What material is a Knee Replacement made of? Titanium?

Another question that I get asked all the time has to do with what the knee replacement implants are made of. Patients often guess that it is made of titanium.

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The answer is yes, some parts of knee replacements are made of titanium, however, it is more complicated than that.
A knee replacement usually contains four separate parts that are combined. From top to bottom the components are:
1. Femoral component - usually made of Cobalt Chrome
2. Patella component - usually made of Polyethylene
3. Tibial insert - Polyethylene
4. Tibial tray - Titanium or Cobalt Chrome

Here is an illustration:

The reason for the different materials is simple. Metal can not rub on metal so there needs to be polyethylene wherever motion is occurring  The patella rubs on the metal femoral prosthesis, so it is made of polyethylene. The Femoral prosthesis rotates on the tibial insert so the insert is also made of polyethylene. The parts that attach to bone are metal. The reason why the femoral prosthesis is made of cobalt chrome is because it is hard and smooth and therefore causes a low rate of wear. The tibial tray component can be made of either titanium or cobalt chrome since it is not part of the joint, there is no movement involving the tibial tray, it supports the liner and sits on the bone. Titanium is often preferred for metals that come in contact with bone because it is less stiff and more bio-compatible.
I will review the components of a hip replacement implant next.