In the April 1998 issue of the AJODO, Dr. Peter Buschang¹ and Dr. Ary Santos-Pinto published an article pertaining to condylar growth and glenoid fossa displacement during childhood and adolescence.

In the opening paragraph they stated that forward displacement of the chin is dependent on the growth of the condyle and the displacement of the glenoid fossa. They further state that, "Both condylar growth and fossa displacement must be evaluated to fully understand mandibular growth changes occurring with or without therapy."

The authors clearly defined the manner of measurement of the position of the fossa, but the exact manner of measurement of the condylar growth was not completely clear. They stated that, "Condylar growth was evaluated on the basis of mandibular superimposition"-not demonstrating exactly how the measuring was done.

To "fully understand" mandibular growth changes there is another very important factor which must be included. The vertical growth of the molar teeth, upper and lower, is very important and must be factored in to explain the behavior of the position of the chin.

Fig. 1 A, B, C & D

Please see Fig. l, A, B, C & D. This shows the growth of two individuals, both having had 16 mm of condylar growth. This same 16 mm of growth produced quite different effects on the positions of the chins. In A the chin moved forward 13 mm, while in B the chin moved forward only 4 mm. In A the lower first molar did not grow vertically, while in B this molar grew 4 mm vertically.

In Fig. 1, C, we added 4 mm of lower molar height and this changed a strong Class I molar relation into an end on molar relation. In D we removed 4 mm of lower molar height and this changed an end on molar relation to a strong Class I relation. This is graphic proof that molar height dramatically affects chin position.

There is still another important factor which influences the forward positioning of the chin and the length of the mandible. The forward bending (reduction) of the gonion angle, due to growth, in some cases affects the mandibular length, condylion-gnathion distance.

In 1972 in his post doctoral thesis, George Schudy² first called attention to this phenomenon. He explained that the bending of this angle, from growth, limits the condylion-gnathion distance and the forward positioning of the chin. When the gonion angle reduces 17° it has a 13 or 14 mm effect on the length of the mandible. When the gonion angle is reduced 10° it has a 7 mm effect on the length of the mandible. That is, the condylion-gnathion distance fails to increase in proportion to the vertical growth of the condyle, by reason of the reduction of the gonion angle.

Buschang and Santos-Pinto found 30% to 40% of the sample to have anterior horizontal growth. It is known that in the latter years of growth, condylar growth becomes more anteriorly directed. Then, if the authors had been measuring the period from 13 to 17, the percentage might have been as much as 50% to 60%.

In a post treatment study of 72 subjects age 13 to 17, George Schudy² found average horizontal growth to be 1 mm posteriorly; and only a very few individuals had anterior condylar growth.

The wide difference of horizontal condylar growth found in those two studies was due to the way they were measured. Buschang and Santos-Pinto measured the condyle and fossa horizontally from a vertical base line, not being concerned where the symphysis was located at the time of the measuring.

Schudy superimposed on SN, registered at sella, punched a pin through both tracings in the region of the condyle, then superimposed on the mandibular plane, registered on the lingual outline of the symphysis and measured between the pinholes both vertically and horizontally. These two methods of measuring would obviously give different results.

While the authors were thorough in measuring the position of the fossa and condylar growth, their findings would not necessarily reflect the position of the chin, for the reasons expressed above. By measuring vertical growth of the ramus from gonion to "sella horizontal" perpendicular to Frankfort (please see Article 24 of the Schudy Chronicles on the Internet) it is not necessary to know exactly how much bone was added to the condyle, and exactly how much the fossa moved. These changes are exactly reflected in the overall measurement. Measuring is difficult enough when it is simplified.

Thus, in addition to condylar growth and fossa position, we need (1) vertical position of molar teeth and (2) the possible reduction of the gonion angle, to explain and fully understand the changes in the position of the chin.

Bibliography

¹Peter H. Buschang, PhD, and Ary Santos-Pinto, DDS, PhD: Condylar growth and glenoid fossa displacement during childhood and adolescence. Am J Orthod Dentofacial Orthop 1998; 113:437-42.

²Schudy, George F.: A Longitudinal Cephalometric Study of Post Treatment Craniofacial Growth: Its Implications in Orthodontic Treatment. Am Jour of Orthod 1974; 65:39.

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