During the "cephalometric era" (1930 to the present time) the vertical position of the mandibular incisor has been almost completely ignored in the American orthodontic literature. There has been an inordinate amount of discussion pertaining to the anteroposterior position of this tooth in the craniofacial complex; and volumes have been written pertaining to how the mandibular incisor changes labiolingually with relation to its base. However, very little has been written pertaining to the vertical position of the mandibular incisor with relation to the mandible.

Early in the evolution of the orthodontic profession the position of the mandibular incisor in the dentofacial complex became of special interest. As the treatment of malocclusion evolved, the anteroposterior position of this tooth was seen as an important concept. Innumerable authors have voiced their opinions about how this tooth should be related to the mandible, to the dentofacial complex and to the musculature. The position of this tooth has a critical effect on facial aesthetics, and it can be said that it deserves all of the attention it has been given.

However, from the standpoint of stable treatment, the prime objective of orthodontic treatment, the vertical relation of the mandibular incisor to the mandible has been almost completely ignored for the last seventy years. The volumes of American literature pertaining to the position of this tooth seldom if ever mention its vertical relation to the mandible.

Many orthodontists pay no attention to whether the mandibular incisor is intruded in treatment. When this tooth is appreciably intruded it is the most unstable of all anatomical changes. Also many orthodontists purposely intrude the mandibular incisors and then wonder why the overbite relapses.

There has been much interest in stable treatment results in recent years. Many articles have been written pertaining to the characteristics of the stable treated case, but the word intrusion is never mentioned; at least we do not see it mentioned. This is a real puzzle!

The interincisal angle is an important cause of overbite. When a deep overbite develops, accompanied by a large interincisal angle, it tends to force the crowns of the mandibular incisors lingually and the apices of the maxillary incisors labially. This in turn increases the interincisal angle which causes still more overbite. Thus, a symbiotic relationship develops between the size of the interincisal angle and the depth of the overbite. See Figure 1:

Figure 1. This is a three year growth cycle in a female from 9 to 12 years of age, (A and B respectively) The vertical overbite increased from 4mm to 6mm, and the interincisal angle increased from 148 degrees to 156 degrees. The large interincisal angle was thought to be the principal causal factor in the increaseof overbite.

Thus, it seems that Nature can make necessary compensations and adjustments in axial inclinations of incisors only when good occlusal relations of teeth are present. This is why it is of transcendent importance to correct overbites until incisors are end to end, obtain the correct interincisal angle (about 135°), and secure the integrity of the mandibular arch with a cemented retainer from cuspid to cuspid. When this has been done a harmonious axial relationship, though not necessarily a constant one, can continue to exist throughout the remainder of the growing period.
Figure 4. Pretreatment and post treatment tracings superimposed on the palatal plane. There was no increase in anterior dental height but considerable growth at the condyles. A shows that mandibular incisors were excessively intruded and that the maxillary incisors were moderately intruded. B shows the posttreatment reactions.
Note that the mandibular incisors again moved up and closed the bite accordingly.


(This is a verbatim quotation from a publication in 1965.)
"Merritt made a study of vertical overbite in 30 treated cases. Measurements were made before treatment, after treatment, and from two to three years posttreatment. See Figures 2 and 3:
Figure 3. Pretreatment and posttreatment tracings superimpose on the palatal plane. During treatment anterior detel hieght increased 14mm and has remained the same through 4 years of posttreatment observation. This vertical change was due to 5.5 mm of maxillary molar growth and 7.5mm of mandibular molar growth. This is shown in A. In B note that during the postreatment period the manibular molar remained at the same level and the maxillary grew downward even more.

He found a high correlation between intrusion of mandibular incisors and relapse of vertical overbite. He also found a high correlation between vertical movement of mandibular molars and stability of overbite. He selected the ten individuals in which the overbite correction was most stable and the ten in which overbite was least stable, leaving out the 'middle' ten. Paired 't' tests were then done on the twenty cases. For mandibular molar elevation between the two groups he found a 't' test reading of 8.69 with a probability of .001. For mandibular incisor intrusion the 't' test reading was 4.62 with a probability of .001. In other words, when mandibular incisors are intruded, they usually tend to extrude subsequent to treatment inviting a return of the overbite; and when molars are moved occlusally, they remain at this level and prevent a relapse of overbite. These findings of Merritt confirm the author's observations on hundreds of cases. Thus, it can be said that molars almost never are intruded into the bone by muscle pressure, and if they can be induced to move occlusally, they will remain at this level in almost all instances. If during treatment anterior dental height (ANS to menton) is increased at least the amount of the vertical overbite, we may be assured of a successful overbite correction in most instances. When this dimension is not appreciably increased, we may expect a return of the overbite."

The author has been admonishing colleagues for the last thirty-four years to avoid intrusion of mandibular incisors.

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