Anchorage loss of the posterior teeth under different extraction patterns in maxillary and mandibular arches using clear aligner: a finite element study

Qiang et al. BMC Oral Health (2024) 24:1204 https://doi.org/10.1186/s12903-024-04951-x BMC Oral Health Open Access RESEARCH Anchorage loss of the posterior teeth under different extraction patterns in maxillary and mandibular arches using clear aligner: a finite element study Ruyue Qiang1†, Jie Gao1†, Yanqi Wang1, Wei Wang2, Yanning Ma1,3* and Zuolin Jin1* Abstract Background Extracting the premolars is an effective strategy for patients with bimaxillary dentoalveolar protrusion. Clear aligners (CAs) close the extraction spaces through shortening the length of aligners. The contraction force generated by the terminal of aligners makes the posterior teeth tip mesially, which is known as the roller coaster effect. This phenomenon is even worse in the 2nd premolar extraction cases. Posterior anchorage preparation is commonly used to protect the angulation of molars, taking the form of presetting distal tipping value. However, the distal tipping design aggravates the anchorage loss of anterior teeth simultaneously. This study aimed to explore the different anchorage loss of the posterior teeth when the 1st or 2nd premolars were extracted using CAs, respectively in maxillary and mandibular arches, further providing guidance for anchorage preparation design in clinical practice. Methods Two bimaxillary finite element models with different extraction patterns were established to simulate the anterior en-masse retraction process of the CAs. In Model 1, the maxillary and mandibular 1st premolars were extracted, while in Model 2, the 2nd premolars were extracted. Finite element analysis methods were utilized to analyze the tipping angle of the anterior and posterior teeth. Results Compared between two models, the anterior teeth exhibited a greater lingual inclination tendency and the posterior teeth exhibited a slighter mesial tipping tendency in Model 1 regarding individual tooth. The closer to the extraction spaces, the greater the tip, and the distal tipping tendency of the 1st premolars was more evident than the mesial tipping tendency of the 1st molars in Model 2. Compared between the maxillary and mandibular arches, the mesial tipping tendency of individual posterior tooth was more evident in the maxilla. In addition, the highest hydrostatic stress of the periodontal ligaments was concentrated on the cervical and apical parts directly adjacent to the extraction spaces, and it exhibited relatively uniform distribution in Model 1. † Ruyue Qiang and Jie Gao contributed equally to this work. *Correspondence: Yanning Ma Zuolin Jin Full list of author information is available at the end of the article © The Author(s) 2024. 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To view a copy of this licence, visit http:// creativecommons.org/licenses/by-nc-nd/4.0/. Qiang et al. BMC Oral Health (2024) 24:1204 Page 2 of 13 Conclusions The individual posterior tooth showed the same mesial tipping direction but to different degree when the 1st or the 2nd premolars were extracted during clear aligner treatment. Presetting anchorage preparation design for the posterior teeth is essential to alleviate the roller coaster effect, especially in the 2nd premolar extraction cases. Furthermore, larger anchorage preparation value should be proposed for the maxillary posterior teeth. Keywords Clear aligners, Anchorage preparation, Tooth extraction patterns, Maxillary and mandibular arches, Biomechanics Background Bimaxillary dentoalveolar protrusion patients often turn to the orthodontists complaining about unsatisfied appearance and easily injured incisors [1]. To improve the lateral profile, extracting the premolars is recommended. Whether to extract the 1st or the 2nd premolars should consider a combination of factors, including arch crowding, lateral profile protrusion, vertical dimension, and endodontic diseases [2–6]. It’s crucial to use all the premolar extraction spaces for anterior retraction without undesired mesial movement of the molars [7]. Clear aligner treatment (CAT) is gaining popularity due to its improved aesthetic value and comfort. By virtue of its reliability and validity [8], CAT is also suitable for the malocclusion with complex mechanical principles [9], such as premolar extraction cases. However, clear aligners (CAs) close the extraction spaces through shortening the length of aligners. The contraction force generated by the terminal of aligners makes the posterior teeth tip mesially, especially when the 2nd premolars are selected [10]. This phenomenon is known as the roller coaster effect. Enhancing the anchorage management through overcorrection is the cornerstone to deal with this side effect. Previous studies adopted multiple strategies to control of anterior torque. For instance, adding intrusive activation for the incisors, designing power ridge with a specific height [10–12]. In order to compensate the delayed tooth movement with aligners, optimization of posterior anchorage management should be strengthened simultaneously, taking the form of presetting distal tipping for the posterior teeth before retraction process [8]. To date, although there is consensus relating to posterior anchorage preparation in premolar extraction cases using CAs, the specific value under different extraction patterns is still controversial [8, 13–15]. Clinical observation finds that the roller coaster effect is more serious in the maxillary arch. Liu et al. demonstrated that the maxillary posterior teeth experienced greater levels of tipping than the mandibular teeth during retraction [16]. To our knowledge, the density of the maxilla and mandible is different [17]. Base on this, a hypothesis is raised that posterior anchorage design varies in the maxillary and mandibular arches owing to the anatomical structure. Finite element analysis (FEA) is an effective discretized numerical computation technology that simulates the in vivo situation by controlling a number of experimental conditions, including finite elements, nodes, and degrees of freedom [18]. Over recent years, FEA has been widely used in the ort (...truncated)