It is a common myth that all knee implants are the same. However, this is not the case. Knee implants differ in terms of materials, the amount of flexion they enable, longevity, clinical outcomes, availability of sizes and more. It is important for the patient, caregiver and the orthopedic surgeon to jointly discuss and decide which types of knee implants is best suited for the case at hand.
Knee Implant Components
A knee implant for total knee replacement has three components to fit the three bone surfaces that are replaced during the surgery. This is why an implant used in a partial knee replacement surgery (which involves only replacing one damaged portion of the knee) is different from that used in a total knee replacement surgery.
Femoral Component: This component is attached to the end of the femur. It ensures that the knee cap or the patella slides against the femur smoothly as the knee bends and moves.
Tibial Component: This component comprises a flat metal tray with a cushion made up of polyethylene, which is a sturdy form of plastic. However, some implants, known as rotating platform implants, don’t have the metal portion and attach the polyethylene to the bone directly. A stem inserted into the centre of the tibia may be used to augment and ensure stability of this type of implant. This type of implant has certain advantages which may be discussed with the orthopedic surgeon.
Patellar Component: This component goes on the knee cap and is usually dome shaped and made of polyethylene.
Knee Implant Designs
Knee implant designs are of two key types: cruciate retaining or posterior stabilizing.
The orthopedic surgeon will examine the patient’s posterior cruciate ligament—the large ligament that runs along the back of your knee and provides flexion. Depending on its condition, the surgeon may recommend preserving or removing it during a total knee replacement surgery. Different types of implants exist to address either option.
Cruciate-Retaining: The posterior cruciate ligament helps the patient flex his/her knee. The cruciate retaining implant design enables the surgeon to preserve the cruciate ligament while operating. This type of implant is favored by surgeons who are familiar with a particular type of surgical technique.
Posterior Stabilized: The posterior stabilized implant design uses a cam and post system to substitute for the posterior cruciate ligament once the ligament is removed. The implant design provides additional support to the back of the knee and enables flexion.
Knee Implant Materials
While implant materials do vary from company to company, it is likely that the metal parts of the implant are made of titanium or cobalt chromium. Polyethylene is usually used for the plastic portions. An average implant weighs about 0.4 to 0.5 kgs.
An implant can be fixated (all the components can be attached together) using something called bone cement. Most orthopedic surgeons use bone cement for fixation. However, cementless fixation is also an option. Cementless fixation involves implants, which can be fixated by promoting natural bone growth onto its surface, enabling a more natural fixation. These implants are coated with a porous material, which promotes bone growth. Alternately, screw and pegs can be used and then later removed to stabilize the implant initially.
According to some surgeons, the recovery period can be longer with the use of cementless fixation implants. In case the patient has osteoporosis, cementless fixation is usually not advised since the success of the surgery depends on the ability of the body to promote natural bone growth.