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The knee joint involves three bones; the femur, the tibia and the patella. In Total knee replacement, each one of these bones is resurfaced with a separate component. To “resurface” the bone the surgeon uses a saw guide and small power saw to trim off the outer worn surface of each bone in the exact shape of the inside of the corresponding implant.

The resected bone can be several millimeters thick (about 1/4-1/3 inch). The separate components work together to form the complete prosthetic device. In some cases the surgeon may choose not to replace the patella and let your native patella mate with the other artificial components (this is quite common in Europe, less common in the US).

The Femoral Component

Typically this is a Cobalt-Chrome metal component. The highly polished outer surface serves as the end of the femur, the other side is either “cemented” or “press-fit” onto the prepared bone surface.

Most implants in the US today are cemented. A polymer based compound is mixed in the OR forming a putty which hardens. The surgeon puts the putty in between the implant and bone to secure it.

In the “press-fit” variety, a roughened surface on the inside of the component has a porous 3-D structure designed to promote bony ingrowth.

There are pros and cons to each type of fixation. As mentioned, the cemented version is more common, but both have well established clinical histories

The Tibial Component

The tibial component is usually a two-piece metal and plastic implant although there are all plastic versions available.

The metallic portion of the implant can be titanium or cobalt-chrome and it is fixed to the tibial bone in much the same way as the femoral implant.

The plastic component is known as a “tibial bearing” or “tibial spacer” or simply “tibial insert.” It is a plastic component made of a special grade polyethylene (UHMWPE). Manufacturers have begun to “treat” the plastic with gamma radiation to improve wear properties.

The Patellar Component

The patellar component is typically an all plastic component that is fixed to the cut surface of the underside of the patella or knee cap.

The component is secured with bone cement and articulates or “joins up” with the femoral component, as it would with the normal knee.

Because the patella moves up and down across the surface of the femoral component, the relative alignment of the two components is critical. Patella problems are among the most common complications following total knee replacement, although the overall complication rate is small.

Primary Knee Replacement

Primary Knee Replacement, as the term suggests, the name given to a family of knee implants designed to be used as a first knee replacement. This type of knee replacement allows the remaining soft-tissue to provide some of the constraint to the knee joint and is usually implanted in patients who have not had a previous Total Knee. The term constraint, does not imply limiting movement, rather it simply allows some of the normal stabilizers in the knee (namely the collateral ligaments, and sometimes the PCL) to still perform their function.

Revision Knee Replacement

Revision Knee Replacement is designed to replace a worn-out and/or loosened Primary Knee Replacement. This type of knee replacement has more intrinsic constraint and can “play the role” of some of the natural stabilizers which may be absent or no longer functioning. In some cases, a revision knee implant may be used as a patient’s first knee implant if the disease is advanced enough.

PS Knee Implants

PS, Posteriorly Stabilized, or PCL Substituting Knee Implants are designed to be implanted with the Posterior Cruciate Ligament or PCL removed. In this type of knee replacement, a protrusion on the tibial insert is designed to mate with a specially shaped bar (cam) on the femoral component. This “cam and post” interaction substitutes for the normal function of the PCL by:

Limiting anterior or “frontward” movement of the femur (thigh) relative to the tibia (shin).

Promoting posterior or backward movement of the femur relative to the tibia as you bend or flex your knee. This movement is necessary to allow for deep flexion of the knee.

Studies have shown that PS knees tend to more predictable and provide for slightly higher degrees of flexion compared to CR knees (see CR section). This is because the function of the knee is dependent on a fixed metal and plastic mechanism rather than the native PCL whose function and balance can be variable in a diseased knee.

For some photos and description of a PS knee you can visit:
http://www.stryker. com/jointreplacements/sites/scorpioknee/scorpiops.php

CR Knee Implants

CR or Cruciate Retaining Knee Implants are designed to be implanted with an intact Posterior Cruciate Ligament or PCL. Often this ligament is still functioning to some degree in patients with advanced osteoarthritis. In this type of knee replacement, the PCL is allowed to act as the primary stabilizer limiting anterior or “frontward” movement of the femur (thigh) relative to the tibia (shin). This is the primary function of the PCL in the normal knee. Another function of the PCL is to promote posterior or backward movement of the femur relative to the tibia as you bend or flex your knee. This movement is necessary to allow for deep flexion of the knee.

Studies have shown that CR knees tend to have less predictable and slightly lower degrees of flexion compared to PS knees (see PS section). This is because the function of the knee is dependent on the native PCL whose integrity and “health,” if you will, can be somewhat variable. That being said, there are studies that have shown that the performance of CR knees and PS knees are comparable. If you are so inclined, you can view abstracts of publications at:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi

For some photos of CR knee implants you can visit:http://www.stryker.com/jointreplacements/sites/scorpioknee/scorpiocr.php

femoral component knee replacement implants revision knee replacement

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