Having revised several of those problematic knees, Dr. Pellegrini found that when he converted them to a fixed bearing prosthesis, the problem was typically solved. “I observed that the flexion gap with the knee at 90 degrees was asymmetric and not balanced medial to lateral. In all those that I revised, the lateral side was more lax, so it seemed that the issue was not so much balancing the extension gap to the flexion gap, but rather balancing the medial and lateral sides of the flexion gap.”
“This is because, for example, in single leg stance or when you sit, the medial compartment of the knee is compressed and the lateral compartment may be slightly gapped open. This situation, where the resulting lateral gap is slightly larger than the medial side, is not uncommon after TKA in a varus knee; we balance gaps, but it’s an imperfect art and patients can end up a bit more lax laterally than medially.”
“For this study, we performed TKA with an rotating platform implant in six fresh-frozen cadaveric limbs using a gap balancing technique, followed by sequential femoral component revision with variable-thickness polyethylene inserts to systematically represent 5 flexion-extension mismatch and asymmetry conditions.”
“Each configuration was subjected to mechanical loading at 0°, 30°, and 60°. Rotational displacement of the insert on the tibial baseplate, lateral compartment separation, and insert concavity depth were measured with use of a digital caliper.” Yield torque was used as a surrogate for ease of insert rotation on the tibial baseplate and escape of the femoral component from the poly insert.
“We suspected that laxity on the lateral side allowed the poly to rotate because the larger gap allowed the femoral condyle to ‘escape’ from the concavity of the insert. When you have a fixed bearing knee, the femur cannot ‘escape’ because of subtle insert rotation under the condyle and the margin for error for the femur to stay aligned with the tibia is the height of the post. In the rotating-platform TKA design, the depth of the insert concavity, which in some cases is only a few millimeters, defines the amount of gap asymmetry that is tolerated before insert rotation out from under the femoral condyle can occur.”
“This results in a very narrow margin for error and a rather unforgiving technique for a successful TKA operation. The depth of concavity of the poly defines how much lateral opening of that gap you can tolerate before the plastic escapes from underneath the femur.”
It is a demanding technique, says Dr. Pellegrini…and the ultimate question is: “Can this technique translate easily and broadly to a community orthopedic practice?”
Dear Dr. Pellegrini,
The NJ LCS Rotating Platform Total Knee System was released in 1977 by Depuy and was designed by my father, Frederick F. Buechel, Sr. MD and his Professor Micheal J. Pappas, PhD. The LCS has over 40 years on the market and worldwide utilization with success rates at over 98% at 30 years of in-vivo. I applaud your research and always encourage scientific evaluation of medical devices to educate the public.
Worldwide the LCS is the most successful rotating platform in history. There are specific steps that are required to perform a successful total knee replacement for fixed and mobile bearing knees. One step is that flexion and extension gaps should be balanced. If you perform a fixed bearing knee with a loose flexion gap, you will not get a spin out or subluxation of the poly as you were alluding to, because the poly doesn’t move, however, you will have flexion instability in the fixed bearing knee. This flexion instability is one of the most common causes of revision of a fixed bearing knees and probably one of the most common reasons for patient dissatisfaction in fixed bearing total knees. In patient satisfaction studies of fixed bearing total knees, 20-40% of patients are dissatisfied with their outcome. I would not say surgeons are doing a good job with balancing their fixed bearing knees either when critically looking at these statistics.
This being the case, I would say that whatever the technique some surgeons are using for fixed bearing knee installation is obviously not very easy to reproduce either. So in fairness, you state the mobile bearing technique must be very hard to do, I state the fixed bearing technique of many surgeons is equally hard to do or harder looking at the dissatisfaction rates and number of flexion instability cases in fixed bearing knees.
I do not personally find the technique of either a fixed or mobile bearing knee difficult. And I have used both designs often over my 20 years as a knee replacement specialist. I have also not had a spinout of my mobile bearing knees. I also do not get flexion instability of my fixed bearing knees.
I will give the reason for not having this problem for those reviewing your fine work here and my response to it.
I was trained by the designer of the LCS knee, my father, and a fixed bearing knee master in my residency and fellowship, Dr. Robert E. Booth, MD in Philadelphia. Both master surgeons are tibia cut first and flexion gap first surgeons.
They both cut the tibia in neutral, tension the collateral ligaments of the knee at 90 degrees which then allows them to set the rotation of the femoral AP cut to create a rectangular flexion gap. Once the posterior femoral cut is made, a spacer block is placed to measure the flexion gap. This stable gap determines the final insert thickness. Next is the extension gap. The distal femoral cut is made placing the distal femoral cutting guide on to create the same space as the flexion gap spacer. To avoid a loose extension gap, we make the distal cut conservative, or right on. If after making the distal cut it is too tight, and does not allow the spacer block to fit in the space that matches the flexion spacer block, we just move the block up another millimeter and recut again until it matches the flexion gap. This is a simple and reproducible technique for both fixed and mobile bearing knee designs that avoids the perils of flexion instability in both fixed and mobile bearing knee designs.
If you are a femur first, extension gap first total knee surgeon, you should not be a mobile bearing knee installer. It was not made to be installed with that technique despite what any company or surgeon may say. These surgeons are the ones that have reported spinouts and had difficulties with mobile bearing knees, and it is their improper sequence of cuts and balance that cause the problem, not the design or the difficulty of the technique.
If you are a femur first, extension gap first fixed bearing total knee surgeon, you will inevitably be a surgeon that has a dissatisfied patient here and there, or often. This is due to flexion instability, which will have all the same issues you described for a poorly installed misbalanced mobile bearing knee, including difficulty down stairs, crossing legs, difficulty with lateral movements, painful collateral ligaments, and chronic pain and dissatisfaction.
The only thing you won’t have with a poorly installed fixed bearing knee is the possibility of a spinout or subluxation of the bearing, but you will still have the dissatisfied patient, and a patient needing a revision.
As I have evolved over my career from manual, to navigation, and now Robotic Arm Assisted Total Knee replacement, I can state unequivocally, that precision balance is the key to a satisfied, properly functioning, stable total knee. With documented balance reproducible within 1mm medial and lateral in flexion and extension, patients are even more stable, recover faster, and do not have the ligament pain that can be associated with imbalances of tension throughout the range of motion. This is the precision technique of cutting edge surgeons of today, and will be the standard of care of knee surgeons of the future.
I think we are all capable of doing great jobs with our total knees, but good and great surgeons must perform them with the correct steps that logically result in stability on every case. I think the viewers should know that Dr. Booth and Dr. Buechel, Sr. and myself are not special people that can always get good balance, but we follow a process that results in every case getting good balance. That is flexion gap first, followed by extension gap to match second. It is not a difficult technique at all, just a proper sequence that results in reproducible balance.
In Robotics, we can now do all the intra-operative balancing prior to the precision robotic cuts and achieve the same or better level of balance. This can be the holy grail for femur first cutting surgeons to achieve balanced knees if they don’t want to be a tibia first flexion gap first surgeon.
Thank you for your paper that stimulated this response and I hope it is insightful to you and your readers.
It would be my pleasure to discuss further with you any time.
Warmest regards, and best wishes.
Fred Buechel, Jr. MD
Robotic Joint Center
New York City * Taiwan
http://www.RoboticJointCenter.com