Differential Growth and Treatment
This is a commentary on an article which was published in the Angle Orthodontist in June 1999, entitled "Evaluation of Differential Growth and Orthodontic Treatment Outcome by Regional Cephalometric Superimpositions."
To introduce this commentary it seems logical to point out that the stated aim of the article was to evaluate the effects of regional superimposition (cranial versus maxillary) on interpreting mandibular displacement. Both methods were discussed in relation to a pre- and post-treatment sample of 22 patients treated for Class II Division I malocclusion. According to the authors, since the maxilla is subject to rotational changes, it must be measured from the palatal plane.
The authors prefer to superimpose on the best fit of a number of anatomical structures at the cranial base. They object to lines because of the possibility of the movement of the points connecting the lines.
I contend that we must establish lines to eliminate the differences of opinion by different investigators. By establishing lines vertically and horizontally which are parallel and perpendicular to the earth's surface, we have completely stable landmark for measuring and assessing growth and treatment changes.
When the posterior portion of the maxilla carries the maxillary molars downward, this will cause the mandibular plane to become steeper, unless the condyles grow an equal amount. This simple relationship proves the validity of the "cause and effect" relationship-proves that we can predict the movement of the chin by measuring two basic growth increments. Please see Fig. 1A and B. When are we going to start measuring growth increments instead of the effects of growth increments?
Fig. 1A and B: These two growth records illustrate that condylar growth, as related to molar growth, is the key to changes of the occlusal plane and the mandibular plane. In A it is shown that the condyles grew 23 mm and the molars grew vertically 18 mm (9 + 6 + 3). The result was an 8° change of the occlusal plane and 5.5° of mandibular change. In B is shown that condylar growth was 4 mm and the molars grew 10 mm (4 + 2.5 + 3.5). This resulted in a 5° posterior rotation of the occlusal plane and a 7° posterior rotation of the mandibular plane. (From the growth study of the University of Michigan)
Isaacson is the only American university professor who has ever reported measuring basic growth increments. These concepts were discovered in 1960, comprehensively discussed in the Angle Orthodontist in 1964 and 1965, and rediscussed in 1998 and 1999 in the Schudy Chronicles on the Internet.
The article under discussion, though complicated and difficult to follow, in general contains valid concepts; but they refer to the effects of growth increments, not the increments themselves. American orthodontic literature has never acknowledged that we know anything about cause and effect. As long as we refuse to believe that nothing is known about cause and effect, we will never make any real progress.
Please see Fig. 2A, B, C, and D. It is plain and simple to see that the relationship between just two basic growth increments controls the movements of the mandible-controls the rotation of the mandible, the translation of the mandible, the forward movement of the chin, and the vertical movement of the chin. When just two basic growth increments, easy to measure, control all of the spacial changes of the jaws, why do we use all sorts of complicated and sometimes invalid methods to explain mandibular changes? (The reduction of the gonion angle affects the condylion/gnathion distance, but we will not discuss it here.)
Fig. 2A, B, C and D: Here is shown growth studies of two patients, each having 16 mm of condylar growth.
In A the molars grew 11 mm (8 + 3 + 0) which caused the chin to move forward 13 mm and the mandibular plane to rotate forward 7°.
In B the molars grew 15.5 mm (7.5 + 4 + 4), and the chin moved forward 4 mm. The mandibular plane rotated forward 1°.
In C we added 4 mm to the lower molars and this changed a full Class I relation to an end on molar relationship.
In D we removed the 4 mm which the lower molars grew, and this changed an end on relation to a full Class II relationship. (From the growth study of the University of Michigan)
These two basic growth increments (condylar growth and vertical molar growth) will only explain the behavior and function of the mandible. They will not explain the forward development of the maxilla. The forward growth of the maxilla is due to local foci and sutural growth. It will be noted in Fig. 3A and B, by comparing A and B, that superimposing on the palatal plane causes the mandible to rotate forward, due to the fact that growth above the palatal plane is not included. This leads to increased effectiveness of condylar growth, which in turn causes forward rotation of the mandible.
Fig. 3 A and B: Shows growth changes which occurred in the jaws and facial complex as well as the trajectory of the growth of the chin. I would like to have seen more detail. A shows what happened to the face and jaws. Vertical and horizontal growth were equal, as indicated by no rotation of the mandibular plane in the cranial superimposition. B shows what happens only when we superimpose on the palatal plane. (From Efstratiadis and associates)
The finding that the vertical dimension is more responsible for the difference between cranial superimposition and superimposition on the palatal plane, than the horizontal dimension, is as one would expect, since the vertical growth above the palatal plane is not counted.
Even though the maxilla undergoes rotation, it need not be measured separately because it is surrounded and made to melt into the total by the basic growth increments. Superimposing on regional landmarks is valid so long as the effect is within the same bone; but when the effect of this superimposition extends across the occlusal plane to another bone, its effect becomes invalid. When two bones interface at a line such as the occlusal plane, their relationship can only be correctly recorded when superimposed on the same landmarks.
While it is true that the palatal plane rotates in growth and in treatment, and it is good to have it demonstrated in Fig. 3B, still this demonstrated change is isolated from what really happened. This illustration shows that the mandibular plane rotated several degrees and the chin grew forward several millimeters. This did not happen, except in maxillary superimposition. Fig. 3A shows what really happened.
These two methods should not be compared; they are not comparable. Superimposition on the cranial base correctly shows us what anatomical changes have occurred during treatment. Superimposition on the palatal plane merely shows us how the two jaws changed with relation to one another, within the big picture. The inexperienced reader will get the idea that we have a choice between the two methods of superimposition, which is not true. In Fig. 3A the mandibular plane did not change even though it was shown to change by superimposition on the palatal plane. It is misleading when we use the term "cranial verses maxillary"
In Fig. 2A and B it will be noted that the two patients shown each grew 16 mm at the condyles, but this same amount of growth had quite different effects on the positions of the mandibles. In A the mandible rotated forward 7° and the chin moved forward 13 mm; while in B the mandibular plane rotated 1° and the chin moved forward 4 mm. To produce these movements the molars grew 11 mm (8 + 3 + 0) in A, while in B the molars grew 15.5 mm (7.5 + 4 + 4).
In Fig. 2C we have added the 4 mm which the lower molars failed to grow. This changed a full Class I relationship to an end on relationship. In D we removed the 4 mm which the lower molars grew. This changed the molar relationship from an end on to a full Class I relationship.
This objective evidence is all that should ever be needed to convince the profession that basic vertical and horizontal growth increments and their mutual relationships is a complete explanation for depicting mandibular movement. Also the lines "sella horizontal" and the "great divide," discussed in Article 24 of the Schudy Chronicles, should convince any reasonable minded person that it is valid and superior to all other analyses.
Article 1 of the Schudy Chronicles contains an illustration which shows why the great divide, divides structures which grow forward from those which grow posteriorly. This illustration shows objective evidence to explain why line sella horizontal is parallel to the earth's surface and why it is based on completely stable landmarks.
It was a great disappointment to me that there was no discussion of the effects of post treatment growth, even though that was not the specific subject. As I have stated of recent date, from the standpoint of treatment, post treatment growth is probably the most important segment of total growth. If we ever expect to improve our treatment we must begin to discuss specific characteristics of a well treated malocclusion. Superior treatment is our only defense against the effects of post treatment growth. Post treatment growth greatly improves superior treatment, but causes disaster to the poorly treated case. This growth will usually translate the upper molars and bicuspids forward, tip the incisors forward, improve the overjet and improve the interdigitation. For the poorly treated case with inadequate torque, too much overbite along with a high interincisal angle, crowding of lower incisors continues to increase over time and never stops.
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