ggThe knee is a complex joint because the surfaces roll and glide as the knee bends. The first implant designs used the hinge concept and literally included a connecting hinge between the components such as in the Geupar prosthesis. Newer implant designs, take into account the complexity of the joint, attempt to allow for the more complicated motions of the knee and take advantage of the posterior cruciate ligament (PCL) and collateral ligaments for support.
     Up to three bone surfaces may be replaced during a TKA: 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 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 or unicondylar knee replacement) to resurface just that part of the bone. Some designs (posterior stabilized designs) have an internal post with a circular-shaped device that works with a corresponding tibial component to help prevent the thighbone from sliding forward too far on the shinbone when you bend the knee. The tibial component is a flat metal platform with a polyethylene cushion. The cushion may be part of the platform (fixed) or separate (mobile) with either a flat surface or a raised, sloping surface. Other designs use metal-metal, ceramic-ceramic or metal-ceramic tibial and femoral interfaces rather than metal on polyethylene.  The patellar component is a dome-shaped piece of polyethylene that duplicates the shape of the kneecap anchored to a flat metal plate [3].


Diagram of the knee with a duocondylar implant [3]