Cyclic compression fatigue behaviour of PET-G tooth aligners

Mispositioning of teeth is a medical condition which is treated by the dentist by the application of a properly designed system of forces that progressively move the teeth to the correct position. Nowadays this is often obtained by the so called invisible aligners. Those are devices which are put in contact with the dental structures and act in the direction of progressively shifting the teeth. The passage from the initial position to the final desired one is obtained by a series of intermediate steps which are achieved by a specific aligner so that, usually, each aligner is made to work for about two weeks. Based on this important consideration, in this paper, the mechanical properties of Polyethylene Terephthalate-Glycol (PET-G), one of the most used materials for manufacturing aligners, were studied. In particular, the fatigue behavior was analyzed both in normal environment and under simulated intraoral conditions. In this work, moreover, two different thicknesses of the aligners were analyzed. Typically, during their lifetime, aligners are subjected to 22500 loading cycles, this condition was replicated in our study and during each cycle aligners were loaded up to 50 N. For testing purposes, the aligners were mounted directly on a dental cast and this was clamped to a loading machine. Results of the test show that aligners exhibit an increasing stiffness as the cycling loading progresses, this is observable by a reduction of the crosshead displacement indicating the occurrence of cyclic hardening phenomena. Moreover, at least for the analyzed range of cycles and loads, the aligners display a tendency to strain accumulation. Finally, an effect of residual strain recovery was also observed subsequent to removal of the applied load.