The importance of the reduction of the gonion angle, resulting from growth, seems to have been completely overlooked by the entire specialty of Orthodontics. I know of only one reference1 to this subject in orthodontic literature.As is commonly known, this angle of the mandible almost always undergoes a reduction during the growing period.
What effect does the reduction of this angle have upon treatment? The forward bending of the ramus has a profound effect on the length of the mandible, and therefore on the Class II correction. The reduction of the gonion angle can be as much as 14° and as little as zero. When this angle changes from 111° to 97°, this has about a 13 to 14 mm effect on the length of the mandible.
As the gonion angle becomes smaller and smaller, less and less condylar growth is used to increase the length of the mandible -- to increase condylion-gnathion distance. The remainder of the condylar growth merely moves the gonion angle downward. The effect of treatment on the size of this angle is not well understood. The biologic purpose of this phenomenon is not known.
The extent of the reduction seems to depend on the size of the angle. The lower the angle, the more the reduction. The higher the angle, the less reduction.
In case no. 14 in Bjork's growth study of twenty-one subjects, the gonion angle changed from 114° to 104°. The length of the mandible (condylion-gnathion distance) failed to increase 7 mm by reason of the bending of the gonion angle. Seven millimeters will usually mean the difference between Class I and Class II. Thus, it can be seen that the reduction of the gonion angle plays an important role in the treatment of malocclusion. Had this knowledge been included in the discussions by Bjork, it would have more accurately depicted the growth changes.
1Schudy, G. F., Posttreatment Craniofacial Growth: Its Implication in Treatment. Am. Jour. Of Orthod. 65: 39-57, 1974. Bibliography
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