Knee Replacement Overview

The Knee Prosthesis

­Attempts at removing portions of bone to alleviate pain in knee joints were made as early as the 1860s, but the first artificial replacements were implanted in the 1940s. These early attempts focused only on the femur, but by the 1950s replacements that affixed to both the femur and tibia were being used.

These early models wrongly assumed that the knee worked basically like a hinge. With all the rolling, gliding, sliding and recentering that occurs with each knee movement, these models weren't successful. More recent designs have made greater strides toward replicating the knee's complicated design while also maintaining the structural integrity and efficiency of the artificial implant.

Implants share three common components: an attachment to the femur, an attachment to the tibia and a replacement for the back of the patella. They're usually made of metal like cobalt chrome or titanium and they cup the femur where damaged tissue has been removed from the end, front and back. A specialized type of acrylic cement can seal the device to the bone end, or (depending on the type of prosthesis) pegs or a long stem may be inserted into holes drilled into the bone. Other methods of attachment include using implants that have a coating designed to promote the bone tissue to grow into it. Sometimes both methods -- cement and bone-growth attachment -- are used.

For the tibia, a metal plate may be screwed to the flattened bone end, to which is attached the tibial insert, usually a polyethylene plastic piece that will act as the articular surface (the part of the bone end that is spongy) and prevent the tibia's metal plate and the femur's attachment from rubbing together.

A polyethylene plastic implant is used to replace tissue from the back of the patella. The patellar implant will move against the femoral implant, so a precise fit is crucial.

Another type of implant is known as a rotating platform or mobile bearing knee. As opposed to a fixed bearing knee in which the tibial insert is in a fixed position, the mobile bearing knee has a tibial insert that moves between the two fixed points on either bone end. This allows greater rotation, but can also lead to an increased likelihood of dislocation.