A joint, such as a hip or shoulder, is a ball-and-socket joint. In the hip, for example, the ball part of the hip joint is on the upper end of the thigh bone. It's a smooth, round piece of bone covered by cartilage and fluid. The socket part of the hip joint in the pelvis matches the ball exactly. The joint is held together by muscles and ligaments.

Sometimes through disease, stress or simply age, the cartilage covering the ball becomes damaged so the joint can't glide smoothly. This causes pain and stiffness. Joint replacement can correct the problem.

The goal of joint replacement is to ease pain and restore motion to joints damaged by disease or injury. When a joint needs to be replaced, the deteriorated bone and cartilage is removed from both the ball and socket. In the case of a hip replacement, a new socket made of high-impact plastic is implanted in the pelvis. A new ball, made of highly polished metal alloy, is implanted in the thigh bone. When necessary, these new elements are cemented in place.