An artificial knee is a replacement for the human knee. An artificial knee typically has a metal shell on the end of the thigh bone (the femur), a metal and plastic trough on the shin bone (the tibia), and sometimes a plastic button in the kneecap (the patella). An artificial knee is exchanged for the human knee by a surgical procedure known as total knee replacement. The artificial parts are cemented into place. Knee replacement is done on people, usually over 55, whose knees have been so damaged by arthritis, trauma, or other destructive diseases of the joint that they have severe pain, stiffness, instability or deformity (lock-knees or bowlegs) of the knee. The most common reason for knee replacement is osteoarthritis. An artificial knee is not a normal knee nor is it as good as a normal knee. The main long-term problem is loosening which occurs because the cement crumbles or the bone breaks.