All joint replacement designs have at their core the work of pioneer orthopedists that culminated in the quintessential prototype of the artificial joint which was the hip. In classic terms this involved a cup made of polyethylene, a ball (or head) of cobalt chrome chosen for its hardness and a stem to mount the ball into the femur. As these concepts evolved many of the other modifications to the joint concepts were added and modified to accommodate the knee, shoulder, elbow, wrist, ankle, digits and even intervertebral disc of the spine. Which is to say that while John Charnley's concepts are as relevant today as they have ever been, they have had to have been modified somewhat to accommodate the rigours of these specific joints. In all instances the ultimate tradeoffs are range of motion(ROM), stability and wear. In other words, a joint designed with high range of motion typically is less stable and results in more wear. A joint designed with high stability has less ROM and more wear outside the joint. A joint with less wear has less range of motion and but is very stable all other things remaining the same. The only way to to get high range of motion, with high stability as in newer concepts in the hip is to change the material so they are resistant to wear (eg.ceramic on ceramic articulation).

The Hip
At its core, the hip is a ball and socket joint. The socket or cup can be an all plastic cup or a metal cup into which sits a plastic ceramic or metal liner. The ball is typically cobalt chrome, steel or ceramic. The classic articulating couple her is a metal head on a polyethylene cup. The head at 28mm was classically thought to offer the best balance of volumetric and linear with stability. In recent times with higher patient demand there has been a vogue to increase the size of the head. This results in increased range of motion and stability but much greater wear. To overcome wear harder materials have been developed resulting in ceramic on ceramic, metal on metal and ceramic or metal on highly cross linked polyethylene. All these materials are prone to cracks by virtue of their hardness.

The Knee
While the same concepts of material wear may be applied to the knee replacement endoprosthesis, size increase to increase range of motion is not so relevant. The knee is a hinge joint but it also has a rotational element to it. To make a knee flex more deeply one needs to create an increased curve on the condyle in the back and in the frontal plane make it more ball like (Scorpio NRG). Alternatively a rotating platform may be created on the tibia (Depuy RPF flex). Bothe these approaches have their proponents. Changing materials can improve wear rates but this has received less immediate attention as present designs are very resilient (ie. if it isn't broken don't try to fix it).