Worn knee joint replacement. Tibial component of a knee joint replacement showing severe wear. This plastic plate fits onto the top of the tibia (shin bone) and articulates with a plate attached to the femur (thigh bone). Such components only have a life span of a few years before they need replacing. Prosthetic knee joints are needed when the protective cartilage surrounding the joint is lost. Cartilage can be lost due to injury or osteoarthritis.