Anchorage consumption of the posterior teeth under different extraction patterns in bimaxillary dentition using clear aligner: a finite element study

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 second 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 consumption of anterior teeth simultaneously. This study aimed to explore the relatively suitable distal tipping value of posterior teeth when the first or the second premolars were extracted in bimaxillary dentition using CAs, further providing guidance for anchorage preparation design in clinical practice. Methods: Two bimaxillary finite element models (FEMs) with different extraction patterns were established to simulate the anterior en-masse retraction process of the CAs. In Model 1, the upper and lower first premolars were extracted, while in Model 2, the second premolars were extracted. Finite element analysis (FEA) methods were utilized to analyze the tipping angle of anchorage consumption and anchorage provided teeth. Results: Inter-group comparison revealed that the anterior teeth exhibited a greater lingual inclination and the posterior teeth exhibited a slighter mesial tipping tendency in Model 1 in response to the same tooth site. Intra-group comparison revealed that the closer to the extraction space, the greater the tilt, and the distal tipping tendency of the first premolar was more evident than the mesial tipping tendency of the first molar in Model 2. Compared between the bimaxillary dentition, the mesial tipping tendency of each tooth was more evident in maxilla. As for the posterior segments, the total mesial tipping tendency of maxillary posterior segments was heavier in Model 2, while regarding the mandibular posterior segments, it was heavier in Model 1. In addition, the highest hydrostatic stress of the periodontal ligament(PDL) was concentrated on the cervical and apical parts directly adjacent to the extraction spaces, and it exhibited relatively uniform distribution in Model 1 . Conclusions: During clear aligner treatment (CAT), the posterior teeth showed the same mesial tipping direction but to different degree when the first or the second premolars were extracted. Presetting the anchorage preparation design for the posterior teeth is necessary to alleviate the roller coaster effect, especially in the second premolar extraction cases. The anchorage consumption attributed to anatomical structure counteracts the effect of the number of anchorage provided units to a certain extent, so larger anchorage preparation value should be proposed for the maxillary posterior teeth.