This is an inflammatory problem. It often runs in families. It affects several joints in the body to varying degrees. It begins as an inflammation of the joint lining (not the cartilage that lines the joint surfaces but the sac that sits tightly around the joint to keep in the lubricating synovial fluid). The inflammation results in release of chemicals and enzymes that are harmful to the cartilage surface which is then rubbed away. If the inflammation is controlled then the cartilage surfaces are preserved.