Hip dysplasia is a congenital disease that primarily affects large dogs. Eventually the dog develops weakness, degenerative joint disease, or arthritis, a chronically painful hip and lameness. When a dog has hip dysplasia, the joints develop abnormally. The head of the femur or thigh bone, does not fit properly into the acetabulum or hip socket. These "ball and socket" joints become malformed and unstable, causing inflammation and weakness. Depending on the severity of the problem, this can lead to painful and sometimes crippling arthritis. Hip dysplasia is diagnosed based on a history of weakness or lameness of the rear legs, especially when going upstairs or standing up slowly, particularly in the morning. Some young dogs will hop like a rabbit while running, and might lie down on their stomachs with their legs stretched behind them. It is possible to palpate joint laxity (Ortolani sign) in some dogs that are anaesthetized. Radiography is the definitive way this disease is diagnosed. It is not perfect though, since a dog can be hip dysplasia-free on the radiograph, but can carry genetic traits that will cause it to develop hip dysplasia later in life. A total of 1727 dogs in Lithuania were examined in a span of 10 years (1994-2003) to determine the prevalence of hip dysplasia. Of these, 714 dogs (41.3%) were affected. Hip dysplasia was rather frequent in Neapolitan Mastiffs, Caucasian shepherds, Rottweilers and English bulldogs. Young diseased dogs up to 4 months of age had evident clinical signs of hip dysplasia.