In this post, I discuss a central tenet of orthodontics... the Equilibrium Theory.
This theory came in 1972 from Prof William Proffitt, a legendary figure in orthodontics who sadly passed away in 2018. I was lucky enough to see him at the British Orthodontic Conference in 2014, where he spoke eloquently about his on-going research and academic focus. I was just a year into my specialist training at the time, and so I grabbed a selfie opportunity with orthodontic legends Professors Proffitt and O'Brien.
What is the Equilibrium Theory?
The theory proposed that teeth sit in a zone of equilibrium. What does this really mean? In a nutshell, if an object is subjected to a set of forces but does not move, then the forces must be in equilibrium. Proffitt explained that forces are constantly applied to the teeth by the soft tissues which sit around them, such as the tongue, lips and buccal mucosa. Despite the forces being applied, we don't see the teeth moving in a normal & healthy patient. That is because the forces being applied to the teeth are balanced, or in equilibrium. A parallel can be drawn to the concept of the neutral zone, a concept used in denture design by prosthodontists.
A final component of this concept is the periodontium, comprised of the gingival tissues, periodontal ligament and alveolar bone. These supporting tissues determine where forces will become balanced and therefore where the teeth will sit within the alveolus. A change in these supporting tissues will affect the position of the equilibrium, and the tooth position will change as a result. This can be seen in patients with historical periodontal disease, where the incisors often become splayed and spaced due to a change in the ability of the periodontium to maintain the original tooth positions, as in the images below.
Why is equilibrium so important?
During orthodontic treatment, the teeth should normally be maintained within this zone of equilibrium. If teeth are pushed outside of this zone, for example through incisor proclination or inappropriate expansion, the forces acting on the teeth will no longer be in balance, and the stability of the orthodontic treatment is much reduced. For example, if the incisors are proclined, then the pressure from the lips with be increased, and teeth will be pushed back towards their original position.
There is a higher risk of relapse if the teeth are pushed out of the zone of equilibrium.
So when planning tooth movement to achieve alignment, it is important to relieve crowding in an appropriate manner, and not to simply expand the arch or procline incisors when this is neither indicated nor appropriate.
What is the evidence for this?
Whilst the research cited below is dated, it is still relevant as the basic treatment mechanics, tooth movement and biology have not changed. Methods to treat patients and resolve crowding have also not changed, and nor have methods to research changes in archform. The evidence is focussed around arch form changes in the long term.
De La Cruz et al, 1995, examined long term changes in arch form after orthodontic treatment. They found that the "greater the treatment change, the greater the tendency for postretention change". They noted there was considerable individual variation. In my opinion, a key finding was that whilst not a guarantee, "the patient's pretreatment arch form appeared to be the best guide to future arch form stability".
Little & Riedel have extensively research arch form and long term changes. Their 1990 paper found that arch length and arch width both reduce in untreated individuals as well as treated patients. For me, I feel clinically this means we should look to reduce the amount of space the teeth need (i.e. through IPR or extraction) rather than increase the amount of space available (i.e. through expansion and proclination), particularly in the lower arch.
There are also other papers, but there is no need to pile on additional evidence!
Proffitt re-visited the theory a few years after his original paper, and this seminal work can be found here.
How Can we maintain equilibrium?
The way to do this is to respect the original arch dimensions (the width and length of the arch), and relieve crowding through IPR, extraction or other method. There are some instances were some arch expansion or incisor inclination changes can be tolerated, which are listed below:
Class II/2
Class III camouflage
Decompensation for orthographic surgery
Lip trap with retroclined lowers e.g. caused by digit sucking habit
Bimaxillary proclination.
Practically speaking, it is straightforward to determine if equilibrium has been disturbed. If using fixed appliances, a simple measurement of the arch width, for example, will identify if expansion has occurred. Orthodontic being planned digitally has made it possible to identify arch changes without even starting treatment.
Digital superimposition of the start and planned tooth positions facilitates a pre-treatment assessment of arch form changes.
Orthodontic Learning Points
Respect the original arch form
Do not expand the lower arch
Minimal incisor proclination is desirable
The above will result in more stable long term results.
How can Orthodontia help you?
Come along to our course to learn this and other key skills for treating your patients.
Read our up-coming blogs on treatment planning
Check out our orthodontic assessment template on the Resources page.
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