Three-dimensional changes of the dental arch form and the inclination of the first molars: Comparison between crowding-improvement and crowding-aggravation groups

The aim of this study was to understand the differences in the growth of the dentition between persons with normal occlusion and crowding. Two groups of subjects, one with crowding-improvement group (Im-group) and the other with crowding-aggravation group (Ag-group) were analyzed longitudinally. Im-group consisted of 26 maxillary arches (14 boys and 12 girls) and 30 mandibular arches (11 boys and 19 girls), and the Ag-group consisted of 22 maxillary arches (10 boys and 12 girls) and 24 mandibular arches (12 boys and 12 girls). Dental casts were scanned with a contact-type 3D scanner. Arch form and inclination of the first molars were measured at 7 and 12 years. Our materials at the age of 7 years showed no significant differences in the arch form and inclination of the first molars between the two groups. In the Im-group, regarding changes in the amount of the dental arch form during the observation period, the maxillary arch width increased with an average of 5.0° first molar distal rotation, and the mandibular arch width increased with an average of 9.4° first molar buccal upright. As a result, maxillary and mandibular arch widths were large in the Im-group compared with the Ag-group at 12 years. Therefore, a characteristic of the difference between both groups was three-dimensional changes of the inclination of the first molars.