Parts of a Total Knee
The knee joint involves three bones; the femur, the tibia and the patella. In Total Knee Replacement, each one of these bones is resurfaced with a separate component. To "resurface" the bone the surgeon uses a saw guide and small power saw to trim off the outer worn surface of each bone in the exact shape of the inside of the corresponding implant.
The resected bone can be several millimeters thick (about 1/4-1/3 inch). The separate components work together to form the complete prosthetic device. In some cases the surgeon may choose not to replace the patella and let your native patella mate with the other artificial components (this is quite common in Europe, less common in the US).
The Femoral Component
Typically this is a Cobalt-Chrome metal component. The highly polished outer surface serves as the end of the femur, the other side is either "cemented" or "press-fit" onto the prepared bone surface.
Most implants in the US today are cemented. A polymer based compound is mixed in the OR forming a putty which hardens. The surgeon puts the putty in between the implant and bone to secure it.
In the "press-fit" variety, a roughened surface on the inside of the component has a porous 3-D structure designed to promote bony ingrowth.
There are pros and cons to each type of fixation. As mentioned, the cemented version is more common, but both have well established clinical histories
The Tibial Component
The tibial component is usually a two-piece metal and plastic implant although there are all plastic versions available.
The metallic portion of the implant can be titanium or cobalt-chrome and it is fixed to the tibial bone in much the same way as the femoral implant.
The plastic component is known as a "tibial bearing" or "tibial spacer" or simply "tibial insert." It is a plastic component made of a special grade polyethylene (UHMWPE). Manufacturers have begun to "treat" the plastic with gamma radiation to improve wear properties.
The Patellar Component
The patellar component is typically an all plastic component that is fixed to the cut surface of the underside of the patella or knee cap.
The component is secured with bone cement and articulates or "joins up" with the femoral component, as it would with the normal knee.
Because the patella moves up and down across the surface of the femoral component, the relative alignment of the two components is critical. Patella problems are among the most common complications following total knee replacement, although the overall complication rate is small.