Knee replacement surgery. Surgeon fitting a prosthetic joint component (white) onto the end of the shinbone (tibia) during a total knee replacement operation. The kneecap has been moved aside to expose the knee joint. The component fixed to the end of the tibia is aligned with the component fixed to the end of the femur (thigh bone), forming an artificial knee joint. Knee joints need replacing when the protective cartilage that lines them is lost. Cartilage can be lost due to injury or arthritis. See clips K004 6229 - K004 6255 for entire sequence.