An excerpt from an article entitled "The Control of Vertical Overbite in Clinical Orthodontics," presented before the 1967 Annual Meeting of the AAO and published in the Angle Orthodontist in 1968.For many years there has been a prevailing opinion among most orthodontists that when treatment produces excessive anterior facial height, then subsequent functional forces will intrude the teeth and cause a relapse of the vertical height of the jaws. This is not true. Teeth are never intruded from normal functional forces.
For the last thirty-five years I have been challenging colleagues to send me evidence of where teeth have been intruded from normal muscle functional forces. I am still waiting!!
In the August 1998 issue of the AJODO, Ryan, Schneider, Begole and Muhl published an article entitled "Opening Rotations of the Mandible During and After Treatment." In this article they studied opening rotation by relating it to the y-axis and the inclination of the mandibular plane in a sample of sixty treated patients-30 males and 30 females.
The y-axis tells us nothing about cause and effect. It merely tells us that the symphysis has moved downward and forward a given amount, but it does not tell us how and why. We must measure vertical and horizontal condylar growth, and compare this growth to that of (1) the vertical growth of the maxilla plus (2) the vertical growth of the upper molars and plus (3) the vertical growth of the lower molars. Please see Fig. 1 and Fig. 2.
Fig. 1: Here is shown a treated case in which the Y-axis increased three degrees. This was a very favorable growth reaction from every standpoint (aesthetics, function, overbite, and stability). Increase in anterior facial height was four times greater than increase in facial depth. Condylar growth was ample to balance vertical growth and to keep the mandibluar plane parallel to its original inclination.
Fig. 2: This 3.5° increase in the Y-axis was a very unfavorable growth reaction. There was very little increase in S-Gn distance, pogonion went downward and backward and the mandibular plane became 7° steeper. The cause of this was primarily a marked deficiency in condylar growth.
Condylar growth thrusts the chin forward and vertical growth of the molar teeth moves the chin downward. When vertical condylar growth exceeds total vertical molar growth the mandibular plane rotates forward; when vertical molar growth exceeds vertical condylar growth the mandibular plane rotates backward. Both vertical condylar growth and vertical growth of molars are measured from "sella horizontal." Please see Article 24. Thus, the above explanation tells us how and why the mandible rotates forward and backward.
There exists at the present time considerable difference of opinion regarding the importance of the Y-axis. Some feel that it is undesirable to increase the Y-axis with treatment; however, a careful study of the facts do not confirm this opinion. It must be remembered that the Y-axis angle increases on average from 8 to 15 years of age. Also it must be remembered that the average is made up of extremes in both directions. It can be said that most of these extremes represent normal growth for these individuals.
If faces were square, that is, if depth and height were equal, and if faces normally grew equally in vertical and horizontal directions, then normal growth would in fact be down the Y-axis. However, we know that faces are not square but that depth is from 66 to 85% of height. Also we know anterior facial height increases from two to three times as much as facial depth. Figure 1 shows that the Y-axis can only open when the face grows more vertically than horizontally. It illustrates why this angle does and should become more obtuse. The more vertical growth exceeds horizontal growth, the more the Y-axis must drop posteriorly. If during treatment vertical growth far exceeds horizontal growth, the Y-axis must move backward just as it would do if the individual were not being treated orthodontically. (Fig. 2).
Perhaps the most desirable behavior of the Y-axis from the standpoint of vertical overbite correction is a backward swing, provided there is enough condylar growth to keep the mandibular plane parallel and enable the mandible to keep pace with the forward growth of the maxilla.
While it is important to note and record the anatomical changes which cause the Y-axis to change, I do not feel that it is necessary to actually record the Y-axis. There are other places at which these changes can be recorded more meaningfully. The Y-axis angle merely tells us where the chin is situated with relation to the cranium, but does not tell us by what route it traveled to arrive there. It does not tell us whether we have a square or an obtuse gonial angle. An increase in the Y-axis angle may accompany normal growth as well as abnormal growth.
One encounters on every hand the belief that molars frequently are intruded into the bone by muscle pressure subsequent to treatment and that this is accompanied by a decrease in the lower face height (ANS to menton). In hundreds of cases observed, the author has never seen more than a very slight reduction in ANS-menton height and that was in the first few days following band removal. It can be said that molars almost never are intruded into the bone subsequent to treatment. Thus, if molars can be induced to move occlusally, they will remain at that attained level in almost all instances.
We must not think of posttreatment occlusal plane changes as a rebounding reaction, as though something returns to where it once was. This seldom if ever happens, but intruded incisors in some cases do subsequently extrude. The same kind of growth which causes this plane to flatten before treatment also causes it to flatten after treatment. Please see Fig. 3 and Fig. 4.
Fig. 3: This illustrates that condylar growth (as related to vertical growth) is the key to changes of the occlusal plane. The posterior growth analysis shows that the condyles grew 23 mm and the vertical growth in the molar area was 18 mm (9+6+3). The result was an 8° change of the occlusal plane. (From the growth study of the U. of Mich.)
Fig. 4: Again showing that condylar growth when related to vertical growth is the key to the behavior of the occlusal plane. Poor condylar growth (4mm) could not keep pace with 10 mm of vertical growth (4+2, 5+3, 5). The result was a 5° change of the occlusal plane. (From the U. of Mich. Growth Study).
Please see Fig. 5 and Fig. 6. Also see Fig. 7 and Fig. 8. These illustrations verify the fact that opening rotations are permanent when all growth has been terminated. The author is aware that some individuals grow into the third decade of life, but this latent growth will seldom if ever affect the vertical opening.
Fig. 5, A & B: This shows records of two males both aged 9 to 15. Both individuals grew 16 mm at the condyles. In A this 16 mm of growth produced 7° of forward rotation and moved the chin forward 13 mm. In B this same 16 mm of growth produced 1° of rotation and moved the chin forward 4 mm. How could this be? Why so much difference in the effect of 16 mm of condylar growth? Please note that the maxilla grew 8 mm vertically, and the maxillary molars grew 4 mm. The maxillary structures grew about the same but the difference was in the mandibular molars.
Fig. 6, C & D: Here are the same cases shown in Fig. 5, A & B. In C we have added 4 mm of lower molar height. This rotates the mandible down and back and changes a strong Class I molar relation to an end on molar relation. In D we removed 4 mm of molar height. This rotated the chin upward and forward and changed an end on molar relation to a strong Class I relation.
Fig. 7: This is a treated case in which there was no appreciable condylar growth, necessitating the removal of two maxillary bicuspids. In four years of posttreatment observation there was no reduction of the SN-MP angle.
Fig. 8: This is a record of the treatment of a female. There are several interesting changes which we will point out. (1) The SN-MP increased 8° during treatment, (2) the SN-MP increased 4° posttreatment, (3) condyles grew 3 mm total, (4) vertical growth was 10 mm (3+4+3), (5) chin moved downward 15 mm and back to 10 mm, (6) OM angle increased 6°, (7) the most remarkable reaction was that the alveolar processes, the palatal bone, the nose and the upper lip all adjusted to the mandibular change. Please note that the relation of the lip to the teeth was the same at age 17 as it was in the beginning. The occlusion was stable throughout the posttreatment period. The mandible obviously fulcrumed around the second molars. As this happened the teeth (particularly the lower incisors and cuspids) moved occlusally as rapidly as the mandible moved down. This changed the OM angle 6°, and increased the occluso-palatal angle. This same phenomenon probably happens to nearly all excessively hyperdivergent individuals. Again, this points out and helps to verify the concept that the OM angle is a good indicator of a vertical dysplasia.
Only vertical condylar growth in excess of vertical molar growth can cause a closure of a vertical opening. Again, it is the relationship between vertical condylar growth and vertical molar growth which causes a rotation of the mandibular plane. The authors reported that the average SN to mandibular plane angle of the sample was 23.9°. This is a quite low average angle where one would expect a relatively large vertical overbite, and a tendency toward forward rotation of the mandibular plane.
There is evidence that treatment proficiency might not have been the very best. The interincisal angle improved only 2.66° and lost 2.19° in retention. This indicates that torque was inadequate. Only .83 mm of overbite improvement was reported. The ANS-menton distance increased 8.02 mm, which indicated that there might have been too much Class II elastics or cervical headgear used.
The authors further state, "Opening rotations of the mandible are frequent during orthodontic treatment. It has been postulated that this rotation is a result of molar extrusion insufficiently compensated by the amount of growth in posterior facial height. Yet, many clinicians believe that this rotation is generally of little concern because the mandible usually returns to its original position after treatment. However in some cases after treatment this opening rotation may persist or even increase. The above quote in general is correct, but in 1998 we should not need to postulate-we should know. The mandibular plane returns to its original inclination only if the condyles grow sufficiently.
While the above quotes are in general true, they do not explain why and how these changes are produced-how they are caused. If we cannot explain exactly the cause, we are woefully inadequate with our analysis. Since these principles have been discussed in detail for the last thirty-five years, it is about time the profession begins to understand in detail the cause and effect involved.
The authors state that there was a significant increase in overbite post treatment, but there was no attempt to explain the cause. There was no mention of whether the lower incisors were intruded in treatment. If we hope to ever explain the cause of overbite return, we must not ignore the intrusion of the lower incisors. This is the most frequent cause of overbite return and crowding of lower incisors. I have literally been harping on this concept for the past thirty-five years and have been largely ignored.
In 1968 Fred Schudy stated, "It makes a lot of difference in what we believe. If we are unaware that intrusion of mandibular incisors is undesirable, we will have no qualms about producing it. We will go along year after year affecting this undesirable tooth movement and continue thinking that there is no need to correct overbite for it won't work anyway." Please see Article 16.
Early in this discussion it was stated that the Y-axis is of little value in explaining vertical opening. This concept was verified by the statistics referred to in this article. The authors stated, "The conflicting values seen in the changes in the mandibular plane angle and the Y-axis during retention are probably due to the mandibular plane's value being more sensitive(emphasis added) to the remodeling changes acknowledged in the lower border of the mandible with additional growth." Also it was stated that the angle of Y-axis to SN and the mandibular plane to SN are not interchangeable indicators of vertical change. This indicates that one is ineffectve.
It is hoped that this objective evidence will help to convince orthodontists worldwide that vertical opening created during orthodontic treatment is permanent unless vertical condylar growth exceeds vertical growth of molar teeth in the post treatment period. Also, it is hoped that colleagues will comment-for or against.
Bibliography 1. Creekmore, T.D., Inhibition and Stimulation of Vertical Growth of the Facial Complex, Angle Orthod. 1967; 37: 285-97.
2. Schudy, George F., A. Longitudinal Cephalometric Study of Post Treatment Growth; Its Implications in Orthodontic Treatment, Am. J. Of Orthod. 1974; 65:39.
3. Robert L Isaacson, D.D.S., Ph.D., Richard L Zappel, D.D.S., M.S., Frank Worms, D.D.S., M.S.D., Richard R. Bevis, D.D.S., Ph.D., and T. Michael Speidel, D.D.S,, M.S.D., The Effects of Mandibular Growth on the Dental Occlusion and Profile, Angle Orthodontist April 1977.
4. Schudy, Fred F., Vertical Growth Versus Anteroposterior Growth as Related to Function and Treatment, Angle Orthodontist vol.34, April 1964.
5. Peter H. Buschang, Ph.D; Joel Martins, D.D.S., Ph.D, Childhood and Adolescent Changes of Skeletal Relationships, Angle Orthodontist, vol. 68 no. 3, 1998
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