GROWTH INCREMENTS
The question has been asked many times in orthodontic journals and on the Internet,
Why do we not measure growth increments???
When just two growth increments will explain all movements of the mandible, why does not someone either challenge the concept or begin to use it? In 40 years no one has ever questioned a single word of the concept; yet not a single head of an orthodontic department in the U.S.A. has published a word concerning this important subject, except Isaacson and associates. But in Europe Pancherz and associates have recognized the importance of this concept. Please see Fig. 1.
Fig. 1: These two growth increments shown, cause all movements of the mandible - cause the mandible to rotate, cause the mandible to translate, cause the chin to move downward and with the help of the length of the corpus and the reduction of the gonion angle, cause the chin to grow forward.
Fig. 1 shows that condylar growth and vertical molar growth largely control the growth of the lower face. In this growth study these two increments are equal. This is why there was no rotation of the mandible. Posterior horizontal condylar growth (not shown here) causes the translation of the mandible. When condylar growth exceeds vertical molar growth, the mandible rotates forward. When vertical molar growth exceeds condylar growth, the mandible rotates posteriorly. Downward movement of the chin is caused by vertical molar growth. Forward movement of the chin is caused by condylar growth plus the length of the corpus, plus the reduction of the gonion angle. It should now be obvious to the reader why we need to discuss growth increments. Some heads of orthodontic departments believe that we know nothing about cause and effect. Alas!!!
The entire profession of orthodontics is aware of the possible movement of sella turcica and nasion during growth. So, given this possibility, we will never be able to exactly measure sella nasion.
Our profession has been studying the possible movement of sella and nasion for the past 65 years, and has done a wonderful job of determining the growth of the cranial base. While we will continue to study and learn more about this anatomical complex, it now seems important to change the focus and begin to try to select the one most scientific analysis which we can standardize for the future.
We should not continue to harp on the shortcomings of S-N and FR, but turn our attention to establishing stable landmarks for accurately measuring vertical and horizontal growth. Please see Article 24 of the Schudy Chronicles on the Internet. Frankfort plane is parallel to the earth's surface, and we should base our analysis on a line parallel to Frankfort. Then a line perpendicular to Frankfort through sella turcica will serve as a stable landmark from which we can accurately measure, both anteriorly and posteriorly from this line. The first Frankfort line of a series of films will serve as a basis to which subsequent Frankfort lines can be transferred.
It is not necessary for the clinical orthodontist to exactly understand these anatomical details, but the proficient members automatically correctly apply these principles. Everyone uses electronic devices (telephone, radio, television, e-mail, and Internet) that they do not understand; but the scientists who developed these devices understood. It is the university professors who do not seem to understand these principles; and it is extremely important that these principles be taught to the interns. How will we teach them unless the teachers understand?
Sella Horizontal and the Great Divide
In Fig. 1 we have tried to clarify the function of the primary growth increments, condylar growth and vertical molar growth. In Fig. 2 we have tried to confirm the validity of the great divide - to explain why it divides anterior growth from posterior growth.
Sella horizontal is parallel with Frankfort, which makes it parallel with the earth's surface. This provides a horizontal line which is completely stable, to which we can relate the vertical growth of all structures in the craniofacial complex.
Fig. 2: Illustrating the principle of the great divide. The small circle represents age 3 months, and the large circle represents age 16 years, of the same patient's growth. As the cranium grows concentrically around sella turcica, any diameter divides growth on each side of it. Therefore the great divide, a diameter of the circle, divides the growth which occurs on each side of it. This has been verified by actual measurements. A 7 mm section of the circumference at age 3 months becomes 14 mm at age 16. (The patient, shown from 3 months to 16 years, is No. 3092 from the growth study of Western Reserve University, Cleveland, Ohio.)
The great divide is a line drawn downward from sella turcica perpendicular to Frankfort, which is completely stable. This line divides anatomical points which grow forward from those which grow posteriorly. Fig. 2 shows why anatomical structures grow away from the great divide. When the skull grows concentrically around sella turcica, any diameter acts as a divide to the growth which occurs on each side of it. Since the great divide is a diameter of the skull, then anatomical points move away from it. Thus, we have given a scientific explanation for two lines to which we can relate vertical and horizontal growth.
In conclusion, I hereby challenge any colleague anywhere in the world to show me, with objective evidence, why this analysis is not accurate and valid.
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