Total knee replacement, or total knee arthroplasty, involves resurfacing the knee joint and the implantation of prostheses or knee implants. Knee implant components may be ceramic, metal or plastic. Most total knee replacements are made of a combination of metal and plastic. The components are designed so the metal moves against plastic for smooth movement and less wear, according to the American Academy of Orthopaedic Surgeons.
Your surgeon chooses the type of implant for your total knee replacement from 150 different styles and designs based upon performance record, cost, experience with the product and your physical characteristics.
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Metal and Plastic Knee Implants
The AAOS explains that metal components of a knee implant are made of titanium or colbalt/chromium alloys. Plastic parts are made of high-density polythene. The total weight of such a knee implant is between 425 and 567gr.
The materials used to construct a knee implant have to be biocompatible: they cannot cause a rejection response in the body. They must be strong enough to bear weight and flexible enough to bend without breaking. The parts of a knee implant must move smoothly against each other and the materials must retain their shape and strength for about 15 to 20 years, says the AAOS.
Not Just a Hinge
The first knee implants were constructed like a hinge, but the knee moves in far more complex ways than a simple hinge joint does. New knee implants, says the AAOS, attempt to move in these ways and use the ligaments in the knee for support.
In a total knee replacement, parts of the thighbone (the femur), shinbone (the tibia) and kneecap (the patella) are replaced. The femur component is usually metal and has a groove for the kneecap to move in. The tibia component is metal with a plastic cushion, and the patella component is high-density plastic.
Fixed-Bearing Knee Implants
Fixed-bearing prostheses are the most common knee implants in total knee replacements. The components consist of a metal shell, two plastic pieces and a metal tray.
Extra weight and extra activity can wear out some parts of a fixed-bearing prosthesis so that it loosens from the bone.
Mobile-Bearing Knee Implants
Mobile-bearing implants have a plastic insert that the thigh-bone and shinbone components move against. This reduces wear and lessens the risk of loosening. More rotation can occur in the knee.
Mobile-knee implants are good for younger, more active people or those who are overweight, but they cost more than fixed-bearing knee implants and can increase the chance of dislocation.
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