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 uni-compartmental 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

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 or bow legs):

• In non-constrained (e.g. cruciate-retaining) prostheses, the upper and lower parts 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 stable enough because they still coordinate the movements of the knee – like 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).

Full knee replacement surgery is almost always done using a non-constrained or semi-constrained prosthesis. 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, knee prostheses are usually cemented in place. Sometimes one component is cemented and the other is cementless.

What are the pros and cons of partial and total knee replacements?

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

Both types of knee replacement have their own pros and cons, as summarized in the following table:

Sources

• Beard DJ, Davies LJ, Cook JA et al. Total versus partial knee replacement in patients with medial compartment knee osteoarthritis: the TOPKAT RCT. Health Technol Assess 2020; 24(20): 1-98. [PMC free article: PMC7232134] [PubMed: 32369436]
• Grimberg A, Jansson V, Lützner J et al. Endoprothesenregister Deutschland (EPRD): Jahresbericht 2020. 2020.
• Wilson HA, Middleton R, Abram SG et al. Patient relevant outcomes of unicompartmental versus total knee replacement: systematic review and meta-analysis. BMJ 2019; 364: l352. [PMC free article: PMC6383371] [PubMed: 30792179]

• IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

  Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

  Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.