This week’s Orthopaedic Crossfire® debate was part of the 34th Annual Current Concepts in Joint Replacement® (CCJR®), Winter meeting, which took place in Orlando. This week’s topic is “Medial Pivot Designs Avoid the Need for Ligament Balancing” For is William L. Walter M.D., F.R.A.C.S., Ph.D., Sydney Hip & Knee Surgeons, Waverton, Australia. Opposing is Denis Nam, M.D., Rush University Medical Center, Chicago, Illinois. Moderating is Thomas S. Thornhill, M.D., Harvard Medical School, Boston, Massachusetts.
Dr. Walter: The survival rate for total knee arthroplasty at 15 years is very good at over 95%. Can we improve things further? Well, of course, the problem is satisfaction with only 80% of patients being satisfied.
Medial ball and socket knees or medial pivot knees have something unique. They have sagittal stability and patella mechanics that are different from other knee designs and I think that this helps improve patient satisfaction.
Mike Freeman, who recently passed away, and Vera Pinskerova looked at a series of MRI scans and projected the centers of rotation of the femur onto the tibia and found that the medial side doesn’t move much but the lateral side translates as a voluntarily but not obligatory rotation.
In full extension, there’s no rotation. In the mid-range there’s an optional rotation of 20-30 degrees of the knee—and this occurs around the medial side.
The knee really has two degrees of freedom.
It has a flexion axis, which we all know about, but it also has a rotation axis which goes through the medial compartment, through the medial femoral condyle.
The medial pivot concept takes this concept and puts it into a knee design with a spherical medial condyle. The lateral side which is a conforming cylinder which permits some laxity and rotation and acts like an outrigger. The medial ball and socket knee is stable throughout a full range of motion and does not allow any anterior translation on the medial side.
The other thing is that it’s fully conforming throughout a full range of motion, so you have full contact throughout a full range of flexion up until the posterior flange leaves and then it reduces a bit. It has this stability not just in parts of the rotation but in a full rotation range and it’s the anterior and posterior lips that give it the stability.
What about the other types of knee design?
We have posterior stabilized knee designs which have a cam and post, but these only engage in deep flexion, so in functional positions such as 30-50 degrees of flexion, there is no anterior/posterior stability.
We have cruciate retaining types which are a round on flat. We have mobile bearings, bi-cruciate substituting which is the same as posterior stabilized, and the medial rotation knee.
