Effects of different designs of orthodontic clear aligners on the maxillary central incisors in the tooth extraction cases: a biomechanical study

Meng et al. BMC Oral Health (2023) 23:416 https://doi.org/10.1186/s12903-023-03106-8 BMC Oral Health Open Access RESEARCH Effects of different designs of orthodontic clear aligners on the maxillary central incisors in the tooth extraction cases: a biomechanical study Xuehuan Meng1,2,3†, Chunjuan Wang1,2,3†, Wenjie Xu1,2,3, Rui Wang1, Leilei Zheng1,2,3*, Chao Wang4*, Raffaella Aversa5 and Yubo Fan4 Abstract Background Controlling the 3D movement of central incisors during tooth extraction cases with clear aligners is important but challenging in invisible orthodontic treatment. This study aimed to explore the biomechanical effects of central incisors in tooth extraction cases with clear aligners under different power ridge design schemes and propose appropriate advice for orthodontic clinic. Methods A series of Finite Element models was constructed to simulate anterior teeth retraction or no retraction with different power ridge designs. These models all consisted of maxillary dentition with extracted first premolars, alveolar bone, periodontal ligaments and clear aligner. And the biomechanical effects were analysed and compared in each model. Results For the model of anterior teeth retraction without power ridge and for the model of anterior teeth no retraction with a single power ridge, the central incisors exhibited crown lingual inclination and relative extrusion. For the model of anterior teeth no retraction with double power ridges, the central incisors tended to have crown labial inclination and relative intrusion. For the model of anterior tooth retraction with double power ridges, the central incisors exhibited a similar trend to the first kind of model, but as the depth of the power ridge increased, there was a gradual decrease in crown retraction value and an increase in crown extrusion value. The simulated results showed that von-Mises stress concentration was observed in the cervical and apical regions of the periodontal ligaments of the central incisors. The clear aligner connection areas of adjacent teeth and power ridge areas also exhibited vonMises stress concentration and the addition of power ridge caused the clear aligner to spread out on the labial and lingual sides. † Xuehuan Meng and Chunjuan Wang contributed equally as first authors. *Correspondence: Leilei Zheng Chao Wang Full list of author information is available at the end of the article © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Meng et al. BMC Oral Health (2023) 23:416 Page 2 of 12 Conclusions The central incisors are prone to losing torque and extruding in tooth extraction cases. Double power ridges have a certain root torque effect when there are no auxiliary designs, but they still cannot rescue tooth inclination during tooth retraction period. For tooth translation, it may be a better clinical procedure to change the one-step aligner design to two-step process: tilting retraction and root control. Keywords Clear aligner, Extraction case, Power ridge, Finite Element Method, Biomechanics Background With the increasing demand for cosmetic treatments, traditional fixed appliances can no longer meet the aesthetic needs of orthodontic patients. The concept of modern orthodontics not only requires improving the patient’s aesthetic appearance after orthodontic treatment, but also aims to achieve aesthetic standards in the process of treatment. In early years, lingual appliances were popular among orthodontic patients due to their excellent aesthetic effects [1]. Clear aligners have emerged with the development of computer science and technology. Of all appliances, clear aligners are the most beautiful, comfortable, convenient, and beneficial for oral hygiene. Additionally, using clear aligner treatment greatly reduces the amount of time spent in the doctor’s chair [2–6]. Therefore, the clear aligners have quickly become the favourite of the orthodontic industry. Clear aligners are a series of transparent, elastic, movable correction devices designed and manufactured by digital technology. Each step of the aligners differs slightly from the current position of the teeth. The teeth are gradually moved to the target position by the elastic restoring force generated when wearing the aligner on the crown, and each-step aligner is designed to move teeth by 0.1-0.3 mm. Patients are required to wear eachstep aligner for approximately two weeks, and for at least 20 h per day [7, 8]. Of course, clear aligners also have some limitations. For example, the lack of specific force application points can lead to insufficient force on teeth, and materials are prone to aging in the oral cavity. Therefore, tooth movement efficiency is not always as good as predicted. Moreover, some patients may require multiple stages of intermediate refinement or additional aligners, and even need to switch to fixed orthodontics before completing the treatment [9–11]. However, because the advantages of clear aligners far outweigh the disadvantages, invisible orthodontics continues to thrive and improve. Early studies have shown that clear aligners perform well in distal movement of maxillary molars, alignment of anterior teeth, and expansion of the anterior arch. However, they are not effective in extruding anterior teeth, rotating round teeth or translating teeth [11–16]. With the rapid development of materials science and manufacturing technology, invisible orthodontic technology has been continuously improved, expanding its indications from initial mild to moderate malocclusion to complex malocclusion cases that include tooth extraction [17–19]. In orthodontic clinic, crowded dentition and protruding profile are common complaints that often require tooth extraction. En-mass retraction of anterior teeth is a commonly used method for closing the extraction space [20]. Whether in fixed orthodontics or invisible orthodontics, the accuracy of 3D control of upper incisors is crucial as the posi (...truncated)