Stomatological Hospital of Chongqing Medical University, No.426 Songshibei Road, Yubei District, Chongqing, 401147, China.
Chongqing Key Laboratory of Oral Diseases and Biomedical Sciences, No.426 Songshibei Road, Yubei District, Chongqing, 401147, China.
BMC Oral Health. 2023 Jun 22;23(1):416. doi: 10.1186/s12903-023-03106-8.
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.
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.
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 von-Mises stress concentration and the addition of power ridge caused the clear aligner to spread out on the labial and lingual sides.
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.
在隐形正畸治疗中，用透明牙套控制拔牙病例中中切牙的三维移动非常重要，但也极具挑战性。本研究旨在探讨不同弓丝曲设计方案下透明牙套拔牙病例中中切牙的生物力学效应，并为正畸临床提供合理建议。
构建了一系列有限元模型，以模拟不同弓丝曲设计的前牙内收或不内收。这些模型均由上颌牙列、拔除的第一前磨牙、牙槽骨、牙周膜和透明牙套组成。并对每个模型的生物力学效应进行了分析和比较。
在前牙内收无弓丝曲和前牙不内收单弓丝曲模型中，中切牙表现为冠舌倾和相对伸长。在前牙不内收双弓丝曲模型中，中切牙有冠唇倾和相对压低的趋势。在前牙内收双弓丝曲模型中，中切牙表现出与第一种模型相似的趋势，但随着弓丝曲深度的增加，冠内收值逐渐减小，冠外展值逐渐增大。模拟结果表明，中切牙牙周膜的颈、根尖区出现 von-Mises 应力集中。相邻牙齿的透明牙套连接区和弓丝曲区也出现 von-Mises 应力集中，弓丝曲的加入导致透明牙套在唇、舌侧展开。
在拔牙病例中，中切牙容易失去转矩和外展。双弓丝曲在没有辅助设计时具有一定的根转矩效应，但仍不能挽救牙倾斜在牙内收期间。对于牙齿平移，将一步法牙套设计改为两步法（倾斜内收和根控制）可能是更好的临床程序。
