularly prone to kidney diseases. The School of Veterinary Medicine at U.C. Davis initiated the Renal Transplantation Program in 1987. Cats can tolerate transplantation from unrelated cats, but must be supported with immunosuppressive drugs for the rest of their lives. When the transplanted kidney is implanted in the recipient the original kidneys are usually left in place. There are currently ten centers in the world that offer kidney transplants for cats. Most are in the US. UC Davis have since abandoned their program. Kidney transplantation is a controversial treatment option, with longevity, quality of life, as well as ethical considerations. The United Kingdom permitted this procedure in 2003, but a review in 2013 suspended it. Many argue that it causes unnecessary suffering to donor animals. Total hip replacement A good candidate for total hip replacement (THR) must be at least 9–12 months old to be sure he has finished developing and weigh at least . The hip implant for dogs is similar to its human counterpart, but it is much smaller. X-rays are used to determine the dimensions of an appropriately sized implant. The femoral stem is usually made of cobalt chrome or stainless steel, while the cup is made of ultra high molecular weight polyethylene. Polymethyl methacrylate is the cement used with cemented implants, whereas 250-micrometres (diameter) titanium beads usually cover the stem surface of the press-fit implants. The THR is expected to last 10–15 years, which usually surpasses the remaining lifetime of the dog. In less than 5% of cases, the THR will malfunction through loosening of the acetabular cup or femoral fracture. These failures require correction with revision surgery. Pacemaker implants The first pacemaker surgery on a dog was performed in 1968. About 300 pacemakers are implanted in dogs each year, even though about 4000 dogs