Legg Perthes Disease, also known as aseptic necrosis of the femoral head, Legg-Calve-Perthes Disease, Perthes Disease or simply Legg’s Disease, is a condition that involves spontaneous disintegration and collapse of the top of a dog’s long thigh bone, the femur, which connects the knee to the hip. The cause of this disease is not known. It tends to occur most commonly in puppies of tiny breeds and is thought to have a strong genetic component. For some reason, the blood supply to the upper part of the femur of these dogs becomes disrupted, causing the bone and surrounding cartilage to degenerate and die. The affected hip joint becomes disfigured, loses its structural integrity and becomes extremely painful. Eventually, it collapses and no longer can bear weight. Owners of dogs with Legg-Perthes disease typically notice gradual signs of hind-end lameness when their dogs are quite young – usually less than one year old.