Normally the head of the femur fits snugly into the acetabulum and forms the pivot point from which the hind leg swings. In hip dysplasia, this joint is loose. Instead of rotating smoothly, the femoral head bangs around the inside the acetabulum. The dog may show signs of pain around 6 to 12 months of age, the period of most-rapid physical growth. The dog may be lame, may not want to run and play as hard as usual, may be reluctant to jump, or may experience pain when the hips are handled.
Bones respond to stress by growing thicker.
This works well to make a tennis player's arm stronger but extra bone in the hip joint worsens the fit of the joint. The extra bone that results from the stress of hip dysplasia is referred to as arthritic change. Dogs with hip dysplasia may suffer from arthritis after they are a few years old. Diagnosis of hip dysplasia is based on signs of pain in the hip joint and X rays showing poor hip structure. Unfortunately, X rays alone can't predict the progression of the disease or the secondary arthritic changes.