4 types of knee implants

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Osteoarthritis of the knee: Learn More &#; What different types of artificial knee joints are there?

There are different types of artificial knee joints (prostheses). The main difference is whether the prosthesis replaces the entire joint or only a part of it. Both types of prosthesis have pros and cons. The most suitable type will depend on various factors.

The choice of knee prosthesis depends mostly on the condition of the ligaments, muscles, and bones. Age and weight play a role too &#; as well as how physically active you are. If you&#;re allergic to certain metals, coated implants are used. You can talk with your doctor about which options would be most suitable for you. It's also possible to get a second medical opinion before you decide.

What&#;s the structure of an artificial knee joint?

An artificial knee joint is made up of several parts:

• A metal cap that is placed on the end of the thigh bone (the condyle), like a crown on a tooth, after the damaged joint surfaces have been removed. This cap is also called the femoral (upper leg) component.

• A platform that replaces the top surface of the lower leg. This platform also usually consists of a metal alloy, and is called the tibial (lower leg) component. On its underside, there&#;s a short stem that is anchored in the tibia (shinbone).

• A plastic spacer made of polyethylene between the upper leg component and lower leg component. This replaces the joint space and meniscus and helps the artificial knee joint to move smoothly. Some prostheses use a fixed bearing, while others use a mobile bearing that can easily turn around its own axis or glide forward and back.

If the back of the kneecap is severely damaged by osteoarthritis, it can be replaced by a plastic cap as well.

What&#;s the difference between partial and total knee replacement?

Depending on whether one or several parts of the knee joint are affected by osteoarthritis, a partial or total knee replacement is considered:

• In a partial knee replacement, only one side of the lower end of the thighbone (the medial or lateral condyle) is replaced, along with the corresponding part of the top end of the shinbone opposite it. The medial side of the knee is the side that is closer to the other knee, and the lateral side is the one facing away from the other knee. Partial knee replacement surgery is only possible if both cruciate ligaments and the medial and lateral ligaments are still intact. Major misalignments like noticeable knock knees or bow legs can&#;t be corrected with these prostheses. Partial knee replacement surgery is also known as unicompartmental knee arthroplasty.

• In a total knee replacement, also known as total knee arthroplasty, both lower ends of the thigh bone (both condyles) and all of the top end of the shinbone (tibia) are replaced.

Front view of the right knee; Left: Partial knee replacement; Right: Total knee replacement

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In Germany, about 85% of people who decide to have a knee replacement have a total knee replacement. About 10% have a one-sided knee replacement. Doctors estimate that a partial knee replacement is an option for 25 to 50% of all people who have osteoarthritis of the knee.

What are the different types of total knee replacement prostheses?

Different types of prostheses can be used in total knee replacement surgery. The most suitable type will depend on things like the stability of the ligaments, the condition of the bones and muscles and any misalignments (knock knees, bow legs):

• In non-constrained (e.g. cruciate-retaining) prostheses, the upper and lower part of the prosthesis aren't connected to each other. For this type of implant to work, the medial and lateral ligaments (and, for some implants, also the posterior cruciate ligament) have to be relatively stable because they still coordinate the movements of the knee &#; bending, straightening, and rotating it. For most types of prostheses, the anterior cruciate ligament is removed before they are implanted.

• Semi-constrained (e.g. posterior-stabilized) prostheses are used if the posterior cruciate ligament isn't stable enough and has to be removed. In these prostheses, the two components are connected by a type of hinge that provides stability in place of the cruciate ligaments. The hinge determines how far the knee implant can be bent and straightened.

• In constrained (or "hinged") prostheses, the two components for the upper and lower leg are linked together with a hinged mechanism and attached to the bone on each side using a long stem. Constrained prostheses are used if both the medial and lateral ligaments aren't stable enough. Other reasons include poor bone condition and severe misalignments (knock knees or bow legs).

When doing full knee replacement surgery, a non-constrained or semi-constrained prosthesis is almost always used. Often, constrained models are only needed if the first artificial knee joint has to be replaced by a second one.

How is the prosthesis fixed in the joint?

Knee prostheses also differ in how the components are fixed in place:

• Cemented prostheses are fixed in place with a special two-component adhesive, which is also known as bone cement. But this name is misleading because &#; like the material used to fix a dental crown in place &#; the "cement" used is a type of synthetic glue and not really cement.

• Cementless prosthesis parts are pressed onto the bone. A special, rough surface or coating makes sure that the bone then slowly grows onto it.

In Germany, cemented knee prostheses are mainly used. Sometimes one component is cemented and the other is cementless.

What are the advantages and disadvantages of partial and total knee replacements?

Some people need to decide between a partial or total joint replacement. It's a good idea to talk with the surgeon about which one is better for you. Getting a second opinion can help too.

Both approaches have different advantages and disadvantages, as summarized in the following table:

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  Partial (unicompartmental) knee replacement Total knee replacement How long does the prosthesis last?
About 5 to 10% of these prostheses have to be exchanged within five years. The advantage: It is then often possible to use a non-constrained or semi-constrained prosthesis.

If the surgeon has a lot of experience with partial knee replacements, it's not very likely that the prosthesis will need to be exchanged. About 3 to 4% of these prostheses have to be exchanged within five years. Then a semi-constrained or constrained prosthesis is often needed. How long does it take to recover from surgery? The rehabilitation treatment often only involves outpatient physical therapy. You can usually return to most activities after about six weeks.
The surgery and hospital stay are followed by three weeks of rehabilitation treatment. You can usually return to most activities after about three months. But it can take up to one year for the muscles and ligaments to fully recover. How satisfied are people one year after having surgery?
About 90% of people who have had this type of surgery would choose a partial knee replacement again.
About 80% would choose a total knee replacement again.
How common are problems like infections or loosening of the prosthesis?
About 3% have a complication in the first year after surgery. About 5 % have a complication in the first year after surgery. How satisfied are people five years after having surgery? About 90% of people who have had this type of surgery would choose a partial knee replacement again. About 85 % would choose a total knee replacement again.
How common are complications during or after the procedure, such as blood loss that needs to be treated?
About 5% have a complication after or during the procedure. About 8% have a complication after or during the procedure.

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