Forces and moments generated during extrusion of a maxillary central incisor with clear aligners: an in vitro study

Abstract

Objective: To assess the possibility of extrusion of a maxillary central incisor with the use of buccal and lingual pressure columns in the absence of attachments, and to evaluate the forces and moments experienced by the teeth using both thermoformed and 3D-printed clear aligners.

Materials and methods: A three-axis force and moment sensor (Aidin Robotics, Anyang, South Korea) was used to measure the forces and moments during extrusion of an upper left central incisor (UL1) and any forces experienced by the upper right central incisor (UR1) using thermoformed aligners and 3D-printed aligners. For the thermoformed aligners, the materials used were ATMOS® (American Orthodontics, Sheboygan, WI) and Zendura FLX® (Bay Materials LLC, Fremont, CA). 3D-printed aligners were fabricated using TC-85 clear photocurable resin (Graphy Inc., Seoul, South Korea). For each material type, three conditions were tested: Group 1: No attachment or pressure columns (control); Group 2: Attachment only; and Group 3: Pressure columns only. Each group was planned for 0.5 mm of extrusion on the UL1.

Results: All force readings collected demonstrated statistically significant differences when compared by materials and when compared by groups, with a P value of < 0.001. In the absence of attachment or pressure columns (Group 1), ATMOS® and TC-85 groups exerted extrusive force on the UL1. However, significantly lower forces and moments were exerted by the TC-85 group in comparison to the ATMOS® and Zendura FLX® groups. In the presence of attachment (Group 2), all three ATMOS®, Zendura FLX® and TC-85 groups exerted extrusive force on the UL1, with the TA group showing different directions of faciolingual force, mesiodistal force and faciolingual inclination on the UR1 when compared to the other two thermoformed groups. Whereas in the presence of pressure columns (Group 3), only the TC-85 3D-printed aligner group exerted extrusive force. Thermoformed aligners generated significantly higher mean forces and moments than 3D-printed aligners. Significant levels of unintended forces and moments were present in all groups.

Conclusions: Force levels generated during extrusion with clear aligners are significantly lower with those 3D-printed using TC-85 than with those thermoformed using ATMOS® or Zendura FLX®. Attachments consistently generated extrusive forces, and may be an effective adjunct in achieving extrusion of incisors. Extrusion may be achieved without the use of attachments by utilizing pressure columns in 3D-printed aligners using TC-85. While different strategies can generate extrusive forces, there are significant unintended forces and moments.

Keywords: 3D-printed aligners; Attachments; Clear aligners; Direct printed aligners; Extrusion; Forces and moments; Pressure columns.

Fig. 1

Attachment design and dimensions in uDesign 6.0

Fig. 2

Pressure column design and dimensions in uDesign 6.0

Fig. 3

TC-85 pressure column group outer and internal views

Fig. 4

a TC-85 attachment group, b Thermoformed aligner attachment group

Fig. 5

a Model for fabrication of thermoformed aligner pressure column group facial view, b Model for fabrication of thermoformed aligner pressure column group lingual view, c Model for fabrication of thermoformed aligner attachment group facial view

Fig. 6

a 3D-printed tooth, b Facial and lingual gingival margin anatomy on printed tooth for sensor model

Fig. 7

a 3-axis force and moment sensor apparatus, b Aligner seated on 3-axis force and moment sensor apparatus

Fig. 8

a Experimental environment setup, b Data collection and software setup

Fig. 9

Conventions used for forces and moments in x,y,z axes

Fig. 10

Results Diagram of the three tested groups; a ATMOS® group no pressure columns or attachments, b Zendura FLX® group no pressure columns or attachments, c TC-85 no pressure columns or attachments

Fig. 11

Results Diagram of the three tested groups; a ATMOS® Attachment group, b Zendura FLX® Attachment group, c TC-85 Attachment group

Fig. 12

Results Diagram of the three tested groups; a ATMOS® pressure columns group, b Zendura FLX® pressure columns group, c TC-85 pressure columns group

Fig. 13

a Faciolingual forces on UL1 and UR1, b Mesiodistal forces on UL1 and UR1, c Occlusogingival forces on UL1 and UR1 by the three different tested materials

Fig. 14

a Faciolingual inclination moments on UL1 and UR1, b Mesiodistal crown angulation moments on UL1 and UR1, c Mesiodistal crown rotation moments on UL1 and UR1 by the three different tested materials