The hip joint is a ball and socket joint. It allows for movement in all directions, which is why dogs are able to run, jump, and turn quickly. The hip joint is composed of three bones: the femur (thigh bone), the pelvis and the acetabulum (hip socket). The femur fits into the acetabulum of the pelvis to form the ball and socket joint.
Hip dysplasia occurs when this ball-and-socket joint does not fit together properly, causing pain and stiffness as well as difficulty walking. There are two types of hip dysplasia: developmental and acquired.