DEAR DOCTOR K:
I am going to have my knee replaced. What types of artificial knee joints are available?
The knee is a joint formed by the bottom end of the thigh bone and the top ends of two bones of the lower leg. When the ends of the bones that form the joint become damaged, they can be removed and replaced. That’s total knee replacement, and it is major surgery.
The damaged parts of the bones that form your knee joint are replaced with an artificial material that is shaped like your bones should be. That will allow your knee to again do all the things it needs to do: bend, twist and bear your weight. The artificial material will be attached to the healthy bone above and below it.
Once you have decided to have a total knee replacement, your surgeon may discuss with you several choices. One choice the surgeon needs to make is which artificial material to use for your new knee. Options include different combinations of hard polished metal, hard ceramic, and tough, slick plastic:
- METAL-ON-METAL. These implants were developed to last longer than other implants. But metal-on-metal implants may cause inflammation and pain. Worse, they may shed harmful microscopic bits of metal into the bloodstream.
- POLYETHYLENE. The major drawback to this type of plastic material has been wear. Tiny pieces of the plastic flake off, provoking the immune system cells in the bone to which the plastic has been attached. Turning on the immune system cells produces substances that weaken bone tissue. As a result, the implant pulls away from the bone and has to be replaced. Manufacturers have modified the plastic to make “cross-linked” polyethylene. The newer materials may be more wear-resistant.
- CERAMIC-ON-CERAMIC. The use of very hard ceramic material in place of some metal parts is another option. So far, ceramic-on-ceramic appears to cause significantly less bone deterioration than metal or polyethylene implants.
You and your surgeon will also have a choice of cemented or cementless implants. Most total knee replacements today are done with cement. They use acrylic cement to quickly glue the implant to the healthy bone above and below it.
Cementless implants have surfaces with small holes in them. The healthy bone grows into the implant to form a solid attachment, and no cement (glue) is required. Despite early hopes, cementless implants have not proven to be more durable than cemented implants. There is also a “hybrid” option that uses a combination of cemented and cementless attachments.
Another decision is between a fixed-bearing or rotating knee platform. A fixed-bearing knee prosthesis is the most common type. In a rotating platform prosthesis, the polyethylene insert can rotate slightly. This allows for better movement and reduced stress and wear.
The best implant design for you will depend on many factors, including your weight, bone quality, age, occupation and activity level. The surgeon’s experience with a particular model or brand often enters into the decision, too. Discuss your options with your doctor.