The rotating component, the polyethylene insert (shown here at 15° of rotation), gives patients more mobility, much like a natural knee. For example, it lets them squat or kneel more easily.
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The Sigma Rotating Platform Knee from Depuy Orthopaedics Inc, Warsaw, IN, features a polyethylene cushion or wear surface between the femoral and tibial components that rotates up to 15° when the patient bends his or her knee. The design decouples flexion-extension and rotation into two unidirectional motions and converts shear forces into compression. This should reduce wear on the insert by 94% and let the replacement knee last longer. (Studies show that even after 20 years, more than 97% of these implants are still in use.) These types of knee implants may be of particular interest to women because their knees rotate more than those of men during normal movement. The components of the replacement come in a variety of sizes to match patient anatomy, and the entire implant weighs about one pound.