This instability, called hip joint laxity, results in abnormal weight bearing within the hip joint. When cartilage disintegrates, from abnormal wear, the femoral head and acetabulum rub together with every step causing pain and eventual osteoarthritis.
Cats are rather stoic creatures and it is speculated that they hide or mask their pain well.
Out in the wilds if a cat showed pain by limping or moving slowly, he would appear vulnerable and be an immediate target for predators. Our domestic cats are still driven by natural instincts that tell them NOT to limp or not to move too slowly. There may be a gradual decrease in certain movements or particular activities that cause pain. For example, a cat in pain may not jump as high as usual, or it may move more and more slowly over time. These may be symptoms that most people simply would not notice or would attribute to other things.