What are the most common reasons for revision? We’ve seen this time and time again in multiple publications: instability of the knee. There are multiple ways the knee can become unstable (varus/valgus), but I believe that AP instability in the PCL retaining knee is one of the more common reasons that these are revised.
As Mike mentioned, the PS knee does have its issues historically. I would argue, however, that improved designs and technique are minimizing those. Patellar clunk and crepitus have been the Achilles heels of PS knees for a generation. However, with improved designs, such as a blended PS box transition extending the trochlear groove, I think we’re seeing a significant reduction in complications.
As for clinical outcomes, a Cochrane database review that was not biased (like some of the studies Mike quoted from his partners), a 2013 study of 17 randomized studies showed very similar results across the board. The only difference was that PS knees had better flexion and higher functional Knee Society Scores by small margins. But if I am choosing between two implants, I think as a tiebreaker, I’ll take the one that moves better and has better function.
In summary, I agree that clinical performance is good with both PS and cruciate-retaining knees. I believe the PS knee leads to more reproducible femoral rollback and knee kinematics, and better range of motion. Complications such as fracture, clunk and crepitus, and post failure can be minimized with improved technique, materials, and design.
Moderator Thornhill: Mike, do you use flat-on-flat knees?
Dr. Meneghini: No, I do not.
Moderator Thornhill: I don’t either. Do you remove the PCL with a CR knee?
Dr. Meneghini: I do not.
Moderator Thornhill: OK, good. John Insall said he thinks that the post in a PS knee—and Bill I’d like you to comment on this—is there for a period of time during the first healing of the soft tissues. You don’t continue to have function from the post after a period of time. Do you think the post always functions after the soft tissues? Or does it teach the soft tissues to get the memory of the PS motion?
Dr. Hamilton: Good question. I’m not sure I know the definitive answer, but I think that based on the studies from Doug and Rick Komistek’s lab, that the post does provide consistent kinematics, allowing rollback not just in the early going, but even beyond. So, I do believe it retains some form of function.
Moderator Thornhill: Mike, do you balance the PCL? How?
Dr. Meneghini: Release it off the femur or the tibia, either way. The key is to leave the OR with a well-balanced knee. The second thing is that we spent a lot of time years ago and as a med student, resident, and early faculty listening to the debates of rollback and anterior paradoxical translation. None of that was ever correlated to clinical outcomes. I don’t want to discredit the work that Rick and Doug did, but the gap is that it was never correlated to clinical outcomes and I think that’s a step we have to take. The registry data provides a powerful testament to adding more mechanical congruity into our body and I’m not sure that’s the right answer. I think we do need to evolve for the reasons I outlined.
Moderator Thornhill: Bill, could you live with not using a cam-post but sacrificing the PCL and using a more conforming insert like a deep dish?
Dr. Hamilton: I think so. There are certainly people who are moving towards that direction. I have a junior partner from Philadelphia and what they have taken to doing is cutting the PCL in all patients and using one of these highly congruent inserts and it seems that clinical outcomes are quite similar. That doesn’t make a ton of sense to me because that’s probably giving you the least rollback of all situations, and I think rollback is what God designed and what helps to facilitate deep flexion. While the clinical outcomes seem similar it’s probably because our measurement tools are not sensitive enough to figure those differences out. As long as you put the knee in well and balance it well you can probably get good outcomes with that approach.
Moderator Thornhill: Great debate. I appreciate both of you.
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Senior Editor: Jay D. Mabrey, M.D., whose 35 year career in orthopedics included residency at Duke University Medical Center, service in the United States Army Medical Corps, Fellowship at the Hospital for Special Surgery and a long, distinguished career at Baylor University Medical Center where, in addition to his overall leadership at that institution, developed the Joint Wellness Program that helped patients get up after surgery more quickly, developed the first virtual reality surgical simulator for knee arthroscopy and chaired the FDA Orthopaedic Device Panel, is Orthopedics This Week’s newest contributing writer and editor.
