Prosthesis Choice

There is constant evolution in knee replacement design but not all of it is for the better. There are many different models to choose from and all of them work very well nearly all the time. Your surgeon will probably choose a prosthesis for you based on his own experience and what he is comfortable using.

Prosthesis Types

Cruciate Retaining (CR)

Firstly, there is the long tried and proven round femoral on flat tibial style which requires that the posterior cruciate ligament is preserved. So the design is called Cruciate Retaining (CR). In fact, the evidence suggests that that ligament does not function about 80% of the time when its preservation has been attempted, but remarkably, most of the time, that doesn’t seem to significantly alter the final outcome, so this version of the procedure is still very popular an effective.

Posterior Cruciate Substituting (CR)

The second type of knee replacement works on the principle that the design makes up for what is expected to be an ineffective posterior cruciate ligament. This ensures that the joint rolls and glides in a relatively normal way, so that a better range of movement is achievable, provided that the soft tissues around the knee also allow the extra degree of bending. These knees are called Posterior Cruciate Substituting (PS) designs. So, if the function that you want from your knee requires 120 degrees or so of movement, and you knee is currently able to bend at least that far, there is an argument in favour of this design. It may also give better strength for stair climbing. But it is not for everybody, and statistically, this design is not quite as reliable as the cruciate retaining or rotating platform designs.

Rotating Platform (Posterior Cruciate Sacrificing)

The third type of design works on the principle that the most important thing is not necessarily extra range of movement, but durability. This style has greater congruity between the plastic liner, and the curve of the femoral component, so that there is less pressure on the plastic, and therefore a slower rate of wear.

This design is particularly applicable for younger, heavier, more active people, and for those people whose knees would not be expected to be able to bend past 110 degrees or so anyway, because of the soft tissues around the knee. To allow the normal small amount of rotational movement of the joint, these designs have the facility for the plastic liner.

Finally, there is still controversy over whether it is a good idea to routinely insert a plastic surface for the back of the knee-cap. Those who advocate this argue that there is a lower rate of residual pain from the patella as a result. Those who prefer not to routinely resurface the patella, argue that the resurfaced patella tends to be a source of more problems in later years, and that only 5-10% of people have significant patella pain problems if the patella is not resurfaced. 

We know that the appearance of the patella at the time of the operation is no guide to whether there will be pain from that area afterwards, but maybe it makes sense to select those people who have pain from the patella before the operation to have their knee-caps resurfaced, rather than to have a blanket rule one way or the other. 

In addition, people with inflammatory joint disease such as rheumatoid arthritis should generally have the patella resurfaced, or there would still be part of the joint for the disease to attack. Overall, in Australia, about half of all total knee replacements have the patella resurfaced, and half don’t, based on the preference of the surgeon for the most part, but also on surgeons’ best attempts to predict who would need this.