Lower Incisor Torque Compensation in Class II Correction
Dr. Michel Champagne, BA, DMD, MAGD, IBO, CDE, Fellow of the Quebec Dental Academy
It is an accepted common fact that we cannot increase mandibular growth on an adult nor on a growing child (Cochrane Database of Systematic Reviews 2018, Issue 3. Art. No.: CD003452. DOI: 10.1002/14651858.CD003452). Unlocking is always possible and unlocking will maximise the growth expression of the patient and probably allow the repositioning of the lower jaw. The amount of repositioning by unlocking will vary from patient to patient and we must see it as a gift from nature. I would estimate it (this is by no way scientific), around 2 mm. If repositioning is around 2 mm, where does the balance of the correction come from? The most probable is from dent-alveolar compensation i.e. from the sliding of the dental arches on their basal bone accompanied by tipping of the anterior teeth. With this in mind, we must evaluate which cases can afford such compensation.
Dr. Carlos Mir from the University of Alberta Canada, the author of more than 350 articles and co-editor of 8 textbooks in orthodontic, such a curriculum makes him credible, stated in a recent interview that we should not expect to achieve a real mandibular retrusion correction if the retrusion is more than 6 mm. The results will be achieved by camouflage and all the treatment goals will be difficult to achieve. Most of the camouflage will be in the lower incisor region. If the retrusion is between 2 and 4 mm, we will probably succeed. Remember that more camouflage there is, the more the stability will be compromised. How can we define an excessive camouflage and what is acceptable?
We can start our discussion based on the following logic: the initial position of the lower incisors (/1) is probably in a neutral position, balanced between the centripète forces of the lips and cheek muscles and the centrifuge forces of the tongue. This initial position should be fairly stable. Let us take the following example of /1, initially at 102 degrees to the lower mandibular plane (MPA), displaced at 105 degrees by treatment. This minor movement will probably not put the /1 outside of the neutral zone and they will probably be stable. The same logic does not apply if we move the /1 from an initial position of 90 degrees to 105 degrees. Such a displacement will surely relapse.
We also have to consider the anatomy of the symphysis which varies from patient to patient (Figre1 and Figure 2) A large symphysis will better tolerate some torque modification of the /1 because the displacement will be done inside the symphysis away from the buccal and lingual cortical plates. A narrow symphysis, on the contrary, does not tolerate change as well. The shape of the symphysis is an important factor to consider in the evaluation of treatment stability of the /1. A large symphysis is, most of time, a sign of normal chin button muscle activity.
Figure 1. Large symphysis.
Figure 2. Narrow symphysis.
For the obtention of the famous and ideal /1 inclination of 90 degrees in a major Class II mandibular retrusion case, orthognatic surgery may be the only solution. The solution is available but in reality very few patients will go for it. Most of them will prefer a camouflage compromised treatment when at all possible. The clinician has to evaluate the situation and decide if a camouflage approach will be acceptable or not. We should remember the famous citation by Dr. Openheim : ”I would give half of my fee to anybody who would take care of the retention phase of my patients”. This confirms my own thought: ‘it is very easy to start a case but keeping it stable is the real challenge”. When we chose a camouflage approach, we have to accept that there may be stability issue. The patient must be informed of the long term consequences.
Enjoy your orthodontics and be stable.