State Key Laboratory of Oral & Maxillofacial Reconstruction and Regeneration & National Clinical Research Center for Oral Diseases & Shaanxi Clinical Research Center for Oral Diseases, Department of Orthodontics, School of Stomatology, Air Force Medical University, Xi' an, 710032, China.
Urumql DW Innovation InfoTech Co.Ltd, Xinjiang, 830000, China.
BMC Oral Health. 2024 Oct 10;24(1):1204. doi: 10.1186/s12903-024-04951-x.
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.
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.
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.
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.
对于双颌前突的患者，拔除前磨牙是一种有效的策略。透明牙套（CA）通过缩短牙套的长度来关闭拔牙间隙。牙套末端产生的收缩力使后牙向近中倾斜，这被称为过山车效应。这种现象在第二前磨牙拔牙病例中更为严重。后牙支抗预备通常用于保护磨牙的角度，采用预设远中倾斜值的形式。然而，远中倾斜设计同时加剧了前牙的支抗丧失。本研究旨在探讨 CA 分别在上颌和下颌弓中拔除第一或第二前磨牙时后牙不同的支抗丧失，为临床支抗预备设计提供指导。
建立了两种不同拔牙模式的双颌有限元模型，以模拟 CA 的前牙整体内收过程。在模型 1 中，上颌和下颌第一前磨牙被拔除，而在模型 2 中，第二前磨牙被拔除。利用有限元分析方法分析了前牙和后牙的倾斜角度。
与两种模型相比，在模型 1 中，单个牙齿的前牙表现出更大的舌倾趋势，而后牙表现出更小的近中倾斜趋势。靠近拔牙间隙的牙齿倾斜度越大，第一前磨牙的远中倾斜趋势比第一磨牙的近中倾斜趋势更为明显。在上颌和下颌弓之间，单个后牙的近中倾斜趋势在上颌更为明显。此外，牙周韧带的静水压力最大值集中在与拔牙间隙直接相邻的颈部和根尖部分，在模型 1 中分布较为均匀。
在 CA 治疗中，当拔除第一或第二前磨牙时，单个后牙表现出相同的近中倾斜方向，但程度不同。对后牙进行预设支抗预备设计对于减轻过山车效应至关重要，特别是在第二前磨牙拔牙病例中。此外，上颌后牙应提出更大的支抗预备值。
