Korean J Orthod.  2021 Jan;51(1):32-42. 10.4041/kjod.2021.51.1.32.

Force changes associated with differential activation of en-masse retraction and/or intrusion with clear aligners

• Zhu Y
• Hu W
• Li S

• KMID: 2510616
• DOI: http://doi.org/10.4041/kjod.2021.51.1.32

Abstract

Objective
To investigate the three-dimensional forces created by clear aligners on mandibular teeth during differential activation with en-masse retraction and/or intrusion in vitro.
Methods
Six sets of clear aligners were designed for differential en-masse retraction and/or intrusion procedures in a first premolar extraction model. Group A0 was a control group with no activation. Groups A1–5 underwent different degrees of retractions and/or intrusions. Each group consisted of 10 aligners. Aligner forces were measured on a multi-axis force/ torque transducer measurement system in real-time.
Results
In the en-masse retraction groups (A1 and A2), lingual and extrusive forces were observed on the incisors; the canines mainly received distal forces; intrusive forces were seen on the second premolars; and the molars received mesial forces. In the enmasse retraction and intrusion groups (A3, A4, and A5), incisors also received lingual and extrusive forces; canines received distal and intrusive forces; mesial and extrusive forces were seen on the second premolars; and the second molars received distal and intrusive forces. The vertical forces on the incisors did not differ significantly among groups A1, A3, and A5. However, the vertical forces on the second premolars reversed from intrusion in group A1 to extrusion in groups A3 and A5.
Conclusions
With clear aligners, the “bowing effect” is seen during en-masse anterior teeth retraction and can be partially relieved by performing en-masse retraction accompanied by anterior teeth intrusion. Vertical control of incisors remained unsolved during en-masse retraction, even when intrusive activation was added to the anterior teeth.

Keyword

Aligners; Tooth movement; Orthodontic treatment; Activation amount

Figure

• 

  Figure 1 The force measurement system. A, Three-dimensional-printed resin teeth connected separately with the multi-axis force/moment transducer by hexagonal screws. B, The computer linked with the measurement system. C, The real-time visualization window of Angelalign Mechanical Measurement Software (Wuxi Angel Align Biotechnology Co., Ltd., Wuxi, China). D, The coordinate system for the forces and moments measured. The y-axis runs through the center of tooth and parallel to the long axis of this tooth. The x-axis is oriented parallel to the mesiodistal direction of teeth. The z-axis represents the labiolingual/buccolingual force.

• 

  Figure 2 Comparisons of the three-dimensional forces in groups A1 and A2. A, Forces in the buccolingual direction. B, Forces in the mesiodistal direction. C, Forces in the vertical direction. Group A1, underwent 0.25-mm retraction; Group A2, underwent 0.50-mm retraction. *p < 0.05.

• 

  Figure 3 Comparisons of the three-dimensional forces in groups A3 and A4. A, Forces in the buccolingual direction. B, Forces in the mesiodistal direction. C, Forces in the vertical direction. Group A3, underwent 0.25-mm retraction and 0.25-mm intrusion; Group A4, underwent 0.50-mm retraction and 0.50-mm intrusion. *p < 0.05.

• 

  Figure 4 Comparisons of the three-dimensional forces in groups A1, A3, and A5. A, Forces in the buccolingual direction. B, Forces in the mesiodistal direction. C, Forces in the vertical direction. Group A1, underwent 0.25-mm retraction; Group A3, underwent 0.25-mm retraction and 0.25-mm intrusion; Group A5, underwent 0.25-mm retraction and 0.5-mm intrusion. *p < 0.05.

• 

  Figure 5 Comparisons of three-dimensional forces in groups A2 and A4. A, Forces in the buccolingual direction. B, Forces in the mesiodistal direction. C, Forces in the vertical direction. Group A2, underwent 0.50-mm retraction; Group A4, underwent 0.50-mm retraction and 0.50-mm intrusion. *p < 0.05.

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