Abstract
Invisible aligners are medical devices, which allow repositioning of teeth through a treatment designed by the orthodontists. During this orthodontic treatment, patients use several aligners each for a couple of weeks. The aligner will apply a system of forces on the teeth to shift them into desired position. Since aligners exert forces thanks to their particular shape, it is important that during lifetime's service they do not undergo significant deformations. This research aims to study the mechanical behavior of invisible aligners made by polyethylene terephthalate-glycol (PET-G), which is one of most used the plastic materials to produce these devices. In this study, cyclic compression tests in atmospheric environment (~25°C) as well as in the presence of saliva (to simulate intraoral environment) were performed. The mechanical behavior of two different thicknesses aligners (0.75 and 0.88 mm) was studied. In particular, each aligner was subjected to 22500 load cycles from 0 to 50 N. The number of load cycles simulates the average load history to which an aligner is subjected during its lifetime. The tests were performed on a testing machine, using a hard resin dental casts appropriately fixed to the machine. Analysis of the results shows that stiffness of aligners material increases during the cyclic test. In particular, it was observed a gradual reduction of crosshead displacement during the test, highlighting the occurrence of cyclic hardening phenomena. It was also found that the aligners show a residual strain recovery after removing the applied load. Moreover, in the analyzed load range rate, the aligners show a low tendency to accumulate residual strains as loading cycles progress.
Highlights
Polyethylene terephthalate-glycol (PET-G) is one of the most famous and used thermoplastic materials in the world
Its excellent transparency makes polyethylene terephthalate-glycol (PET-G) copolymer promising in manufacturing removable tooth aligners for orthodontics (Ponitz, 1971; McNamara et al, 1985; Lindauer and Shoff, 1998)
It is noted that, for the sample of 0.75 mm thick (s0.75) tested at atmospheric environment (Figure 4A), the hysteresis loops are almost nonlinear in shape during the entire cycling process
Summary
Polyethylene terephthalate-glycol (PET-G) is one of the most famous and used thermoplastic materials in the world. Mechanical Behavior of PET-G Aligners esthetic characteristics, formability, high mechanical strength, excellent flexibility and high impact resistance (Sheridan et al, 1994; Sheridan and Armbruster, 1999; Oh and Wang, 2007; Lloyd et al, 2001), this material has gained great interest in the last decade in the medical device industry. It is adopted for manufacturing orthodontic retainers, temporomandibular joint splints, periodontal splints, and mouth guards (Ryokawa et al, 2006; Fang et al, 2013; Ma et al, 2016). These may include environmental conditions (i.e., temperature and humidity), thickness of the aligner, time after elastic deformation and forming procedures features (Landel and Nielsen, 1993; Ryokawa et al, 2006)
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