We appreciate Drs Xu and Li’s interest in our article and are happy to have an opportunity to discuss issues related to 3D technology in orthodontics.
In terms of the root movement, we agree with Drs Xu and Li’s opinion that root movement is a concern for the orthodontist. The methodology combining the 3D data of CBCT with the dental cast is effective for analyzing tooth movement. Our group had published a case report using 3D virtual model surgery, with the technology of combining 3D dental casts and 3D reconstructed images from multi-planar CT. Using this methodology, clinicians can obtain the combined images of the bony structure and the roots of the teeth from CT and the crowns of the teeth from high-resolution 3D dental scans. However, since “inclination” and “angulation” defined by Andrews are not related to the roots of the teeth, but to crowns of the teeth, CBCT data for the roots can be used only as a guideline in 3D superimposition studies. In addition, the image of the soft tissues from CBCT might be variable according to window width and level. Precise crown images, especially the occlusal surfaces of the dentition from CT data, are difficult to obtain if CT scans are taken in centric occlusion.
In terms of reference landmarks and areas, Drs Xu and Li have spent much time identifying stable structures during growth. Since the medial and lateral points of the third palatal rugae seem to be reasonably stable in estimating changes in spite of vertical growth, these could be used as reference landmarks for late adolescent and adult patients during orthodontic treatment. However, since these structures are located in the anterior part of the maxilla, small superimposition errors can bring out relatively large discrepancies in the posterior part of the maxilla. Therefore, for better accuracy, we used the palatal rugae in the anterior area of the hard palate and the midpalatal region between the right and left maxillary first and second molars in the posterior area of the hard palate. However, a prospective study will be needed to find more reliable anatomic areas during growth.