A normal joint is lined with a 4 mm layer of cartilage on either side of the joint. Cartilage has no nerves. It provides a protective layer for the bone ends that move against each other in any normal joint. When the cartilage starts breaking down , the joint becomes inflamed and starts hurting. At this stage , the symptoms are mild and medications usually help relieve the pain. However , when the cartilage completely wears through , the bone ends begin directly rubbing against each other. Nerve endings in the bone are stimulated , resulting in significant pain. At this stage , the diagnosis is severe arthritis. Pain medicines usually do not give adequate relief and most people are forced to substantially curtail their activities. The principle of any joint replacement is to provide a new artificial joint-bearing surface that is fixed to the bone on either side of the joint. Motion of the joint now no longer causes the bone ends to rub against each other. As long as the implants remain well fixed to the bone on either side of the joint , motion no longer stimulates the nerve endings in the bone , and motion is painless. Activities can be resumed without pain.