During knee replacement surgery, an orthopaedic surgeon will resurface your damaged knee with artificial components, called implants.
There are many different types of implants. The brand and design used by your doctor or hospital depends on many factors, including:
• Your needs, based on your knee problem and knee anatomy, as well as your age, weight, activity level and general health
• Your doctor's experience and familiarity with the device
• The cost and performance record of the implant
Your surgeon will discuss with you the type of implant that will be used for your knee replacement surgery.
In total knee replacement, damaged bone and cartilage are removed and replaced with metal components that recreate the surface of the joint.
Implants are made of metal alloys, ceramic material, or strong plastic parts. Up to three bone surfaces may be replaced in a total knee replacement:
A total knee replacement implant.
• The lower end of the femur. The metal femoral component curves around the end of the femur (thighbone). It is grooved so the kneecap can move up and down smoothly against the bone as the knee bends and straightens.
• The top surface of the tibia. The tibial component is typically a flat metal platform with a cushion of strong, durable plastic, called polyethylene. Some designs do not have the metal portion and attach the polyethylene directly to the bone. For additional stability, the metal portion of the component may have a stem that inserts into the center of the tibia bone.
• The back surface of the patella. The patellar component is a dome-shaped piece of polyethylene that duplicates the shape of the patella (kneecap). In some cases, the patella does not need to be resurfaced.
Components are designed so that metal always borders with plastic, which provides for smoother movement and results in less wear of the implant.
In a healthy knee, ligaments support the joint.
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.
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.
Gender-specific implants. Many manufacturers have developed components for the end of the thighbone which more closely match the average woman's knee. At this time, there is no research to show that "gender specific" implants last longer or provide better function than standard implants.
One of the most commonly used type of implant in total knee replacement is a posterior-stabilized component. In this design, the cruciate ligaments are removed and parts of the implant substitute for the posterior cruciate ligament (PCL).
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.
- How long a knee replacement lasts depends on several factors, including activity level, weight, and general health.