Growth Displacement of the Mandible Relative to Anterior Cranial Base

Objective: To quantify and compare the relative downward and forward displacement of the human mandible with respect to anterior cranial base between the ages of 8.5 and 15.5 years. Materials and Methods: Longitudinal data were generated measuring the relative displacements of Pogonion, Menton, Gonion and Condyle on a lateral cephalograms taken annually for a sample of 20 treated and untreated children between the ages of 8.5 and 15.5 years. (Mathews Sample: See JDR: 1298-1306, AJO:91;29-45, etc). Data are reported relative to superimposition on Anterior Cranial Base in the Frankfort Plane frame of reference with the origin at Porion. (Horozontal values and deltas are designated X; vertical values and deltas are designated Y.)   Results: The anterior landmarks Pogonion and Menton displaced forward while the posterior landmarks Gonion and Condyle displaced backward. Downward displacement exceeded forward or backward displacement at all landmarks except Condyle. The displacements of the 4 landmarks from the 8.5 year baseline are summarized in the Table below (mean ± SD in millimeters). Conclusion: The mandible as a whole displaced downward with respect to anterior cranial base while growing larger in both the forward and backward directions. Vertical displacements greatly exceeded horizontal displacements at all landmarks except Condyle.