Total Knee Replacement (TKR)

Knee Replacement Implants

Your doctor may recommend knee replacement surgery if you have severe knee pain and disability from rheumatoid arthritis, osteoarthritis, or traumatic injury. A knee replacement can relieve pain and help you live a fuller, more active life.

During the surgery, an orthopaedic surgeon will replace your damaged knee with an artificial device (implant). Although replacing the total knee joint is the most common procedure, some people can benefit from just a partial knee replacement.

Implants are made of metal alloys, ceramic material, or strong plastic parts, and can be joined to your bone by acrylic cement. There are many different types of implants. Your surgeon will discuss with you the type of implant that best meets your needs.

Implant Design

Normal Knee Function

Normal knee anatomy

Your knee is the largest and strongest joint in your body. The knee joint is where the lower end of your femur (thighbone) meets the upper end of your tibia (shinbone). Your patella (kneecap) sits in front of the joint to provide some protection.

A healthy knee lets you move your lower leg forward and backward, and swivel slightly to point your toes in or out. Ligaments and cartilage stabilize and support the joint, preventing your knee from moving too far from side to side.

Types of Designs

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.

The first implant designs used the hinge concept and included a connecting hinge between the parts. Newer implant designs recognize the complexity of the joint and more closely mimic the motion of a normal knee. Some designs preserve the patient's own ligaments, while others substitute for them.
Several manufacturers make knee implants and there are more than 150 knee replacement designs on the market today.
Recent developments in design include "gender specific" implants. A number of studies indicate that the shape and proportions of a woman's knee differ from those of a man's knee. As a result, several manufacturers have developed components for the end of the thighbone which more closely match the average woman's knee. However, there are no studies to show that "gender specific" implants last longer or provide better function than standard implants.

The Right Implant for You

The brand and design used by your doctor or hospital depends on many factors, including your needs (based on your age, weight, activity level, and health), your doctor's experience and familiarity with the device, and the cost and performance record of the implant. You should discuss these issues with your doctor.

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Implant Components

Knee implant components

Up to three bone surfaces may be replaced in a total knee replacement:

• The lower ends 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).

Posterior-stabilized component.

Components are designed so that metal always adjoins with plastic, which provides smooth movement and results in minimal wear.

Posterior-Stabilized Designs

In these designs, the cushion of the tibial component has a raised surface with an internal post that fits into a special bar (called a cam) in the femoral component. The posterior cruciate ligament is removed to fit the components to the bone. The pieces work together to do what the posterior cruciate ligament does: prevent the thighbone from sliding forward too far on the shinbone when you bend your knee.

Cruciate-Retaining Designs

Cruciate-retaining component.

As the name implies, the posterior cruciate ligament is kept with this implant design. Cruciate-retaining implants do not have the center post and cam design. This implant may be appropriate for a patient whose posterior cruciate ligament is healthy enough to continue stabilizing the knee joint.

Unicompartmental Implants

In total knee replacement, large implants are used to resurface the ends of the femur and tibia. If only one side of the knee joint is damaged, smaller implants can be used (unicompartmental knee replacement) to resurface just that side.

In a unicompartmental (partial) knee replacement, only the damaged part of the knee is replaced.