How Does the Mandible Grow Forward,
and What Is the Mechanism of Jaw Function?
In order to resolve some differences of opinion that exist, regarding the forward displacement of the chin, we need to start with early postnatal basics. We must choose landmarks which are present at birth, such as sella turcica, as a basic landmark for relating growth changes.
In order to understand how the jaws grow, we must first understand the mechanism of jaw function. The jaws function by a mechanical principle unrelated to biology. Effective condylar growth moves the gonion angle downward and the chin forward (not downward), while the combined vertical growth of the upper and the lower molars moves the chin downward and backward. The only function which moves the chin downward is the vertical growth of the molar teeth.
This is a reciprocal action between vertical and horizontal growth. It has been described by the term "versus"-an appropriate term to describe the action of these two opposing forces. Thus, the function of the jaws is a synchronized mechanical action known as "chewing."
The effective growth of the condyles has been incorrectly described as moving the chin forward and downward. However, condylar growth cannot move the chin downward. The more the condyles grow, the less the chin moves downward, the less the ans to menton distance increases. The only function which can move the chin downward is molar growth as mentioned above. Please see Art. 12 in The Schudy Chronicles.
To analyze total craniofacial growth from birth to adulthood we must start from sella turcica. A line parallel to the earliest Frankfort line through sella is known as "sella horizontal." All anatomical points are measured vertically from sella horizontal. A line perpendicular to sella horizontal drawn downward from sella is known as the "great divide," which divides anatomical points that grow anteriorly from those which grow posteriorly. Thus, vertical growth is accurately measured from sella horizontal and anteroposterior growth is accurately measured from the great divide. Please see Art. 24 of The Schudy Chronicles.
All points on our globe can be pinpointed and measured in degrees of longitude and latitude. By the same token, all points on the lateral cephalogram can be pinpointed and measured in millimeters and fractions of millimeters of vertical and horizontal growth.
In review we should like to again emphasize the nature of the reciprocal function of the jaws-the manner of the function of the chewing process. Vertical growth of the condyles moves the mandible forward and is pitted against vertical molar growth, which determines the downward movement of the chin. This relationship controls the chewing process.
This suggested measuring process should be enthusiastically received by the entire orthodontic profession, since it is the first time in history that we have had an accurate way of measuring all growth that is recorded on the lateral cephalogram.
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