Korean J Orthod.  2025 Jan;55(1):69-81. 10.4041/kjod24.202.

Evaluation of the effects of the third molar on distalization and the effects of attachments on distalization and expansion with clear aligners: Three-dimensional finite element study

• Kuguoglu A
• Akarsu-Guven B

• KMID: 2565030
• DOI: http://doi.org/10.4041/kjod24.202

Abstract

Objective
This study aimed to evaluate the effects of attachment design on maxillary molar distalization and simultaneous expansion during distalization, and the influence of third molars on distalization in the clear aligner technique using the finite element method.
Methods
Six models were created to evaluate three different attachment designs on the second molars. Model I: employed a vertical rectangular attachment; Model II: used a vertical rectangular attachment with the presence of the third molar; Model III: used a combined semi-elliptical attachment; and Model IV: featured an opposed semi-elliptical attachment with buccal and palatal components. Models I through IV focused on distalization. Models V and VI were created by adding expansions to Models III and IV. The displacement amounts, clear aligner deformations, and stress distributions were analyzed using Ansys 19.2.
Results
The presence of a third molar reduced maximum total displacement by 17%. Models I and III demonstrated similar distal displacement and tipping, both of which were more pronounced than in Model IV. Model IV achieved the most parallel tooth movement, with the least distal and buccal tipping and minimal distopalatal rotation. In the comparison of Models V and VI, Model VI exhibited greater distal and buccal displacements and more tipping than Model V.
Conclusions
The presence of a third molar reduced distalization efficiency and increased clear aligner deformation. For both movements, the attachment that provided the most parallel movement showed the least displacement. When selecting attachments, it is essential to consider factors such as tooth rotation, inclination, bone support, and root health.

Keyword

Attachment; Distalization; Expansion; Clear aligner

Figure

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  Figure 1 Patient records used in maxilla digital modeling; A, Maxillary arch intraoral scan image of the patient in standard tessellation language (STL) format. B, Cone-beam computed tomography image of the patient in digital imaging and communications in medicine (DICOM) format. C, D, Patient anatomical model. E, Mesh structure of the finite element model, front. F, Mesh structure of the finite element model, side.

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  Figure 2 A, Vertical rectangular attachment. B, Combined semi-elliptical attachment. C, Opposed semi-elliptical attachment.

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  Figure 3 Boundary condition applied to the finite element model; A, Yellow-colored arrow is the direction of the distalization force. B, Yellow-colored area is the clear aligner region where 0.25 mm distal displacement is applied. C, Yellow-colored arrow is the direction of the expansion force, yellow-colored areas are the clear aligner region where 0.50 mm buccal displacement is applied. D, Yellow-colored area where 0.25 mm distal and 0.50 mm buccal displacement is applied.

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  Figure 4 A, Measurement of XY-direction rotation. B, Measurement of X-direction displacement. C, Measurement of Y-direction displacement. D, Measurement of Z-direction displacement.

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  Figure 5 Total deformation and values of rotation of the second molar teeth for six models.

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  Figure 6 Displacement (deformation) observed in the clear aligners for six models.

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  Figure 7 Stress distribution on the teeth and attachments for six models.

Reference

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