Legg-Calvé-Perthes Disease in dogs
This is also known as aseptic necrosis of the femoral head. Yorkshire Terriers, Pugs, Jack Russell Terriers, and Dachshunds can be affected. Hip pain is usually seen by the age of 6 to 8 months. The disease is bilateral in 12 to 16 percent of cases. X-rays are necessary to make the diagnosis and show increased opacity and focal lysis in the head of the femur, and later in the disease, collapse and fracture of the neck of the femur. The recommended treatment is surgical removal of the head of the femur, but conservative treatment (rest, exercise restriction, and pain medication) may be effective in a limited number of cases (less than 25 percent, according to some studies). The prognosis is excellent with surgery. 

What is it Legg Perthes (from WestieClubAmerica)?
Legg-Calve-Perthes Disease, also known as Legg-Perthes, is caused by the disruption of the blood supply to the head of the femur (the round bone which fits into the hip socket). Legg-Calve-Perthes results when the blood supply to the femoral head is interrupted, resulting in the death of bone cells. This is usually followed by a period of revascularization, where the femoral head is subject to remodeling and/or collapse, creating an irregular fit into the hip socket. This process of bone cells dying and chipping followed by new bone growth and the remodeling of the femoral head and neck, leads to stiffness and pain in the rear leg or legs.

What causes Legg Perthes?
In some cases, trauma, such as a twisting injury or fracture, can be the initiating factor. Other speculative predisposing causes can be inflammation, nutritional factors, hip dysplasia, circulatory problems and possible excessive hormones. Although no specific causes of the disease are known, it is generally believed to have a genetic mode of inheritance, and is not believe to be caused by trauma alone. Because there is a genetic component, it is recommended that dogs affected with Legg-Perthes not be used in breeding programs. It is considered a polygenic trait in several small breeds of dogs. Onset can occur from three to eleven months of age.

What are the Symptoms?
The young dog will begin to favor one hind leg, or will walk or limp with a stilted gait. In the rare cases where both hips are affected, the dog will sometimes "bunny-hop". As the disease progresses, the dog may suspend the affected limb in the air, and may or may not exhibit pain when the leg is flexed. In very mild cases, often no symptoms are apparent, while in severe cases; the dog can be in great pain and become very irritable.

How is Legg Perthes Diagnosed?
Diagnosis is accomplished through radiographs (X-rays) of the hips, often done under light anesthesia.

What is the treatment?
Current veterinary thinking is that a general rule of thumb is to allow non-surgical therapy, limiting activity and treatment with non-steroidal anti-inflammatory drugs for one month. In severe cases, the dog may become totally lame and avoid use of the affected leg. Furthermore, the leg muscles may begin to atrophy after extended periods of non-use. If a good response is not seen, surgical repair by excising the head of the femur should be considered. By removing the femoral head and neck, the bone on bone contact that is the source of pain and discomfort is eliminated. Within two months of surgery, a false joint of scar tissue will form by muscle and tissue, and the dog may again walk without pain.

What is the prognosis?
Usually excellent, and within several months, the dog can again walk and run. In some cases, the affected leg may remain slightly shorter and the muscles may be somewhat atrophied. Some veterinarians feel there may be an increased risk of arthritis as the dog ages.