This week’s Orthopaedic Crossfire® debate was part of the 18th Annual Current Concepts in Joint Replacement® (CCJR®), Spring meeting, which took place in Las Vegas. This week’s topic is “Posterior Stabilized Knee Designs: Vestigial Organs.” For is R. Michael Meneghini, M.D., Indiana University School of Medicine, Indianapolis, Indiana. Opposing is William G. Hamilton, M.D., Anderson Orthopaedic Research Institute, Alexandria, Virginia. Moderating is Thomas S. Thornhill, M.D., Brigham and Women’s Hospital, Boston, Massachusetts.
Dr. Meneghini: So, my charge is defending the position that posterior stabilized designs are a vestigial organ.
This was a classic debate—as a medical student, resident, early in my practice—it was awesome to watch the greats fight back and forth over keeping the PCL [posterior cruciate ligament].
Why did that debate fade away?
Probably because there has been no clear advantage over cruciate-retaining or posterior-stabilized designs based on the literature; the pros and cons were largely theoretical. We focused on survivorship, and the metrics weren’t sensitive enough to determine a difference…so why are we bringing the debate back?
I would argue that newer tibial inserts with enhanced sagittal conformity (anterior-lipped) designed to substitute for the PCL, render the post-cam obsolete.
The definition of a vestigial organ is a structural organism that has lost all or most of its original function in the course of evolution. And we have evolved in total knee replacement.
There are two fundamental arguments: one is intuitive and one scientific/data-driven. The first argument comes from the observation that the native knee is elastic and fluid. Therefore, it makes little intuitive sense to put a cam and a post in a native knee. Nothing in the human body likes robust or articular constraining devices. So, I would argue that nothing external or internal in the knee really likes slamming into a post.
Why put that inside the knee?
The second argument is based on the post-cam mechanism. There is wear and impingement and fractures across all designs and that is all over the literature. Patella clunk is an issue; condylar fracture (depending on the size of the box, which we’ve seen in other designs) and so forth.
So now let’s look at the science. My debate partner takes out the cruciate ligament on every knee, so we decided to examine them. We found no difference in any functional outcomes between those patients without a PCL whether you replace it with a cam and post or a sagittally conforming polyethylene insert like an anterior lipped.
Brian Parsley, et al., reported the exact same finding over a decade ago, with over 200 knees, in all without the PCL with either an anterior lipped ultracongruent insert or a cam and a post.
In an elegant prospective randomized study presented at AAHKS a few years ago and subsequently published, David Scott of Washington looked at 56 knees with posts and 55 knees with an anterior-lipped insert, all without the PCL. There were no functional outcome differences at two years.
From the Australian registry, at 14 years there is a clear difference between survivorship and revision rates in cruciate-retaining versus posterior stabilized TKAs, with cruciate-retaining having a distinct advantage.
