The hip joint has a ball (top part of the femur) which fits into a joint (hip socket in the pelvis). If the hip socket is not developed properly the ball does not sit in it properly. This causes stress on the cartilage lining the hip joint. With the constant stress on the joint the cartilage thins and becomes stiffer which decreases the joint's ability to handle the daily stress of movement and weight bearing. Over time arthritis will build up in the joint and cause thickening and a decrease in range of motion along with accompanying pain.
This is a congenital, degenerative condition. It is a polygenic disease which means there are several genes operating to produce the condition as well as environmental factors involved such as rate of growth, nutrition (eg high protein, excess calcium), excess weight and too much or too little exercise. Certain breeds, primarily larger breeds such as German Shepherds, Golden Retrievers and Labrador Retrievers have a breed predisposition for this condition though it can occur in all dogs, even in a smaller dog as the Cavalier King Charles Spaniel.
Diagnosis of hip dysplasia is made by x ray. As this is a degenerative disease, breeding stock in dogs with a breed disposition are x rayed after the age of 2 for signs of the disease. An affected dog should not be used for breeding. In the Cavalier it is a breeder's choice to x ray for hip dysplasia as the incidences of the condition at this time seem fairly low. As with many other inherited diseases while x raying may indicate that the individual does not have signs of HD it does not answer the question of whether a dog is a carrier and can pass the condition to its offspring.
Depending upon the severity of the condition treatment varies from the use of nutraceuticals such as glucosamine, Cosequin, shark cartilage, NSAIDS such as Rimadyl or Metacam, buffered Asprin or Bufferin, to surgery. It is also helpful in controlling the symptoms of HD to keep the dog's weight down as the excess weight puts more strain on the hips.