The Mechanism
of Growth and Function
The purpose of this discussion is to repeat a concept which was developed in 1960. This concept of growth and function was presented at the 1964 Biannual Meeting of the Edward H. Angle Society, was published in the Angle Orthodontics in 1965, and was declared a classic by the American Association of Orthodontists in 1991.
Despite the fact that it is valid, simple and easy to apply, it apparently is not well understood. Only a few investigators have used this concept in the thirty-nine years since its introduction.
The "Mechanism of Growth and Function" concept represented the introduction of importance of the vertical dimension of the human face in the treatment of malocclusion. Prior to that time (1960) the vertical dimension of the face had not been considered.
This new concept involved measuring the causative increments, the building blocks, at the posterior aspect of the jaws and noting their effect on the forward positioning of the chin. See Fig. 1. The anteroposterior growth of the maxilla is not involved in this system-only the vertical growth of the upper first molars. The relationship of the effective vertical growth of the condyles with the vertical growth of the upper first molars plus the vertical growth of the lower molars, controls the physical and mechanical function of the jaws.
Fig. 1 A & B: A four year growth study of a female from age 8 to 12, showing that the condyles grew 11 mm, the maxilla 7 mm, upper 6 grew 4 mm, lower 6 grew 0 mm, upper one grew 2 mm, and lower one grew 4 mm. This was very favorable growth. Of special interest is the fact that lower one migrated lingually 4 mm, lower 6 moved mesially 1 mm and did not grow vertically. The mandible did not rotate and was translated forward 3 mm.
This is a pure mechanical relationship of the function of the jaws, having nothing to do with biology-a principle of jaw function, and principles never change. The effective growth of the condyles moves the gonion angle downward and the chin forward, while the growth of the molars moves the chin downward and establishes the trajectory of the chin movement. The posterior condylar growth only moves the chin forward on the trajectory established by the relationship of vertical and horizontal growth.
At the time of the introduction of this system of measurement, we were using SN as a basis for vertical measurement; but due to the downward slope of this line, with relation to horizontal, it causes a minor error in vertical measurements of anatomical points which move anteriorly or posteriorly in growth.
In 1998 we introduced the "sella horizontal line" parallel to the first Frankfort line of a series of films. This made it possible to accurately measure all points in the craniofacial complex vertically, regardless of whether they move anteriorly or posteriorly. Also a line perpendicular to the sella horizontal line through sella (the great divide) serves as a base from which all points can be accurately measured horizontally.
The lines sella horizontal and the great divide now make it possible to accurately measure all points in the facial complex both vertically and horizontally. This method of measuring vertical and horizontal growth has been demonstrated several times, but since it is a part of this discussion we will again explain how the measuring is done.
The two tracings are superimposed on line SN and registered at sella. Then a pinhole is punched through both tracings near the condylion of the first film. Next, the two tracings are superimposed on the mandibular plane and registered on the lingual outline of the symphysis. Then the movement of the pinholes is measured both vertically and horizontally.
The vertical position of the upper first molar (distal surface) is measured from sella horizontal to the occlusal plane, parallel to the great divide. The lower first molar is measured from the mandibular plane to the occlusal plane perpendicular to the occlusal plane.
When the effective condylar growth (measured from the gonion to sella horizontal) is greater than the vertical growth of the molars, the mandibular plane rotates forward. When combined molar growth is greater, the mandibular plane rotates posteriorly. When these two measurements are equal, the mandibular plane remains parallel to its original inclination as it moves downward. The mandible is translated forward, and the chin moves forward the exact amount of the posterior condylar growth, provided there is no posterior movement of the fossa.
It is hoped that this simple explanation will be crystal clear to all readers and will encourage them to use this method of measuring in the analysis of growth and treatment results.
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