ABSTRACT
PURPOSE: The aim was to investigate the effect of aging by thermocycling and mechanical loading on forces and moments generated by orthodontic clear aligners made from different thermoplastic materials. METHODS: A total of 25 thermoformed aligners made from 5 different materials, i.e., Essix ACE® and Essix® PLUS™ (Dentsply Sirona, Bensheim, Germany), Invisalign® (Align Technology, San Jose, CA, USA), Duran®+ (Iserlohn, Germany), Zendura™ (Fremont, CA, USA), underwent a 14-day aging protocol involving mechanical loading (a 0.2â¯mm vestibular malalignment of the upper left second premolar [tooth 25]) and thermocycling in deionized water (temperature range 5-55⯰C). The 3D forces/moments exerted on tooth 25 of a resin model were measured at three time points: before aging (day 0), after 2 days and after 14 days of aging. RESULTS: Before aging, extrusion-intrusion forces were 0.6-3.0â¯N, orovestibular forces were 1.7-2.3â¯N, and moments as mesiodistal rotation were 0.3-42.1â¯Nmm. In all directions, multilayer Invisalign® exhibited the lowest force/moment magnitudes. After aging, all materials showed a significant force/moment decay within the first 2 days, except Invisalign® for orovestibular and vertical translation. However, following thermomechanical aging, Duran®+ and Zendura™ aligners had equivalent or even higher vestibular forces (direction of mechanical load). CONCLUSION: Thermomechanical aging significantly reduced forces and moments during the first 48â¯h. Multilayer aligner materials exhibit lower initial forces and moments than single-layer ones, and were less influenced by aging. Material hardening was observed after subjecting some of the aligner materials to mechanical loading. Thus, orthodontists should be aware of possible deterioration of orthodontic aligners over time. This work also sheds light on how material selection impacts the mechanical behavior of aligners and may provide valuable guidance regarding optimal timing for the aligner changing protocol.
ABSTRACT
The aim of the study is to investigate the effects of artificial aging by thermocycling and mechanical loading on force/torque delivery by thermoplastic orthodontic aligners. Ten thermoformed aligners, made of Zendura™ thermoplastic polyurethane sheets, were aged over two weeks in deionized water by thermocycling alone (n = 5) and by both thermocycling and mechanical loading (n = 5). The force/torque generated on upper second premolar (Tooth 25) of a plastic model was measured before aging (as control), and after 2, 4, 6, 10, and 14 days of aging, using a biomechanical set-up. Before aging, the extrusion-intrusion forces were in the range of 2.4-3.0 N, the oro-vestibular forces were 1.8-2.0 N, and the torques on mesio-distal rotation were 13.6-40.0 Nmm. Pure thermocycling had no significant effect on the force decay of the aligners. However, there was a significant decrease in force/torque after 2 days of aging for both thermocycling and mechanical loading aging group, which is no longer significant over 14 days of aging. In conclusion, artificial aging of aligners in deionized water with both thermocycling and mechanical loading results in a significant decrease in force/torque generation. However, mechanical loading of aligners has a greater impact than pure thermocycling.
