All primary knee replacements bend and rotate. Most also slide. Except for some limited revision, or “re-do” settings, all knees bend, rotate and slide. The mechanism by which these knees bend, and rotate, and slide varies from design to design.
The normal motion of a human knee is a complex combination of bending, rotating and sliding. Theoretically, the artificial knee that most closely approximates this complex motion is the one that will feel most normal. This is in fact the argument put forward by DePuy in their advertising. However, even under ideal conditions, the DePuy knee can never completely replicate normal knee motion.
If one were to plot the contact points between the femur and the tibia for a variety of different degrees of flexion of the knee, one would see a fan shaped distribution, with mostly roll demonstrated laterally (away from the middle of the body), and mostly sliding demonstrated medially (towards the middle of the body). Two knees currently recreate this fan shaped motion, each by a unique mechanism. These are the Journey by Smith and Nephew and the medial Spherocentric from Wright Medical. The DePuy rotating platform knee has a fixed center of rotation in the middle of the knee, so it cannot reproduce this fan motion. That not withstanding, since the patents expired on the DePuy rotating platform knee, Zimmer and others have basically copied it for some of their knee models.
It remains a point of considerable debate, to what extent the patient can actually feel minor differences between the motion of the artificial knee and the normal knee.
DePuy also makes a claim about this knee that by increasing the conformity of the surfaces they reduce wear debris. All knees currently available have at least one plastic piece. This eventually wears out. The question of wear debris is extremely complex, and is not completely understood by anybody--not yet. Most total knee designs have one surface where all of the “slipping and sliding” occurs. Rotating hinge knees (and other “mobile bearing” designs) have two.
The size of the debris is very important. Very small particles cause more trouble with the adjacent bones than do larger particles. Highly conforming surfaces tend to create smaller particles. It may well be that the plastic which performs best in the standard knee design is different from that which performs best in the mobile bearing knee. Again, the “best” is not really known.
The bottom line here is that all current designs are at least “good”. It is far more important to select your surgeon well. If there were a “perfect design” and it were put in poorly, it won’t perform well. Dr. Harris selects the knee model from a variety of manufacturers that would fit the individual patient best. As with all his decisions, the knee model selected for an individual patient is the same as he’d want for his own family in the same setting.
