Canine hip dysplasia is a malformation of the canine hip joint. During development, the femoral head (ball) fails to fit well into the acetabulum (socket). In early phases, say when the puppy is six months old, the joint is lax and on palpation the femoral head can be felt moving up and down and in and out of the acetabulum. In other words the femoral head is not well “seated” into the acetabulum.
Over time, the hip joint becomes arthritic because of these anatomical abnormalities. The joint capsule thickens and becomes inflamed, osteophytes (bone spurs) develop, cartilage erodes, the acetabulum begins to fill in with bone and the femoral head may even luxate (slide completely out of the acetabulum). These secondary arthritic changes (dogs and cats almost never develop arthritis without an underlying anatomical reason) can develop quickly; puppies with severe hip dysplasia can develop severe arthritis before their first birthday.
In cases where the femoral head stays somewhat within the acetabulum, and the dog lives without intervention, the arthritic changes continue to worsen over years.
The primary cause of hip dysplasia is genetic. Parents with hip dysplasia are more likely to yield puppies with hip dysplasia, although breeding normal parents does not ensure the puppies will be free of hip dysplasia. In fact, for decades efforts have been made to identify dogs with hip dysplasia using standard radiography (X-Rays) taken at two years of age and eliminate those dogs from breeding programs. However, hip dysplasia remains prevalent in large breed dogs. More recently, distraction radiographs taken under anesthesia called PennHip™ is being used successfully to identify puppies as young as four months with hip dysplasia before breeding is even considered. Basically the technique involves placing adjustable distraction bars between the puppy’s thighs, then squeezing the knees together to distract the femoral heads.