We read with interest Min-Young Cho and colleagues’ article on three-dimensional (3D) analysis of tooth movement and arch dimension changes by superimposing 3D virtual images (Cho MY, Choi JH, Lee SP, Baek SH. Three-dimensional analysis of the tooth movement and arch dimension changes in Class I malocclusions treated with first premolar extractions: a guideline for virtual treatment planning. Am J Orthod Dentofacial Orthop 2010;138:747-57). As the 3D image superimposition method is more widely used in orthodontic studies, there are 2 aspects of this method to which we want to draw attention.

In orthodontic evaluation and tooth movement analysis, clinicians are interested in the final position of the crown, as well as the kinetic manner of root movement. Root movement control determines the orthodontic procedure, and the final position of the crown determines the esthetic and functional outcomes.

Three-dimensional images of tooth and arch are the substructure of 3D image superimposition methods. Computed tomography (CT) and optical scans are 2 ways commonly used for 3D image reconstruction. An optical scan, which has higher resolution, can provide only the image of tooth crown, whereas the CT scan can provide the images of both crown and root with relatively lower resolution. Can we find a way to get a comprehensive 3D image that has the information about not only the crown with high resolution, but also the root? In our work (funded by NSFC, No. 30801307), we attempted to develop a method to obtain a combination model of images from the CT and the optical scans. First, we get the image of both crown and root from cone-beam CT (CBCT). The high-resolution image of the crown is obtained by optical scan of the dental cast. Then, the crown image from the optical scan was superimposed onto the image from the CBCT scan. The crown part of the image from the CBCT scan was substituted for those from the optical scan. The generated image can be used for further analysis. The individual tooth (including the root and crown) can also be extracted.

When tooth movement and arch dimensional changes are evaluated by the superimposition method, another aspect must be addressed: reference landmarks and areas for superimposition. As Min-Young Cho said in the article, several landmarks and references had been suggested in previous studies. Which are the best and most stable landmarks in orthodontics? Furthermore, the arches and the jaws of teenagers are growing and changing. Which reference landmarks and areas can we choose for them? There are still many unanswered questions.