Mechanical Behavior Analysis of PU and PETG Passive Clear Aligners under Simulated Swallowing Loads

Understanding the mechanical response of clear aligners during their lifetime service is of particular interest in designing a more efficient orthodontic treatment plan. It also helps to quantitatively assess the patient’s response to the treatment and the performance capabilities of the orthodontic device. This study aims to analyze and compare the mechanical performance of thermoplastic polyurethane (PU) and poly(ethylene terephthalate)-glycol (PETG) clear aligners under cyclic loading. The fatigue behavior of the aligners was investigated at room temperature under cyclic compression loading. Aligners with a thickness of 0.75 mm were tested in two conditions: in the presence and absence of an artificial saliva environment. The clear aligner was placed on an upper dental cast and subjected to 22500 loading cycles. Furthermore, the morphological features of the aligners, at the end of the tests, were analyzed by optical microscopy. The results show a ratcheting displacement reduction in the PU aligners during the loading cycles in both test conditions. At the same time, the material becomes progressively stiffer and less viscoelastic. When tested in saliva, the aligners show a greater structural rigidity because of water absorption. Moreover, the experimental results show that the tested PU and PETG aligners exhibit similar fatigue behavior. It has been found that the cyclic loading due to swallowing has a significant effect on the mechanical properties and integrity of the aligners, especially, when the saliva absorption is taken into account.