Late Mandibular Incisor Crowding
This is a commentary on the official bulletin of the American Association of Orthodontists entitled, "Late Mandibular Incisor Crowding," which was mailed to the membership May, 1999. In this discussion the inference is that all orthodontic treatment is adequate, and in case of any shortcomings we must look elsewhere for the answer to the problem.
Why is it that we continue to try to find answers among anatomic factors over which we have little or no control? Why do we spend exhaustive effort in a field where there is questionable success, and ignore an area of human weakness which can be partially remedied? When we, in effect, try to justify poor treatment and refuse to condemn ourselves, we need not expect any real progress.
The article under discussion visited every weak, questionable argument known to the profession and failed to concentrate on what is definitely known by many proficient clinicians. The important concepts are "simple," not complex. To find out what happens to intruded lower incisors, we measure. To find out what happens to extruded molars, we measure. To find out whether the lower molars drift forward, and how much, we measure. To find out what causes the mandible to rotate, we measure the relationship between vertical condylar growth and vertical molar growth. To find out what causes the mandible to translate, we measure posterior horizontal condylar growth. To find out how and why the mandible grows downward without rotating, we measure the number of mm the condyles grow vertically and compare this measurement to the number of mm the molars grow vertically. They will be found to be equal.
To find out what causes the chin to grow downward, we measure the number of mm the molars grow vertically. To find out what causes the chin to grow forward a given number of mm (say 2 mm), we measure and compare vertical and horizontal growth increments. We will find that the condyles have grown vertically 2 mm to 3 mm more than the molars. To find out what causes the chin to grow downward (say 2 mm), we measure and compare vertical and horizontal growth increments. We will find that the molars grew vertically 2 mm to 3 mm more than the vertical condylar growth. To find out what causes an increase in condylion/gnathion distance, we measure the vertical growth of the condyles, we measure the horizontal growth of the corpus, and we measure the reduction of the gonion angle.
To accurately determine the downward movement of the anatomical structures, we measure from "Sella Horizontal." To accurately determine anteroposterior movement of anatomical structures, we must measure from the "Great Divide," both forward and posteriorly. To accurately measure the movements of the mandible, we must measure and reconcile the vertical and horizontal movements of the fossa.
This discussion of the problem of lower incisor crowding is supposed to be comprehensive-the last word, the official opinion of the orthodontic profession. But alas, it is only a superficial treatment of the problem, completely ignoring the important specifics. Generalities will get us nowhere.
In the article under the heading of "Why does it happen? Can it be prevented?", the following statement is made: "Years of study have failed to identify a single primary cause of late incisor crowding." This is a pathetic, shameful indictment of our profession.
In "years of study," I personally have identified a number of primary causes of crowding of lower incisors. Some of these are as follows:
1. Intrusion of lower incisors in growing patients, usually grow upward post treatment and cause bite closure and crowding of incisors.
2. Moving the lower incisors forward usually causes them to move lingually post treatment and encourages crowding.
3. Moving the lower cuspids labially causes them to move lingually post treatment, resulting in incisor crowding.
4. Leaving the lower incisors oversized with relation to the upper incisors leads to crowding.
5. Leaving the contact surfaces rounded instead of flat leads to crowding of the lower incisors.
6. Hesitancy in reducing enamel from the lower incisors results in crowding of lower incisors. I hope I will be forgiven for this personal reference. When my daughter was age 13, I removed more enamel from her upper incisors than I have ever removed from any other patient. Today, at age 60, even though her molars and bicuspids are full of restorations, her incisors are free of restorations. In fifty years of practice I can truthfully say that I have never seen caries develop as a result of enamel reduction.
7. Failure to produce adequate torque of the upper incisors (about 24° to line NA) leads to bite closure and crowding of lower incisors.
8. Failure to produce correct interincisal angle (130°) can lead to incisor crowding.
Another personal reference: As I travel around my city I often run across my old patients. I always inquire about their dental condition and note the smile and appearance of the front teeth. I would estimate that 90% to 95% of them have upper front teeth in good alignment, and if there is a tendency toward crowding it is usually slight. A high percentage of these patients have lower incisors in good or acceptable alignment. Rarely do I find a patient who is concerned about the extent of the lower incisor crowding. These patients may be from forty to sixty years of age and have not worn a retainer in several years.
The portion of the article entitled, "Should it be corrected and how?" makes some very helpful suggestions pertaining to retreatment of cases which have relapsed. I wholeheartedly agree with all of these suggestions.
Post treatment growth was not mentioned in the article, and the quality of treatment was not mentioned. From the standpoint of stability, this phase of growth is probably the most important phase of total growth; yet we have almost no control over it. Our retaining devices can only preserve status quo. The only control we have over post treatment growth is superior treatment. Post treatment growth improves superior treatment, but causes disaster to poor treatment results. To accomplish superior treatment we must carefully execute certain procedures and principles. Following is a list of principles and segments of treatment which must be executed:
1. Maxillary incisors must be torqued to about 24° to line NA, or possibly higher if there is a small root-crown angle.
2. The interincisal angle should be about 130°, in most cases.
3. The maxillary incisors should be intruded if needed (to about 2 mm below the lip line at rest).
4. Maxillary incisors should be slightly inclined mesially.
5. Mandibular lateral incisor apices should be about 15 to 17 mm apart.
6. Mandibular incisors must not be intruded in growing patients.
7. Mandibular arch must be completely leveled.
8. Mandibular molars and bicuspids must be uprighted (90° to occlusal plane).
9. Maxillary first molars must be well rotated.
10. Good cuspid relationship is essential. We should not hesitate to remove enamel from the appropriate teeth to accomplish this.
11. Overbite should be reduced to about 1 mm.
12. Overjet should be reduced to about 1 mm. To leave more than 1 mm of overjet will invite bit closure and crowding of lower incisors.
13. When the extraction of four bicuspids is considered advisable for a given individual and the extraction of four 2nd bicuspids seems feasible, a far better result can be produced when 2nd bicuspids are removed.
14. When these aspects of treatment have been accomplished, then post treatment growth will complement our treatment. Condylar growth will usually translate the maxillary molars and bicuspids forward, tip the maxillary incisors and cuspids forward, reduce the overjet and improve the interdigitation.
The poor treatment scenario is as follows: (1) the interincisal angle is too high, (2) the maxillary incisors are inadequately torqued, (3) overbite is still 3 to 5 mm, (4) the upper incisors have been moved downward when they should have been intruded, (5) upper first molars are poorly rotated, (6) the cuspid interdigitation is poor, and (7) there has been too much increase in lower anterior dental height, suggesting too much Class II elastics or cervical headgear. These conditions invite complete disaster. As post treatment growth ensues, the overbite increases, the lower incisors become crowded, and crowding continues with time.
For forty years we tried to execute every one of these procedures on every patient. Recently, a colleague who was associated with me for three years said, "You always corrected deep bites to an end to end relationship." Please see Article 16 in the Schudy Chronicles.
It is hoped that this discussion will be taken in the kind manner in which it was intended. Even though kindness must sometimes be harsh, these admonitions were expressed in a spirit of love and good will.
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