Broken prosthetic knee joint. Close-up of the knee of a patient showing a broken prosthetic knee joint. The metal surface (upper centre) is the end of the thigh bone (or femur). Underneath this is a plastic surface which forms the joint of articulation between the artificial ends of the femur and tibia (the shin bone). The metal that holds these components together has broken, probably due to vigorous activity or injury. Prosthetic knee joints are used in cases of severe arthritis to relieve pain and lack of movement. Most prosthetic knee joints erode over time.