The largest, strongest and heaviest joints in the body, knees provide support and mobility and carry almost half the body's weight.
Functioning like a hinge where the lower end of the femur (thighbone) rotates on the upper end of the tibia (shinbone) and patella (kneecap), a healthy knee lets you move your lower leg forward and backward, and swivel slightly to point the toes in or out. Ligaments and cartilage stabilize and support the joint, preventing it from moving too far from side to side.
If osteoarthritis wears away a knee joint's articular cartilage, your doctor may recommend total knee arthroplasty (replacement), a common and successful procedure that improves knee motion and lets you resume relatively normal activities without pain. An orthopedic surgeon resurfaces the knee joint, replacing damaged and worn weight-bearing surfaces with a prosthesis (implant) made of metal alloys or ceramic material and high-density plastic parts which may be joined to bone by acrylic cement.
Knee replacement surgery is generally recommended for patients with severe knee pain and disability caused by damage to cartilage from rheumatoid arthritis, osteoarthritis, or trauma. It is highly successful in relieving pain and restoring joint function.
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 surfaces actually roll and glide as the knee bends. The first implant designs used the hinge concept and literally included a connecting hinge between the components. Newer implant designs, recognizing the complexity of the joint, attempt to replicate the more complicated motions and to take advantage of the posterior cruciate ligament (PCL) and collateral ligaments for support.
There are more than 150 knee replacement designs on the market today. Several manufacturers make knee implants. 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), the doctor's experience and familiarity with the device, and the cost and performance record of the implant. You may wish to discuss these issues with your doctor.
Up to three bone surfaces may be replaced during the total replacement of the knee: the lower ends (condyles) of the thighbone, the top surface of the shinbone, and the back surface of the kneecap. Components are designed so that metal always articulates against plastic, which provides smooth movement and results in minimal wear.
The three components include:
- Femoral: a polished, strong metal shell on the lower end of the thighbone.
- Tibial: a high-density polyethylene piece on top of a metal tray.
- Patellar: a high-density polyethylene piece replacing the underside of the kneecap in the center of the knee.
The metal femoral component curves around the end of the thighbone and has an interior groove so the kneecap can move up and down smoothly against the bone as the knee bends and straightens. Usually, one large piece is used to resurface the end of the bone. If only one side of the thighbone is damaged, a smaller piece may be used (unicompartmental knee replacement) to resurface just that part of the bone. Some designs, called posterior stabilized, have an internal post with a center cam. This works with a corresponding tibial component to prevent the thighbone from sliding forward too far on the shinbone when you bend the knee. Designs which do not have the center cam are called cruciate-retaining.
The tibial component is a flat metal platform with a polyethylene cushion. The cushion may have either a flat surface (cruciate-retaining) or a raised surface with a center cam (posterior-stabilized).
The patellar component is a dome-shaped piece of polyethylene that duplicates the shape of the patella (kneecap).
Adapted from the American Academy of Orthopaedic Surgeons (AAOS).