What different types of knee implants are there?

Photo of a patient and doctor during an examination (PantherMedia / Wavebreakmedia ltd) There are different types of knee implants (prostheses). The main difference is whether the implant replaces the entire joint or only a part of it. Both types of implants have advantages and disadvantages: The most suitable type will depend on various factors.

For instance, the choice of knee implant will depend 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. It's best to talk to your doctor about which types of implants would be suitable and what pros and cons they have.

What do knee implants consist of?

Knee implants basically consist 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 implant move. Some implants 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. 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 implants. 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.

 

Illustration: Front view of the right knee; Left: Partial knee replacement; Right: Total knee replacement – as described in the articleFront view of the right knee; Left: Partial knee replacement; Right: Total knee replacement

What are the different types of total knee implants?

Different types of implants 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) implants, the upper and lower part of the implant 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 implants, the anterior cruciate ligament is removed before they are implanted. This ligament is often damaged by the osteoarthritis anyway.
  • Semi-constrained (e.g. posterior-stabilized) implants are used if the posterior cruciate ligament isn't stable enough and has to be removed. In these implants, 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") implants, 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 implants 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).

In Germany, almost 90% of people who decide to have a joint replacement have a total knee replacement. In most cases, the posterior cruciate ligament isn't removed, and a non-constrained implant is used. Often, constrained models are only needed if the first knee implant has to be replaced by a second one. About 10% have a one-sided joint replacement.

How is the implant fixed in the joint?

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

  • Cemented joint implants 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 synthetic adhesive and not really cement.
  • Cementless implant 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 implants 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?

Sometimes a decision needs to be made between 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 advantages and disadvantages, as summarized in the following table:

  Partial (unicompartmental) knee replacement Total knee replacement
How long does the implant last? About 10% of these implants have to be exchanged within ten years. The advantage: It is then often possible to use a non-constrained or semi-constrained implant. About 5% of these implants have to be exchanged within ten years. Then a semi-constrained or constrained implant is often needed.
How long does it take to recover from surgery? The rehabilitation program 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 implant? About 3% have a complication in the first year after surgery. About 5% have a complication in the first year after surgery.