Knee Replacement Implants - OrthoInfo - AAOS

For simplicity, the knee is considered a hinge joint because of its ability to bend and straighten like a hinged door. In reality, the knee is much more complex because the bone surfaces actually roll and glide as the knee bends.

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Current implant designs recognize the complexity of the joint and more closely mimic the motion of a normal knee. For example, ligaments keep the joint stable in a healthy knee. Some implant designs preserve the patient's own ligaments, while others substitute for them.

Several manufacturers make knee implants and there are more than 150 designs on the market today. Although various manufacturers market their implants to the public, and while surgeons may have a preference for a particular implant, there is no strong evidence that one brand or design is superior to the others in terms of function or longevity. There are advantages and disadvantages of each design, which should be discussed with your surgeon.

Posterior-Stabilized vs. Cruciate-Retaining Designs

Posterior-Stabilized Designs. In this design, the cruciate ligaments are removed and parts of the implant substitute for the posterior cruciate ligament (PCL).

In posterior-stabilized implants, the tibial component has a raised surface with an internal post that fits into a special bar (called a cam) in the femoral component. These components work together to do what the PCL does: prevent the thighbone from sliding forward too far on the shinbone when you bend your knee. 

Certain types of posterior-stabilized designs have an extra-large post that helps substitute for the function of the medial and lateral collateral ligaments as well.

Cruciate-Retaining Designs. As the name implies, the PCL is preserved with this implant design (the anterior cruciate ligament [ACL] is removed). Cruciate-retaining implants do not have the center post and cam design. This implant may be appropriate for a patient whose PCL is healthy enough to continue stabilizing the knee joint.

At this time, there is no research to show that posterior-stabilized implants last longer or provide better results than cruciate-retaining implants, or vice versa.

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Mobile-Bearing vs. Fixed-Bearing Implants

Whether they are posterior-stabilized or cruciate-retaining designs, knee implants can also be classified as mobile-bearing or fixed-bearing.

Fixed-Bearing Implants.  In this design, the polyethylene of the tibial component is attached firmly to the metal implant beneath. The femoral component then rolls on this cushioned surface.

Mobile-Bearing Implants. In a mobile-bearing knee, also known as a rotating platform, the polyethylene insert can rotate short distances inside the metal tibial tray. This is designed to allow patients a few degrees of greater rotation to the medial and lateral sides of their knee.

Both fixed- and mobile-bearing designs have many theoretical advantages and disadvantages. However, after more than 20 years of clinical research into each design, neither design has proven to be superior to the other. Talk with your surgeon if you have questions about fixed- or mobile-bearing implants.

Bicruciate-Retaining Designs

In most total knee replacement procedures, the ACL is removed to allow for precise placement of the implant. In bicruciate-retaining designs, both the ACL and PCL are preserved. The rationale for this type of design is that by saving both ligaments, the knee will function and feel more like a non-replaced knee.

Bicruciate-retaining components are relatively new to the market and there are no high quality studies as yet that demonstrate the specific advantages of this design.

Unicompartmental Implants

Although replacing the total knee joint is the most common procedure, some people can benefit from just a partial knee replacement.

If only one side of the knee joint is damaged, smaller implants can be used (unicompartmental knee replacement) to resurface just that side.

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