This week’s Orthopaedic Crossfire® debate was part of the 19th 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 Douglas E. Padgett, M.D., Hospital for Special Surgery, New York, New York. Moderating is Paul F. Lachiewicz, M.D., Duke University Medical Center, Durham, North Carolina.
Dr. Meneghini: It is my pleasure to debate my esteemed colleague Doug Padgett on this topic about the merits of a posterior stabilized knee design.
The topic is whether or not it’s a vestigial organ. The cam-post is obsolete in modern knee replacement. I’m going to describe why I think that’s the truth.
I would argue that newer tibial inserts have enhanced sagittal geometry and conformity, such as an anterior-lipped design, which is designed to substitute for the posterior cruciate ligament (PCL), rendering the cam and post obsolete in modern knee replacement. I’ll go through why that is.
First, what is a vestigial organ? It is a structure in an organism that has lost all or most of its original function throughout the course of evolution. I would argue that we have evolved in total knee replacement.
The normal knee is smooth. It has very fluid motion, so it’s really not intuitive to think that patients would like a cam-post in their knee replacement. Why would anybody want anything to slam into a post? I would say probably not.
In addition, the cam-post mechanism is not benign. Post wear and impingement have been seen in multiple studies. Post fatigue fractures occur in multiple designs. Patella clunk has been an issue over time. Admittedly it has improved with newer designs. Then you have to remove bone. And there is the potential for condyle fractures. All of which have been reported in the literature.
So, let’s look at the science. We’re going to look at four different types of literature: retrospective, matched cohort studies; randomized prospective studies; large institution registry data, and then national registry data. So, the first is a retrospective, matched cohort from my institution (Biyani et al., Surg Technol Int, 2017). My partner resected the PCL in all of his knees. We compared a cam-post mechanism to an anterior-lipped insert. No difference in any of the functional outcomes at a minimum 1-year follow-up.
Another, similar type of study (Parsely et al. JOA, 2006), — 209 total knee replacements; 121 with a cam-post and 88 with an ultra-congruent sagittal confirming bearing. Again, no functional outcome differences at 1 year.
A prospective randomized controlled trial by David Scott was presented at AAHKS and published initially in 2014 (JOA) and in that study the PCL was resected in all cases and, again, cam-post in one arm and then an anterior-lipped device designed to substitute for the PCL in the other arm. No functional outcome difference at 2 years. He just published the 5-year minimum follow-up (JOA, 2018). Same conclusions. Statistically significant. Not surprising.
You see a consistent message through all these studies.
Mayo Clinic registry data (Abdel et al., JBJS-Am, 2011). Over 8,000 total knee replacements. Fifteen-year survivorship—90% for cruciate retaining; 77% for posterior stabilized; and a lower risk of revision for cruciate retaining. Makes sense when you have a less mechanically designed mechanism inside your knee that you’ll get a longer survivorship.
That’s also supported in the Australian Registry data (2016). Out to 14 years, the posterior stabilized knees clearly have a greater risk of revision and the lowest rate of revision is cruciate retaining knees where you resurface the patella.
It’s also something to consider that cam-post mechanisms may render technological advancements such as highly cross-linked polyethylene and cementless knee replacement less than advantageous.
For example, fatigue failure is the Achilles heel of highly cross-linked polyethylene. I would argue that a cam-post mechanism could put that at risk. And in cementless knees, if you look historically, cruciate retaining knees have the longest survivorship and best outcomes. There are few modern PS knee designs that have shown to be viable with cementless, but not many. So, I would argue that’s probably a good avenue to continue doing cruciate retaining knees as we advance technology.
In summary, we have the emergence of anterior-lipped cruciate retaining inserts, which I believe obviate the need for a cam-post mechanism in modern total knee replacement. Surgical technique remains critical. You still must be balancing in flexion no matter whether you use a cam-post or an anterior-lipped. And healthcare reform mandates that we control cost and reduced inventory is a big part of that. I would argue we probably don’t need all these bearings anymore and there’s no scientific evidence to support that a cam-post mechanism is appropriate to replace the posterior cruciate ligament in total knee replacement.
