Incisor uprighting: mechanism for late secondary crowding in the anterior segments of the dental arches.

T he orthodontic literature abounds with speculation on the etiology of incisor crowding as a late (postpubertal) growth phenomenon. Lombardil cites most of the prominently considered factors in his survey of the literature: tooth discrepancies, arch size, interference of canines in lateral excursive movements, overbite, overjet, tooth removal, incisor-mandibular plane angle, type of original occlusion, third molars, and muscular forces. In their growth studies Bj6rk,2 Subtelny and Sakuda,3 and van der Linden4 report or present data revealing lingual uprighting of the incisor segments during late growth. Weinstein,5 citing Bjijrk’s study, speculated on the possible etiologic role of this uprighting in incisor-segment crowding during the typical postorthodontic-treatment growth period. LundstrijmG mentions the possible role of net lingual crown movement of the anterior teeth in decreases in arch circumference. The present study is a mathematical investigation of incisor uprighting, conducted in two parts. In the first part a mathematical method is derived to serve as a vehicle for the interpretation of and correlation between the data available from two growth studies, those of Sillman’ and Bj6rk.2 The method is used to predict the anterior crowding to be expected from the changes in mean anterior arch dimension reported by Sillman during a postpubertal growth period. The changes in mean incisor axial inclination during late growth reported by Bjiirk arc then related to the changes in mean anterior arch dimension of Sillman to determine the possible contribution of the former to the latter. The above investigation is based on mean dimensions and can therefore provide only a typical indication of the actual events. Therefore, the second part of the