Potential Application of 4D Technology in Fabrication of Orthodontic Aligners

Abstract

Objectives: To investigate and quantify forces generated by three-dimensional-printed aligners made of shape memory polymers (four-dimensional [4D] aligner).Methods: Clear X v1.1 material was used in this study. On a custom-made typodont model, correction of maxillary central incisor (tooth 21) malposition by 4D aligners with thicknesses of 0.8 and 1.0 mm was measured by superimposition of subsequent scans. Maximum deflection forces generated by foil sheet specimens were measured at different temperatures in three-point bending (3-PB) tests. In a biomechanical system (orthodontic measurement and simulation system [OMSS]), forces generated on movements of tooth 21 by the 4D aligners were measured at different temperatures.Results: 4D aligners succeeded to achieve a significant tooth movement (2.5 ± 0.5 mm) on the typodont, with insignificant difference between different thicknesses. In the 3-PB test, the maximum deflection forces measured at 20, 30, 37, 45, and 55°C, were 3.8 ± 1.1, 2.5 ± 0.9, 1.7 ± 0.6, 1.0 ± 0.4, and 0.5 ± 0.4 N, respectively. Forces delivered on palatal displacement of tooth 21 at 37, 45, and 55°C by 0.8-mm aligners were 0.3 ± 0.1, 0.2 ± 0.1, and 0.7 ± 0.2 N, respectively, whereas those by 1.0-mm aligners were 0.3 ± 0.1, 0.3 ± 0.1, and 0.6 ± 0.2 N, respectively. A good concordance with movement on the typodont model was shown in OMSS.Conclusion: An initial study of 4D-printed aligner shows its ability to move a tooth by biocompatible orthodontic forces, after a suitable thermal stimulus within the oral temperature range.

Effect of Aging of Orthodontic Aligners in Different Storage Media on Force and Torque Generation: An in vitro Study

“…As an active orthodontic appliance, aligners should basically have the property of exerting force evenly over time [4,7,18]. However, aligners are exposed to various intraoral loads which may affect their mechanical properties and thus their effectiveness [23].…”

“…Force/torque measurements were carried out using the Orthodontic Measurement and Simulation System (OMSS) (Figure 1) [7,[25][26][27] before starting storage (Day 0) and on the 2nd, 4th, 6th, 10th, and 14th day (±2 hours) after starting storage. The OMSS is able to simulate three-dimensional tooth movements and measure force/deflection or torque/rotation ratios.…”

“…Thanks to CAD/CAM and intraoral scanner technology, the workflow has become much easier and faster [4]. The mechanical performance of orthodontic aligners is still vague [5][6][7]. It is assumed that the returning forces of the aligners on the target tooth are generated by two mechanisms: initially by the local elastic deformation of the aligner body in the contact area with the misaligned target tooth and then comes the elastic deformation of the entire aligner, since the entire aligner body is lifted in the area of the misaligned target tooth while it is fixed to the other teeth by friction [8,9].…”

“…For this purpose, the material should have sufficient stiffness to apply the required light force and a sufficiently high elastic limit to avoid permanent deformation [12]. However, the commonly used aligner sheets are made of viscoelastic thermoplastic polymers [4,7,13]. Viscoelastic materials show phenomena of creep and stress relaxation, which means that their mechanical behavior differs significantly over time under different loading conditions [12,14].…”

“…Patients are instructed to wear each aligner for at least 22 hours per day for 14 days and then replace it with its successor [15]. This means that the aligners should only be removed when eating or brushing teeth [4][5][6][7]. During function in mouth, the aligners are subjected to thermal and mechanical alternating loads.…”