I’m going to do a little bit of basic high school physics and imagine that the force is coming down onto the tibia at about 30 degrees (such as when the patient’s walking down a step) with a medial ball and socket, a cruciate retaining, and a cam and post knee design. The medial ball and socket knee is inherently stable in its articulation, but the cruciate retaining and the cam and post are unstable.
So, the force, instead of being normal to the articular surface, is at an angle, let’s say 30 degrees, which means some of the force is shear and some of the force is normal. Whereas in the medial ball and socket, we have zero shear and in the other designs we have a shear force.
If you’ve got ligaments that are well balanced, you have a stable knee. If you have a poorly balanced knee, with ligaments that are loose, the cruciate retaining and the cam and post knees will become unstable, but the medial ball and socket knee will still be stable because it’s an inheritably stable design. This also has implications for contact stress.
The registry results of medial ball and socket knees are all equal to or better than the total knees on the Australian registry. So, my conclusion is that ligament balancing is not critical in a medial ball and socket knee, because the bearing is inheritably stable.
Dr. Nam: We are not discussing whether medial pivot designs are relevant. We are talking about ligament balancing. It doesn’t matter what kind of insert you put in, if it’s not balanced, it’s not going to pivot around the medial side.
We all know that balance in knee replacement is crucial; the goal is to create symmetric and equal flexion and extension gaps and to have ligament and soft tissue balance. Regardless of your technique, whether you do gap balancing or measured resection or some type of hybrid, you want a balanced, stable knee.
Inadequate balancing has several consequences including abnormal kinematics, difficulties with range of motion, instability and recurrent diffusions, and also polyethylene wear because you can get condylar liftoff which can accelerate your wear. You need to have balance in the sagittal and coronal planes, and we know that instability remains a leading cause of early failure after total knee arthroplasty.
Even if you’re using implants with increased intraarticular constraint or even a hinge prosthesis, I would go as far to argue that you still want to try to balance this knee and balance the soft tissues.
So, it’s hard to believe that simply putting an insert with more congruency in the medial side of the knee would obviate the need for ligament balancing.
As Dr. Walter alluded to, the medial compartment is ultra-congruent in this ball and socket design, it allows translation of the lateral condyle, and the goal is to avoid paradoxical anterior slide. This is contrary to the four-bar linkage theory in which the femur symmetrically rolls back on the tibia both medially and laterally.
In theory, this should enhance quad power and improve physiologic kinematics, and I agree there have been encouraging results in several studies demonstrating excellent survivorship, good Knee Society Scores and also potential improvement versus other design concepts.
But I think it’s important for us to determine what these studies show and how they got to these results. If we go to Dr. Walter’s website, looking at knee replacement design concepts, he says that medial pivot designs do better.
A study by Professor Fares Haddad (CORR 2011) compared medial pivot and posterior stabilized knees and showed that medial pivots had improved range of motion and total knee function scores but there is no difference in Knee Society, WOMAC, or Oxford scores.
Looking at the surgical technique, Haddad and his colleagues utilized the mechanical alignment technique. They externally rotated their femoral components, and if you look at the methods of how they achieved these results, they specifically say, soft tissue balance was assessed in flexion and extension using the implant’s respective supplied spacer device and flexion and extension gaps were equalized in all cases using a sequential approach, so they balanced their knee replacements.
Looking at another study which demonstrated excellent long-term survival (Macheras, et al., Knee 2017). The study authors balance the flexion gap based on the tightness of the PCL, they resected it in some and retained it in others. They checked coronal stability with varus-valgus stress tests. They checked sagittal stability with a flexion distraction test and thus they balanced the knee. So the reported results of the medial pivot knee have shown that in the methods of these studies, you still need to balance your knee for this to work.
