Surface knee replacement is an effective surgical procedure for the treatment of all causes of end-stage knee osteoarthritis, and the procedure has evolved to the point where more than 500,000 total knee replacements are performed each year worldwide, with a satisfactory outcome rate of more than 90% within fifteen years. In total knee arthroplasty, the surface is replaced with special metal and plastic in the part of the knee where the bone rubs together, and is fitted in place with precision instruments, using a metal prosthesis that matches the original curvature of the distal femur above and a smooth metal prosthesis with an ultra-high polymer polyethylene spacer on the tibial surface below to act as cartilage. The lower surface of the patella can also be replaced with a disc made of polyethylene.