There are multiple types of knee implants that doctors and patients can discuss before choosing the best option.
Regardless of type, all knee implants have certain things in common.
All consist of a component that attaches to the thigh bone and one that attaches to the tibia, one of the two lower leg bones. A knee replacement implant may or may not have a patellar component mimicking the kneecap.
Some components, particularly the bearings, may be made of ceramics or a mixture of ceramics and metal. The materials have to be “biocompatible,” meaning they cannot trigger the body’s rejection response. Knee implants are relatively lightweight — usually between 15 and 20 ounces.
Deciding on Types
The surgeon makes the decision for the best type based on a patient’s condition, activity level and other factors.
Fixed Bearing
This is the implant most patients receive. Less mobile and not as strong as other implants, it is best suited for patients who are not overweight and who do not have an active lifestyle. It is usually recommended for elderly patients who will not put serious wear-and-tear on the implant.
FACT
In a fixed bearing design, a metal implant is attached to the tibia and a polyethylene component is attached atop it, creating a cushioned surface.
Another component is attached to the femur, where it can roll on the polyethylene.
The drawback is that this contact causes wear on the polyethylene. Studies have shown that this stress is a leading cause of fixed bearing long-term implant failures. It wears away the polyethylene component, causing the implant to loosen. Loosening is not only painful, it is also a major cause of implant failure.
Studies have shown that 95% of fixed bearing implants continue to function properly 10 years after patients received them.
Mobile Bearing (rotating platform)
This design allows the plastic cushion of the tibial component to rotate, giving patients greater flexibility on the medial (inner) and lateral (outer) sides of the knee. Mobile bearing implants work well for young and active patients.
The polyethylene component in mobile bearing implants is designed to fit into the metal tibial tray component in a way that allows limited rotation. The design was introduced to limit the polyethylene wear seen in fixed-bearing designs. It produces less stress between components and in theory should reduce chances of — or at least delay — loosening.
Mobile bearing implants tend to have less stress between the femoral and tibial surfaces, granting the flexibility to benefit a patient’s gait and even deep knee bends. However, their long-term durability is about the same as for fixed bearing implants in older patients.
Like the fixed-bearing design, mobile bearing knee implants have a 95% survival rate 10 years after being implanted.
Medial Pivot (mobile bearing)
The medial pivot design incorporates a mobile bearing to more effectively replicate the natural function of the knee. It rotates, twists, bends and flexes like a natural knee joint. More complicated than fixed or standard mobile bearing designs, it can be more expensive.
Illustration showing up-close view of a medial knee replacement
Instead of a “hinge joint” approach used by more conventional knee replacements, the components of a medial pivot implant incorporate a “ball-and-socket” approach. It tends to be more stable than some other implant designs. Because it allows for a large contact area between components, it reduces wear-and-tear, extending the life-expectancy of the implant. But this design is more dependent on tissue and surrounding ligaments to prevent dislocation. This can result in tissue becoming caught in the implant, which can cause pain.
A 2017 study of 325 patients with a total of 347 medial pivot knee implants showed good long-term results for the design. Researchers conducted follow-ups with patients over an average of 15.2 years. They found that 98.8% of medial pivot implants survived to 17 years and 94% of patients were able to do age-appropriate activities, with an average knee bend of 120 degrees, in their final follow-up sessions.
Posterior Cruciate Ligament (PCL) Retaining (nonconstrained)
The posterior cruciate ligament (PCL) runs along the back of the knee, connecting the femur to the tibia. This design keeps the PCL intact, but it depends on several patient factors: good bone quality, few defects in the bones, intact soft tissue around the knee and a functional PCL.
There are two types of PCL implants: retaining and substituting. The difference between the two comes in part from how the PCL is affected by the implant surgery. A surgeon may remove the ligament to implant a PCL substituting implant. But if the ligament is in good condition, the surgeon may want to preserve it. In that case, he or she will use a PCL retaining implant.
FACT
Preserving the ligament may not necessarily result in better postoperative knee function.
Preserving the ligament may not necessarily result in better postoperative knee function. A simple polyethylene bearing attaches to a metal component implanted in the tibia, and a metal implant in the femur hinges on the polyethylene. It is a minimal design dependent on the PCL to stabilize the implant, but a tight PCL may lead to excessive wear on the bearing.
Less bone is removed with a PCL retaining implant. A PCL retaining implant is less likely than a PCL substituting version to result in a condition called “patella clunk” syndrome. This syndrome results from scar tissue forming near the implant and becoming caught in part of the device as the knee is fully extended.
A 2015 study reviewed 15 years of follow-ups of patients who had received PCL retaining implants. It found 98.7% of the implants survived 10 years and 83.6% survived 17 years.
Posterior Cruciate Ligament (PCL) Substituting (constrained)
This design replaces ligament with plastic components. It is recommended primarily for patients with severely damaged knees or weak ligaments. Also used in revision surgery.
It is indicated for patients with severe knee deformities or who have had previous knee replacement surgery or have had a knee cap removed.
The PCL substituting design has a tall post attached to the tibia that fits into a deep box attached to the femur. The two are not connected with any kind of hinge joint. A polyethylene bearing attaches to the tibial component.
DESIGN
This design may allow for more knee flexibility than the PCL retaining design, but it may also release more polyethylene debris because of friction between its components.