Authors’ response

Abstract

We appreciate the insightful and interesting comments on our article. It has been well recognized that clear aligner therapy (CAT) has poor control over the root during orthodontic tooth movement. This retrospective clinical study aimed to evaluate the accuracy of CAT on 4 different extents of root movement of incisors: pure tipping, controlled tipping, translation, and torque. In particular, the efficacy was calculated with the ratio of the achieved area of root movements over the predicted area. The results showed that the least predictable movement was torque (35.21%), which indicated torque movement was difficult to develop by CAT, as stated in the letter. Considering the forces and moments generated by clear aligners, Simon et al1Simon M. Keilig L. Schwarze J. Jung B.A. Bourauel C. Forces and moments generated by removable thermoplastic aligners: incisor torque, premolar derotation, and molar distalization.Am J Orthod Dentofacial Orthop. 2014; 145: 728-736Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar reported that the initial mean moments delivered by a single aligner in vitro were 7.3 N·mm for maxillary incisor torque, and torque supported by a power ridge would have higher moments than attachments. They reported that the clinical efficacy for maxillary incisor torque was 42% using the surface matching algorithm on the crowns.2Simon M. Keilig L. Schwarze J. Jung B.A. Bourauel C. Treatment outcome and efficacy of an aligner technique--regarding incisor torque, premolar derotation and molar distalization.BMC Oral Health. 2014; 14: 68Crossref PubMed Scopus (85) Google Scholar Castroflorio et al3Castroflorio T. Garino F. Lazzaro A. Debernardi C. Upper-incisor root control with Invisalign appliances.J Clin Orthod. 2013; 47 (quiz 387): 346-351PubMed Google Scholar also reported that close to 99% of the torque movement for maxillary central incisors was expressed using a power ridge (Align Technology, San Jose, Calif). Hahn et al4Hahn W. Zapf A. Dathe H. Fialka-Fricke J. Fricke-Zech S. Gruber R. et al.Torquing an upper central incisor with aligners--acting forces and biomechanical principles.Eur J Orthod. 2010; 32: 607-613Crossref PubMed Scopus (39) Google Scholar pointed out that aligners tend to lift during torquing. As a result, intrusion should be added during the setup when the incisors are designed for root lingual torque. This ensures a close fit of the aligner to the tooth. In addition, it is reported that incorporating intrusion displacement on aligners contributed to the expression of lingual root movement.5Jiang T. Wu R.Y. Wang J.K. Wang H.H. Tang G.H. Clear aligners for maxillary anterior en masse retraction: a 3D finite element study.Sci Rep. 2020; 10: 10156Crossref PubMed Scopus (6) Google Scholar This could also explain why some torque-like movement developed, albeit with no rectangular wires used during fixed appliance treatment. In this study, torque was defined as a movement in which the rotation center was within 2 mm of the incisal edge, which provided some space for intrusion. There is no typo in Figure 2, B, in which the pretreatment tooth (black) is bigger than the other 2 as the coordinate system was set up on the basis of the pretreatment tooth position, and the posttreatment and virtual teeth would seem smaller from the sagittal direction. Accordingly, there was a correction for the length of the virtual incisor position (vir-R’ and vir-C’).