The thigh bone (femur) and the pelvis (acetabulum) join to form the hip joint. The joint surface is covered by the smooth articular cartilage that allows pain free movement in the joint. The cartilage cushions the joint and allows the bones to move with each other smoothly, easily and without pain. This cartilage does not show up on Xray, therefore you can see a "joint space" between the top of the thigh bone and the hip socket. When the cartilage wears out, the bone ends rub on one another and cause pain.