Magnitude of Monthly Pubertal Mandibular Growth in Class II Controls, And the Relationship between Upper and Lower Incisor Movements and Horizontal vs. Vertical Expression of Chin Growth

Objective: To quantify how much the mandibles of untreated pubertal boys and girls grow per month, and to show that the horizontal and vertical expression of that growth of the chin is related to how the incisors change positions during that growth. Materials and Methods: Lateral cephalograms from untreated deep bite class II boys and girls chosen to be of pubertal age were selected. Films were approximately two years apart. The films were fiducialized, and landmarks were drawn on the T1 film and transferred to the T2 film by one experienced investigator. Changes in upper and lower incisor positions and Gnathion were measured for both vertical and horizontal components. The average monthly radial movement of Gnathion was calculated and the relation between incisor movements and Gnathion horizontal and vertical movement was graphed. Results: For boys the average monthly movement of Gnathion was 0.27 mm/mo. For girls, it was 0.18 mm/mo. The horizontal and vertical movements of the lower incisor were then subtracted from the horizontal and vertical movements of the upper incisor, and that sum was graphed against the ratio of the Gnathion horizontal movement vector (or the Gnathion vertical movement vector) to the Gnathion radial movement vector, and a near-linear relationship was found for both boys and girls. Conclusion: The percentage horizontal growth (and vertical growth) changes in Gnathion is related approximately linearly to the sum of horizontal and vertical growth movements of the incisors. Pubertal untreated boys grow the chin 50% more than pubertal girls.