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, ceramic material or high-density plastic parts which may be joined to bone by acrylic cement.
Fixed- vs. Mobile-bearing
Most people get a fixed-bearing prosthesis that reduces knee pain dramatically and may last for many years. Knee prostheses consist of three component parts that function together as a system:
- 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
In certain cases, excessive activity and extra weight can accelerate the process of wear to parts of a fixed-bearing prosthesis, causing it to loosen from the bone and become painful. Loosening is a major reason some artificial joints fail.
If you are younger, more active and/or overweight, sometimes a doctor may recommend a rotating platform/mobile-bearing knee replacement designed for potentially longer performance with less wear. Doctors also consider gender, occupation, disability level, pain intensity, interference with lifestyle and other medical conditions in selecting the appropriate prosthesis.
Difference is Bearing Surface
Like fixed-bearing replacements, mobile-bearing knees use three components to provide a relatively natural and even interface. The difference is the bearing surface. In a mobile-bearing knee replacement, both the metallic femoral component and metallic tibial tray move across a polyethylene insert to create a dual-surface articulation. The insert absorbs forces across a larger contact surface, helping reduce the amount of wear to the bearing and loosening in places where the prosthesis attaches to bone.
Mobile-bearing knee replacements can reduce early wear failure caused by high contact stress and early loosening failure caused by over-constraint. The insert’s mobility ensures congruent contact between the femoral and tibial components and conformity of the surfaces that move together when you bend and rotate your knee during activity. The mobile-bearing insert lets you move the knee from both the thighbone and shinbone. You can also rotate the shinbone slightly.
Compared with fixed-bearing designs, mobile-bearing knee implants are less forgiving of imbalance in soft tissues. They may increase the chance of dislocation and may cost more than fixed-bearing implants.
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