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.
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.
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.