Dr. Dunbar: Absolutely. Thanks very much Rob. Thanks to Seth and the organizing committee for the opportunity. My disclosures are relevant particularly with respect to kinematic alignment. So take everything with a grain of salt.
Neutral mechanical axis is the paradigm. It’s really a paradigm of a remarkable development of evolution that we owe a debt of gratitude for to people like Dr. Ranawat for bringing us to where we are. It still remains a standard of care.
But my question would be, framing this argument, is it really the end of the road for the evolution that we’ve come down to? Are we really at a dead end with respect to the evolution of knee arthroplasty?
Consider the remarkable advances that we’re all witnessing as we go into the display area with respect to imaging, robotics, machine learning and all the other wonderful tools and techniques that we are enjoying.
Right now the paradigm is to ignore the variability and treat all patients as if they are the same and put them all in neutral mechanical alignment.
And like everything in the human condition, mechanical alignment falls upon a normal distribution. The first thing you do in grade school when you’re learning about statistics is you take your classmates and you line them up and measure their heights and you realize that they fall on a bell curve. Well, I would submit that clearly alignment is also on a bell curve and there is a lot of evidence to support that.
Consider a paper by Bellemans, et al.: “Is Neutral Mechanical Alignment Normal for All Patients?: The Concept of Constitutional Varus.”
They recruited a cohort of 500 asymptomatic adult volunteers between 20 and 27 years old for this cross-sectional study and all had full-leg standing digital radiographs on which 19 alignment parameters were analyzed.
Thirty-two percent of men and 17% of women had constitutional varus knees with a natural mechanical alignment of 3 degrees varus or more and when they looked at their alignment, indeed it produced a bell curve just as I postulated.
But more interestingly is that the average overall alignment is 1.3 degrees varus for all comers and it’s actually 1.8 degrees of varus for males. And if we look at our plus or minus 3 degree range, and those that are outside of it, it’s actually a full one-third of patients who are outside of this window that we’re looking at. And clearly many are outside of neutral, plumb down the middle 0 degrees mechanical alignment.
The question then posed from this paper is “Are all knees in neutral mechanical alignment?” Clearly the answer from this paper is “No, they are not.” And in fact the conclusion from this paper is restoration of mechanical alignment to neutral in these cases may not be desirable when, in fact, it would unnatural for these knees.
One of the things that I didn’t tell you about this paper is that it is a Knee Society Chitranjan Ranawat Award winning paper, so clearly it demonstrates that Dr. Ranawat has an open mind as he’s endorsing eponymously that this concept of neutral mechanical alignment actually isn’t the case.
Another senior thought leader, Adolph Lombardi, at this meeting in 2011 reiterated the point that aiming for neutral provides the safest margin for error. The foremost objective of total knee arthroplasty is a durable joint. Not necessarily, and this is the important point, one that replicates normal or the patient’s native condition. We’re not reproducing that bell curve that I alluded to and in fact we’re intentionally ignoring it.
So the concept is, if you want to make something last, you make it straight, you make it straight and it’s going to stand up a long time. But there’s a lot of evidence to refute that now.
