Hip Dysplasia is a hereditary defect in the socket joint (in the pelvis) which implies that the socket is not as deep as it normally should be. Because of this the ball does not fit exactly in the socket joint and the surfaces begin to rub against each other, causing the cartilage to degenerate. The result is then that the joint surface remains solely made up of bone (the cartilage having been worn away) leaving a bone-against-bone joint surface which is painful for the cat. The problem is, that the body cannot renew the cartilage, and the body tries to repair the damage by increasing bone production which only serves to make the problem worse. Cats in general are very good at not showing pain and can suffer from HD without limping at all. Instead they may move more carefully or less than cats normally do, and may also avoid jumping. Cats with a mild degree of HD may not suffer at all.
Feline HD is an inherited trait involving multiple gene pairs, which means that two cats with no sign of HD together can produce offspring that develop HD. Also, two cats having HD can together produce kittens that do not develop HD, but the odds for normal hips in the kittens are of course higher if the parents have good, normal hips.