Evaluation of Tooth Movement and Arch Dimension Change in the Mandible Using a New Three-dimensional Indirect Superimposition Method

Purpose: To analyze the amount and pattern of tooth movement and the changes in arch dimension of mandibular dentition after orthodontic treatment using a new three-dimensional (3D)-indirect superimposition method. Materials and Methods: The samples consisted of fifteen adult patients with class I bialveolar protrusion and minimal anterior crowding, treated by extraction of four first premolars with conventional sliding mechanics. After superimposition of 3D-virtual maxillary models before and after treatment using best-fit method, 3D-virtual mandibular model at each stage was placed into a common coordinate of superimposition using 3D-bite information, which resulted in 3D-indirect superimposition for mandibular dentition. The changes in mandibular dental and arch dimensional variables were measured with Rapidform 2006 (INUS Technology). Paired t-test was used for statistical analysis. Result: The anterior teeth moved backward, displaced laterally, and inclined lingually. The posterior teeth showed statistically significant contraction toward midsagittal plane. The amounts of backward movement of anterior teeth and forward movement of posterior teeth showed a ratio of 6 : 1. Although the inter-canine width increased slightly (0.8 mm, P<0.05), the inter-second premolar, inter-first molar, and inter-second molar widths decreased significantly with similar amounts (2.2 mm, P<0.05; 2.3 mm, P<0.01; 2.3 mm, P<0.001). The molar depth decreased (6.7 mm, P<0.001) but canine depth did not change. Conclusion: A new 3D-indirect superimposition of the mandibular dentitions using best-fit method and 3D-bite information can present a guideline for virtual treatment planning in terms of tooth position and arch dimension.