The process of the eruption of permanent mandibular anterior teeth was observed overall with the use of radiographs. The materials were the serial panoramic tomograms obtained from 85 children ranging from the deciduous dentition stage to the mixed dentition stage.
Before measuring them, a preliminary study was made by using the dry skull, and the reproducibillity of the images on the panoramic tomogram was confirmed.
The results were as follows
1) The chronologic change of the amount of tooth germ formation and rate of eruption of the central incisor, lateral incisor and cuspid showed a curve using a cubic regression equation with the chronological age as an independent variable in both boys and girls. Based on, it is suggested that the growth change of the permanent mandibular anterior teeth is not constant in the course of development. Even though the cuvic regression curves of the central and lateral incisors showed similar shapes, the age of the growth spurt of the central incisor occured about one year later than that of the lateral. The rate of eruption of the lateral incisor and cuspid in girls showed a significantly higher rate than in boys.
2) The extent of overlap among the four incisor germs showed a tendency to decrease gradually after about 80 months of age. This dissolution of tooth germ overlap seemed to be mostly related to the eruption of the lateral incisor.
3) The angle of tooth axial inclination showed no marked change in both central and lateral incisors with age.
4) The root resorption of the deciduous lateral incisor was apt to be delayed compared with those of the deciduous central incisor and canine in relation to the rate of eruption of their successors.
5) The rotation of the permanent tooth germ was not observed frequently in central incisor, but was seen in about 40 to 50 % of the lateral incisors. The rotation of the lateral incisor seemed to show a gradual decrease as the eruption occured.