Concepts to Live By
As we grow older, we become less and less sure of many things. So when we come to the end of a professional career, as I have, and still feel strongly about certain concepts, there is a good possibility that we may be right in these convictions. Here are the seven concepts I feel most sure of after 50 years in orthodontic practice.
1. Molars do not intrude. A common misconception is that when we move molars occlusally, the muscles of mastication will intrude the molars and close the bite. This is patently wrong. Of course, molars will intrude like all teeth if you apply a continuous force to them, but they do not intrude from function.
2. Management of the vertical growth of the alveolar processes is the primary function of the orthodontist. We aren't sure at present whether condylar growth can be stimulated. Vertical growth of the posterior portion of the maxilla cannot be stimulated or inhibited. That leaves only the alveolar processes, which can be affected vertically more than laterally or sagittally. The lower molars seem to have latent growth potential that does not fully express itself unless they are completely uprighted.
When there is a danger of producing too much vertical growth of the jaws, we should not use long Class II elastics for an extended period, even if anchorage has been prepared. Short Class II elastics, from the distal of the lower cuspid to the mesial of the upper cuspid, can be worn indefinitely without producing any vertical development of the molars, because they are attached anterior to the center of resistance of the lower arch.
3. Growth prediction is based on average growth, and average growth needs no prediction. As treatment progresses, the orthodontist needs to know whether to stimulate or inhibit the vertical growth of the molars. It is far more important to know whether an increment of growth comes from the condyles, the body of the maxilla, the maxillary alveolar process, or the mandibular process than to know that the chin moves downward and forward on a certain trajectory at a certain rate.
4. Lower incisors should not be intruded to level the arch in growing patients. This limits vertical development of the lower face and tends to result in a closed bite. The first bicuspid is at the lowest point in the curve and therefore is its weakest link. If the bicuspids and molars can be moved occlusally to the level of the incisors, the occlusal plane will nearly always be stable. Of course, other important relationships--sagittal harmony of the jaws, overbite correction, correct torque of the upper anterior teeth, and any necessary intrusion of the upper anteriors--must already have been established. All this will allow the maximum possible vertical development.
Leveling with round archwires is inadvisable because the apices of the incisors usually move lingually, and it then becomes difficult to torque them back to their previous positions. For the last 20 years, I have used .016" brackets on the anterior teeth and .018" brackets on the posterior teeth. This provides incisor control, molar control, and intrabracket space around the wire in the bicuspid region, making it possible to level with rectangular archwires.
5. There are good reasons for extracting second bicuspids rather than first bicuspids. The treated lower arch is more stable, because the cuspids require less movement and less manipulation, and unwanted cuspid expansion can be avoided. The upper second bicuspid is usually more narrow mesiodistally than the upper first bicuspid, so when it is extracted the molar is positioned farther distally, improving interdigitation with the lower first molar. The lower second bicuspid is wider than the lower first bicuspid, so its removal further improves molar occlusion.
The upper first bicuspid usually has longer, wider roots than the upper second bicuspid, making it much more resistant to lateral forces. The first bicuspid's long buccal cusp helps disocclude the molars in lateral movement, produce good permanent interdigitation, and make the finished result resemble a full complement of teeth. If the lower first bicuspid is extracted and the cuspid is tipped or retracted bodily into the extraction space, the cuspid apex tends to move into the labial bone. Since it is difficult to torque the apex back into the center of the bony ridge, the tip of the upper cuspid is often left contacting the labial surface of the lower cuspid, producing poor function.
Finally, treatment is much easier. If the lower first bicuspids are retracted with sectional arches from first molar to first bicuspid, the cuspids will usually correct themselves. When spaces appear distal to the cuspids, I stop adjusting the sectional arches and apply weak Class II elastics to the closing loops on the archwires. If the elastics were applied to the molar hooks, they would tip the molars forward. A common practice is to pit the anterior teeth against the posterior teeth and let them meet in the center. This can result in the incisors being positioned too far lingually. We can make them meet precisely where we want them by using sectional arches correctly.
6. Differential anchorage is nature's way of harmonizing the temporomandibular joint and the teeth. In natural growth, lower molars seldom move forward appreciably in relation to the symphysis; often they move distally. These teeth offer resistance to forward movement, while the upper molars move forward much more readily. In the first phase of treatment, the upper incisors should be intruded or maintained as the overbite is reduced. If this is done before retraction of the anterior teeth, the upper and lower incisors never occlude with one another, and the condyles cannot possibly be forced posteriorly into the fossae. Such evidence, which can be documented cephalometrically, is what the profession needs to counteract the charge that TMJ dysfunction can be caused by orthodontic tooth movement.
7. The most important part of orthodontic treatment is the management of the six upper anterior teeth. The smile is our acid test. In general, the upper incisors should be held at their original vertical level or intruded, but there are many exceptions. A "gummy smile" or excessive overbite will require incisor intrusion. In any case, if the finished upper model is placed on a table teeth down, the cuspids, bicuspids, and molars should contact the table and the incisors should be ½mm from the table.
The anterior teeth must not only be properly positioned anteroposterioraly, but properly angulated and torqued. You can torque adequately with .018" or .022" single or twin brackets, but it requires many archwire changes. An .016" bracket can be filled with wire much earlier, and you can accomplish all necessary torquing and intrusion of the upper incisors in the eight to 12 months prior debonding
I am so sure of the concepts I have outlined here that ido not hesitate to boldly affirm them. I will trust in posterity to further confirm them.