The Pitchfork Analysis
This is a commentary on an article published in the January 2000 issue of the AJODO, entitled "Johnston Analysis Evaluation of Class II Correction in Patients Belonging to Petrovic Growth Categories 3 and 5."
Let us say at the beginning that any discussion of the analysis of the growth and treatment of the craniofacial complex is sufficiently complex, when simplified as much as possible, but when we make no effort to simplify and clarify the manuscript, it may become difficult to understand.
The illustration on page 87 is unnecessarily complex, not well explained, displays many tables and contains the "usual" computer generated charts. When we understand the causative growth increments, we do not need to categorize malocclusions - we do not need to try to predict future growth. In our literature, we never make a distinction between first bicuspid extraction and second bicuspid extraction. We seldom discuss the effect of post treatment growth, a very important subject. Most post treatment growth studies are done by graduate students and are almost never published.
Also we must state early in this discussion that the Johnston pitchfork analysis does not take into consideration the vertical growth of the jaws. It is based totally on growth parallel to the occlusal plane. Therefore, basing an analysis only on the anteroposterior dimension of the jaws is hopelessly flawed right in the beginning.
When we base a discussion on the superficial measurements such as SNA, SNB, ANB, ML/NSL (SN to mandibular plane) and NL/NSL (palatal plane to SN) we have no chance of explaining the rotations of the mandible. These measurements are largely caused by the two primary growth increments. The anteroposterior dimension of the maxilla is not caused by the primary growth increments.
In treatment, "rotational types" can be either created or prevented, depending on the quality of treatment. In high angle cases, posterior rotation can be caused by moving molars vertically. In low angle cases forward rotation can be prevented by moving molars vertically. This is why the pitchfork analysis is so flawed.
Measuring just two GROWTH INCREMENTS and their relationship one to the other will open a WHOLE WORLD of understanding. These two measurements and their relationship will explain most movement of the mandible. "Fidutial" lines are effects, while vertical and horizontal increments are causes.
When we learn what causes the mandible to rotate, we will have no trouble understanding why the mandible rotates during growth and treatment. These important principles pertaining to the vertical growth of the jaws were discovered in 1960, published in 1964 and 1965, declared a classic in 1990 by the American Association of Orthodontics and discussed in many lectures in the Western hemisphere and in Europe in the sixties.
The rotation of the mandible is caused by the relationship between the effective vertical condylar growth of the ramus and the vertical growth of the molars, lower and upper. When effective vertical condylar growth exceeds vertical molar growth, the mandible rotates forward. When vertical growth of molars exceeds that of the condyles, the mandible rotates posteriorly. This simple principle of growth has been difficult for many orthodontists to grasp, even though we have tried continuously for forty years to get the point across and stimulate some discussion in the literature.
The authors state, "With respect to the growth of the mandible (R) a posterior rotation (P) increases the condylion/pogonion distance by opening the gonion angle." This could be a correct statement if the gonion angle really increases, but it seldom does. They also state, "…an anterior growth rotation (A) decreases the distance by closing the gonion angle." This is incorrect. The condylion/pogonion distance does not decrease, it only fails to increase in proportion to the growth of the ramus.
The reason the authors found it so difficult to explain the growth of the jaws was because they did not understand the "mechanism of growth of the jaws" - did not understand that the relationship between effective vertical condylar growth and vertical molar growth determines the rotation of the mandible. It was quite obvious that the reviewers did not understand these principles. The underlying reason for the difficulty was that the pitchfork analysis does not recognize the vertical growth of the jaws.
The authors state, "So far, it is not obvious why some biologic categories split into two or even more rotational types." It will always "not be obvious" unless we learn what causes rotations.
To engage in a learned discussion of the analysis of the craniofacial complex, yet completely ignore the importance of the vertical growth of the jaws and not try to explain the rotation of the mandible, is most regrettable. To those members who are intensely interested in the future and welfare of our beloved profession, it is a great sadness!
There is now complete lack of knowledge of "cause and effect" in our literature, and when we have no knowledge of cause and effect, erudite discussions become convoluted masses of words.Return to the Index of Articles
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