Knee Replacement

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.

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 polyethelyne component, causing the implant to loosening. Loosening is not only painful, it is also a major cause of implant failure.

Studies have shown 95 percent 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 this 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 or at least delay loosening.

Mobile bearing implants tend to have less stress between the femoral and tibular 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 percent survival rate ten 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.

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 and because it allows for a large contact area between components, it reduces wear-and-tear extending the life-expectancy of the implant.

A 2017 study of 325 patients, with a total of 345 medial pivot knee implants, showed good long-term results for the design. Researchers conducted follow ups with patients over an average 15.2 years.

They found 98.8% of medial pivot implants survived to 17 years and 94 percent 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 (non-constrained)

The posterior cruciate ligament (PCL) runs along the back of the knee, connecting the femur to the tibia. This designs 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.

Preserving the ligament may not necessarily result in better post-operative 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.

A 2015 study reviewed 15 years of followups of patients who had received PCL retaining implants. It found 98.7 percent of the implants survived 10 years and 83.6 percent 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.

The PLC substituting design has a tall post attached to the tibia which 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.

This design may allow for more knee flexibility that the PCL retaining design, but it may also release more polyethylene debris because of friction between its components.

A 2016 study of the PLC substituting design found 88.5 percent of PLC substituting implants used in initial knee replacement surgery survived 10 years. For revision surgery, the 10 year survival rate was 75.8 percent if the revision surgery was due to some cause other than infection, and 54.6 percent if revision was due to infection.

Gender specific

Women undergo more knee replacements than men – accounting for roughly two-thirds of all knee replacement surgeries in the U.S. Surgeons performing knee replacements have long reported anatomical differences between men and women which make a “one-size-fits-all” approach ineffective in knee replacement surgery.

"Female anatomy is a little different," Dr. Anthony Unger, a surgeon who has performed hundreds of knee replacements told NPR in 2007. "The end of the bone called the femur (thigh) is a little different contour."

Modifying the way implants fit between men and women can tailor the device’s performance to the patient.

Many women may be shorter or have smaller body frames than the overall population. Gender-specific implants, engineered to the various, different dimensions of male and female body frames are designed to perform better for specific patients.