There are two concepts held by the basion horizontal system of superimposition, which we think are invalid. These two concepts are as follows:
- The vertical space for the teeth is created first and then the teeth fill in the space. The cranial base area grows upward and the mandible grows downward. The growth of the teeth does not jack the jaws apart.
- Condylar growth moves the entire mandible downward.
It is my opinion that the vertical growth of the teeth literally forces the jaws apart -- jacks the jaws apart. Please see Fig. 1 The angle between the two vectors of growth is supposedly established by genetic decree and the teeth have to fit between the two vectors, and allegedly are limited to this space. This does not seem logical.
Fig. 1
General vectors of craniofacial growth. (From the work of Coben)Without orthodontic treatment the jaws will reach a given amount of vertical opening; however with orthodontic treatment the jaws will usually permit additional vertical opening -- showing that the teeth have control of the vertical dimensioon of the face.
Regardless of the amount of condylar growth, it will not move the entire mandible downward. Only the vertical growth of molars will move the chin downward and backward. The more the condyles grow, the less the chin moves downward when molar height is deficient.
This concept is borne out by Coben's thesis published in 1955. Coben states, "While the vertical growth of the posterior face was only slightly greater than the mean, the the growth of the anterior face was below average, due primarily to the small increase in the upper dental height." No!! The reason was because the molars did not grow vertically sufficiently to force the chin downward.
Coben further states, "The proportion of posterior face height was much greater than average and the lower border of the mandible assumed a more horizontal inclination." If the condylar growth causes the entire mandible to grow downward, why did not the chin move downward also? While the vertical growth of the molars was not compared to the average, the real reason the chin did not move downward was because the molars did not grow vertically sufficiently to force the chin downward.
In the same article it was stated, "The vertical growth of the posterior face was close to average, but the increment of anterior face appeared to be deficient." Again, the reason the anterior face appeared to be deficient was because molar growth was deficient. If he had been conversant with the "mechanism of growth" concept, he could have used millimeters to better relate relationships.
We feel that these quotes are ample evidence to show that condylar growth does not move the entire mandible downward -- only the posterior end of the corpus.
The work of Fred Schudy 1966 and Bjork 1972 has taught us that the growth of the corpus of the mandible causes a reduction in the gonion angle. This was determined by both men entirely by the use of statistics. 1,2 While the growth of the corpus increases the length, ironically it also cancels some of the effective length by the reduction of the gonion angle. Please see Figures 5 & 6 of Article 1.
This narrative obviously runs counter to a well established, well documented analysis of a system of superimposition. Our profession must decide between the two in future years.
Statistics
(an afterthought)Statisticians are agreed that it is definitely incorrect to state that a given relationship is "statistically significant" without stating the level of significance (whether .05%, .01%, .001% or 0.0000%); and that it is also incorrect to report the probability (P. value) of a relationship without also reporting the correlation coefficient (R. value). Many authors are guilty of these omissions. It is important that we keep our literature statistically correct. This should be required for publication.
1Bjork, A., Skieller, V., Facial Development and Tooth Eruption: an implant study at the age of puberty, Am J Orthod 1972;62:339-83. References
2Schudy, Fred F., The Association of Anatomical Entities as Applied to Clinical Orthodontics, Angle Orthodontist Vol. XXXVI, No. 3, July 1966.
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