Effect of arch shapes and missing second premolars on anchorage during maxillary molar distalization with clear aligners
Wang Shiyu1, Huang Yangyang1, Liu Hao1, 2, Yang Li2, Fan Dian1, Yuan Changyong1, 3, Wang Penglai1, 3
1School of Stomatology, Xuzhou Medical University, Xuzhou 221000, Jiangsu Province, China; 2Department of Orthodontics, 3Department of Implantology, Affiliated Stomatological Hospital of Xuzhou Medical University, Xuzhou 221000, Jiangsu Province, China
Abstract: BACKGROUND: The reciprocal force generated by the molar distalization with clear aligners can lead to anchorage loss. The effect of arch shapes and missing second premolars on anchorage has not been reported.
OBJECTIVE: To analyze the effect of arch shapes and missing second premolars on anchorage during molar distalization with clear aligners using the finite element method.
METHODS: Cone-beam CT data from an adult male were acquired from the database to establish the maxilla-upper dentition-periodontium-rectangular attachment-clear aligner model. The distal movement amount designed on the bilateral second molars was set to 0.25 mm. First, there were two groups in the study: second premolar bilateral presence and absence groups. Then, four subgroups in each group were created: tapered arch, ovoid arch, square Class II Division 1 arch, and Class II Division 2 arch groups. The Ansys software was used to calculate the displacement of the anchorage tooth and the stress of the periodontal ligament.
RESULTS AND CONCLUSION: Mesial tipping and extrusion of first molars and premolars, labial inclination and intrusion of anterior teeth occurred during the upper second molar distalization with clear aligners. When the bilateral second premolars were missing, the mesial displacement of first molars increased significantly while that of first premolars and anterior teeth decreased in all groups. The square Class II Division 1 arch group showed the least anterior labial inclination, while the tapered arch group showed the most. There was no significant difference between the ovoid arch group and the tapered arch group. Moreover, the magnitude of tipping in the square Class II Division 2 arch group was slightly higher than that in the Class II Division 1 arch group. The stress of the periodontal ligament of the anchorage teeth was concentrated on the cervical and apical regions of the teeth. And the lowest stress level was detected in the square arch group. Compared with the other groups, the stress on the labial cervical area of the periodontal ligaments was also significantly relieved in the square arch group. To conclude, the square arch is more favorable in terms of anterior anchorage control and periodontal ligament stress distribution. Anterior labial inclination efficiency can be increased in cases of Class II Division 2 by designing the anterior labial inclination in conjunction with molar distalization. If the second premolar is missing during molar distalization, it is not conducive to opening up the space in the area of the missing tooth.
Key words: clear aligner, molar distalization, anchorage, arch shape, second premolar missing, finite element