Bill Proffit: Equilibrium Theory revisited.
Last week I was stunned to hear about the death of Bill Proffit. As a result, I have decided to make this blog post about his classic paper “Equilibrium Theory Revisited”.
Like many orthodontic students my first exposure to Bill Proffit and his teaching was in seminars and his book. I also heard him lecture several times. I first met him when we were setting up the early Class II treatment study and he was incredibly helpful. We met again many times and had great conversations about orthodontics, trials, evidence, Forest plots and crossing the USA by train. His influence on orthodontics was immense and he had a great effect on my career. This paper made me “sit up and think” and I decided to become an academic. So as a tribute to Bill Proffit, I am going to blog about it in my usual style.
Equilibrium Theory Revisited: Factors influencing the position of the teeth.
WR Proffit. Angle Orthodontist: July 1978.
An academic orthodontist from North Carolina wrote this paper. The Angle Orthodontist published it.
What did he ask?
He considered the role of the soft tissue environment in the aetiology of malocclusion.
What did he do?
He simply put forward his opinion based upon his interpretation of his own and others research.
We can interpret this paper in many ways. This is my understanding of it.
Aetiology of Malocclusion
Malocclusion is caused by an interplay of our genetics and the environment. As a result, if you feel that tooth position is mostly influenced by the environment, then it is more likely that you will tend to treat non-extraction. This is because, you hope to influence the environment. If you believe that genetics is more important then you are probably more likely to extract teeth, as you believe that the environment cannot be modified to any significant extent. This is a simple and clear philosophy.
He then suggests that when we consider the forces that act on the teeth, there is no doubt that there is equilibrium. The forces of the tongue on the inside of the teeth and lips and cheeks on the outside of the teeth are balanced. When we carry out orthodontic treatment we apply forces to the teeth which changes the equilibrium and the teeth move.
The effect of force
This then led to a discussion in which he states that the duration of any applied force is more important than the magnitude of the force. I take this to mean that if we move teeth out of equilibrium zone, then the lighter but longer duration forces will result in the teeth moving back, and relapse is a strong possibility.
During the middle part of the paper, he points out that other external forces also have an effect. These forces arise from for example digit sucking habits, the force of occlusion and head posture also has an influence. Again, these are long acting and can significantly modify the position of the teeth.
Summary
My overall interpretation is that the environment of the soft tissues, forces from the teeth and possibly respiratory needs influence the position of the teeth. It then follows that we move teeth by disrupting the equilibrium of forces. As a result, treatment may relapse if the equilibrium is not significantly modified.
However, before all the non-extractionists, expanders and orthodontic breathing physicians get excited about this theory, we need to remember that we have no evidence that we can modify the equilibrium. It is for that simple reason that this brilliant paper is still relevant to contemporary practice. In the current orthodontic world of spin, extreme claims, advertising and the effects of Key Opinion Leaders, we need to remember the basics.
I hope that you follow my interpretation of his paper. You may have a completely different understanding, but that is what scientific discussion is all about.
And this issue of scientific debate is where Bill Proffit was without equal.
Emeritus Professor of Orthodontics, University of Manchester, UK.
Very sad when the great & good are no longer with us.
What this does show though is that the credible evidence, scientifically, has not really progressed in many decades !!
As you say Kevin these are still mainly ‘Opinions’ and thus it is not surprising, some will have different opinions to others that STILL fits in with the paucity of published credible evidence.
Indeed even the Cochraine review of Class 2 div 2 cases updated recently, still acknowledges there is no credible evidence for any approach being superior or indeed that any treatment is better than doing nothing !!
So……. whilst we like to think everything we do is based in science first, the truth is whilst we wait for the credible science to become irrefutable (absence of evidence is not evidence of absence!) it is clear, clinical experience and skills are STILL the most reliable & credible predictor of outcomes, when coupled with the patient’s wishes & cooperation.
I acknowledge one can say that about other sectors in Dentistry too – thus logically, Dentistry is still more Art than Science, overall.
Yours also reflectively,
Tony Kilcoyne.
Very sad when the great & good are no longer with us.
What this does show though is that the credible evidence, scientifically, has not really progressed in many decades !!
As you say Kevin these are still mainly ‘Opinions’ and thus it is not surprising, some will have different opinions to others that STILL fits in with the paucity of published credible evidence.
Indeed even the Cochrane review of Class 2 div 2 cases updated recently, still acknowledges there is no credible evidence for any approach being superior or indeed that any treatment is better than doing nothing !!
So……. whilst we like to think everything we do is based in science first, the truth is whilst we wait for the credible science to become irrefutable (absence of evidence is not evidence of absence!) it is clear, clinical experience and skills are STILL the most reliable & credible predictor of outcomes, when coupled with the patient’s wishes & cooperation.
I acknowledge one can say that about other sectors in Dentistry too – thus logically, Dentistry is still more Art than Science, overall.
Yours also reflectively,
Tony Kilcoyne.
Well said Tony!
A sad loss of a great thinker. Equilibrium theory dictates where we put the teeth when making dentures so there’s no reason why it should be different for real teeth. The idea that we can alter the soft tissues around the teeth as well as the bones (by growing them) strikes me as rather unlikely. The only soft tissues I do manage to alter are thumbs. This means that we have to move the teeth within the limitations of bones and tissues around them and plan accordingly. This simple theory really is all you need to explain where the teeth are and where they can go and why they move back. Until good evidence of something better comes along I’ll stick with Proffit.
A classic paper which has given us much to think about during the years, form and function – function and form. Your blog post is fine homage to a great Orthodontists.
He was on of the giant orthodontists on the earth.
Malocclusion is caused by an interplay of our genetics and the environment. As a result, if you feel that tooth position is mostly influenced by the environment, then it is more likely that you will tend to treat non-extraction. This is because, you hope to influence the environment. If you believe that genetics is more important then you are probably more likely to extract teeth, as you believe that the environment cannot be modified to any significant extent. This is a simple and clear philosophy.
Is this really simple and clear. Is it not true that you can you believe that genetics plays a huge part BUT understand that the environment can be manipulated to your advantage and then utilise this?
I love the theoretical thinkers in ortho-considering both sides of the equation. One thing I would add-we don’t really know if equilibrium today is the same as equilibrium tomorrow or twenty years from now. And how much of what we call relapse is simply adaptation to a new environment. Seems we know alot less than we don’t know… Excellent post.
I was greatly saddened to hear of the passing of Dr. Proffit. Unfortunately, as I get older, more of my heroes are leaving us. I do have a comment regarding equilibrium, stability, etc. I’ve always thought that the most stable position is usually the original malocclusion. I put my faith in retention.
Leading academics – and Bill Proffit was certainly one of those – have an easy knack of cutting problems down to size and addressing theoretical and practical questions in an understandable and memorable way.
As a post grad student at the Eastman, London in the 1980’s, we were expected to read Bill Proffit’s articles and watch some of the ‘slide-tapes’ he had produced.
We were very much influenced by equilibrium theory, in our case, especially by the cephalometric studies of JRE Mills on the stability of the lower labial segment.
There is a story about one postgraduate with more spirit than most who, when asked by Professor Mills in an exam, if the lower labial segment would be stable after proclination, replied “Well Prof, only time will tell!”
This was felt to be the wrong answer …
I think Equilibrium Theory still gives us pause for though in orthodontic diagnosis and treatment planning – and concerning retention strategies.
In this particular context, I am disquieted by the concept of routine life long retention which has become more frequently recommended in recent years.
If we could plan to move teeth into stable positions, the burden and risk of life long retention might be avoided.
I think you mean to say that we have only low levels of evidence that we can modify the equilibrium. Case studies, and smaller sample size studies, which show changes in growth direction and stability resulting from postural changes certainly exist. Equilibrium can certainly be changed. The question is how to do it predictably, and how to document it. Rest oral posture is almost impossible to measure and demonstrate, although ultrasound studies during sleep may show promise.
I would suggest that functional MRI,s or possibly CBCT,s would demonstrate this more appropriately but nothing has been shown to be particularly useful to me and certainly not worth the extra effort.
With great respect and admiration. Dear Dr Proffit. We will all miss you
It’s always a question I had in my mind. How do we measure the soft tissue forces? In case we alter the teeth position, then once the teeth are in the new position. How do we know the forces are balanced?
Very sade to know about the death of Mr Bill proffite through your blog . Their contribution to orthodontics unforgetable and Will remember through out my career.
Regaring equilibrium theory your interpretation you right because the whole system is working under equilibrium if there is no balance it will create stress strain and lead to relapse.
The question is how we judge or calculate the soft tissue forces that keep it in equilibrium and reduce chances of relapse.
RIP—-one of the few orthos. that could ,fully ,reconcile academic studies with practical ,clinical reality ! He will be greatly missed.
Really sad to hear of the passing of Dr Bill Proffit. I’m glad I had a chance to meet him and even present my work at UNC. Dr Proffit’s paper was written about 40 years ago – about the time I was graduating high school. Dr O’Brien’s interpretation is written with the benefit of work done since that time, so I find it surprising to read “we have no evidence that we can modify the equilibrium”. Two technical revolutions have occurred in our lifetimes that make me ponder that statement. First, the human genome has been sequenced, and second, there have been major advances in digital technologies. In view of those developments, I wonder if the concepts espoused by orthodontic leaders such as Dr Proffit et al. still stand the test of time? The utility of a theory is that it can be used make predictions and forecasts, which can then be scientifically tested for accuracy. The notion of ‘equilibrium’ as noted in the equilibrium theory is a prime candidate for this type of assessment. On re-reading the paper, it seems that ‘equilibrium’ is based on a balance of forces. This is not surprising since orthodontic theory was largely based on Newtonian physics and Darwinian genetics. Newton taught that action and reaction are equal and opposite, but that rule does not apply to a biologic system, because of its innate behavior (to provide necessary functions). Later generations of physicists talked about quantum physics, where (physical) behavior is studied on an entirely different scale. Similarly, the quanta of biology are genes, representing an era of quantum biology. So how do these advances affect the notion of ‘equilibrium’? Put simply, the holy grail is now biologic equilibrium otherwise known as homeostasis. In the same way that blood pressure, body temperature, etc. are subject to physiologic control, the craniofacial region, including the teeth, is no different. Thus, a malocclusion is a solution for a complex, adaptive system to maintain homeostasis or (biologic) equilibrium. IMHO the prime reason why malocclusions are stable conditions is partly because of sutural homeostasis. While previous generations were taught about the periodontal ‘ligament’, more recent data suggests that the periodontium is most likely a modified suture and, as such, is subject to sutural homeostasis, which is itself genetically-encoded and epigenetically-regulated. Taking Moss’ functional matrix hypothesis as a starting point, and adding newer concepts, such as temporo-spatial patterning, novel theories are precipitated. One such theory is my Spatial Matrix hypothesis, which successfully predicted how “we can modify the equilibrium”. Craniofacial homeostasis is the new norm, by addressing the hard tissues, soft tissues, dental tissues and functional spaces, such as the upper airway. By so doing, surgical and non-surgical solutions to orthodontic dilemmas, such as relapse, start to come into focus, standing on the shoulders of giants such as the late Dr Proffit.
Professor Singh has hit the nail on the head; Bill Profitt took the notion of equilibrium, or homeostasis, and applied it to the problem of malocclusion, providing us with a simple and clear explanation of why and how teeth acquire their ultimate position within the oral cavity.
But equilibrium is not peculiar to dentistry; equilibrium exists and works within just about every physiological and functional system in the body. It is the principle of equilibrium that gives us stereoscopic vision and stereophonic hearing. Equilibrium causes us to shift out weight slightly onto the left foot when we lift our right arm. Dentally, when we move the mandible forward into a Class 1 relationship, equilibrium rebalances the skull by moving the head back and straightening the neck spine (and, dare I say it, opens up the airway!).
Equilibrium is not a ‘force’ or a ‘theory”, it is a natural phenomenon that is essential to our successfully maintaining our upright posture. If we mess with it carelessly, i.e. make changes without addressing its parameters, we are inviting failure, especially in orthodontics.