Biomechanics of the mandible and growth extension

One way of determining the direction of growth of the mandible is to consider the temporomandibular joint and movement of the mandible as a four-joint gear system, regarding growth then as an extension of the gear system. Our aim was to examine any correlations between the type of biomechanical growth extension and change in the maxilomandibular relation after Class II therapy.

A total of 130 lateral cephalograms-before and after orthodontic treatment-were available from 65 adolescent class II patients with open bite or deep bite. The two lateral cephalograms from each patient were superimposed on the occlusal plane. Cephalometric values and the vertical base point deviation were determined from biomechanical analyses, together with three distances and three angles.

No correlation between the cephalometric data and distances or angles were observed. Although there were no significant differences in the distances, we did note significant differences in all three angles (p < 0.05).

If gear system extension during growth is considered, this can be interpreted as meaning that the occlusal plane of those patients with an initially open bite dropped during treatment, but that it rose in patients with an initially deep bite.