The term congenital means present at birth. Hip dysplasia ranges in severity depending on the degree of malformation of the hip socket and the position of the thigh bone.
Mild dysplasia means that the hip socket is somewhat shallower or flatter than normal.
The shallower the socket the more likely that the head of the femur will slip out of the socket completely i.e. dislocate. More frequently the head of the femur is only partially out. This is known as a subluxated hip. In order for the hip to develop normally the head of the femur must be held securely within the hip socket. If it is not held in this position the joint will be very prone to arthritis and other degenerative changes in later life.