In 1947, Shiers, a British Orthopaedic Surgeon, developed the first true knee joint replacement, which was a simple hinge joint, pivoting around a central pin. From this early design, other types of knee prostheses evolved, but they all functioned in much the same way as a basic hinge joint, with no attempt made to replicate the complex motion of a normal knee. This was primarily because there was little knowledge of the way in which a physiologically normal knee moves.
In most types of total knee replacement devices there are four parts:

a metal “femoral” component which is placed in the lower end of the femur
a metal “tibial” component which is placed on the upper end of the tibia
a plastic “insert” which is placed between the two metal components and effectively replaces the cartilage
a plastic “patella” component which is sometimes (though not always) used to resurface the back of the knee cap

Today, there are three main types of knee replacement devices:

Fixed Bearing Total Knee Systems:
both sides of the knee (the inside or medial compartment and outside or lateral compartment) are replaced in a single total knee replacement procedure, and with this type of device the plastic insert is fixed in place.

Mobile Bearing Total Knee Systems:
similar to that above, but with a plastic insert which is able to move within the joint, allowing more natural and complex knee movements and possibly creating less wear.

Unicompartmental knee systems:
this type of replacement is used where disease is limited to one compartment in the knee, and is effectively a ???half-knee??? replacement