Cause and Effect
This is a commentary on the publication by Jean Lavergne and Nicole Glosson at the University of Bergen Orthodontic Department of the Dental School, Bergen, Norway. The title of the publication is, "The Influence of Jaw Rotation on the Morphogenesis of Malocclusion." The authors have "cause and effect" reversed.
The title should have been, "The Influence of Morphogenesis of Malocclusion on the Rotation of the Mandible." The rotation of the mandible has been completely misunderstood for many years. Having had a manuscript pertaining to the subject of "cause and effect" rejected by six different reviewers, I am well aware of the lack of appreciation for the subject. The rotation of the mandible is an effect, not a cause.
The rotation of the mandible was comprehensively discussed in 1960, was published in the Angle Orthodontist in 1964 and 1965, and has been discussed in many lectures. When we understand what causes the mandible to rotate forward and posteriorly, we will have no problem understanding "WHY" the mandible rotates.
This is the opening paragraph of an article written in 1960 and published in 1965 in the Angle Orthodontist: "An intensive study of the growth of the human head will inevitably lead to the realization that it involves the most complicated anatomical complex in all creation. The interrelationships are infinite and the causes and effects of these relationships are almost imponderable. The more our knowledge increases the more our ignorance unfolds. The vast stretches of the unanswered and unfinished still outstrip our collective comprehension. It is little wonder that the allied forces of medical science are making such slow progress in gaining an understanding of the growth of the jaws." Almost the same condition exists today!!
In the same article it was stated, "Rotation of the mandible involves primarily the vertical growth of the dentocephalic complex. It is this growth on which we would like to focus your attention. Documented evidence will be presented in this study to corroborate the validity of this thesis."
The rotation of the mandible is caused by the relationship between the effective vertical condylar growth and the combined vertical growth of the first molar teeth. These two primary growth increments will explain all movements of the mandible - all rotation of the mandible, all sagittal movement of the chin, all downward movements of the chin. When effective vertical condylar growth exceeds the vertical growth of molars, the mandible rotates forward. When vertical first molar growth exceeds condylar growth the mandible rotates posteriorly.
When the two primary causative growth increments are equal, the mandibular plane grows downward, remaining parallel to its original inclination, and is translated forward exactly the amount of posterior horizontal condylar growth.
There is another rotation regarding the increase of condylion/gnathion distance. The increase of the length of the mandible is affected by closure of the gonion angle. I have postulated that where there is a tendency toward Class III malocclusion, that condylar growth moves the chin forward and the closure of the gonion angle keeps it from moving too far forward. Taken together, these two growth entities are a marvelous phenomenon - keeping the growth of the face and jaws integrated.
The authors state "...that mandibular rotation may be a mechanism of the adjustment of the mandible to the maxillary complex." No! The causative increments are responsible for the vertical adjustment of both jaws.
It was also stated that, "Anterior mandibular rotation seems to be a process of shortening of the mandible related to specific changes in the gonion angle and condylar direction, and posterior relation seems to determine the maximum length of the mandible." The mandible never becomes shorter, it only fails to increase in proportion to the growth of the ramus. Posterior rotation may be caused by vertical growth of molars, unrelated to mandibular length. In conclusion the authors state, "Malocclusions in the vertical dimension appear to be more related to the degree of mandibular rotation." With this statement I totally agree.
A knowledge of the cause of rotations would have spared the authors from producing tables, computer generated charts, and an inordinate amount of work. Also, misguiding students is most regrettable.
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