Effect of thermomechanical aging on force system of orthodontic aligners made of different thermoformed materials : An in vitro study.

Abstract

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. Atotal of 25thermoformed aligners made from 5different 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 a14-day aging protocol involving mechanical loading (a0.2 mm vestibular malalignment of the upper left second premolar [tooth25]) and thermocycling in deionized water (temperature range 5-55 °C). The 3D forces/moments exerted on tooth25 of aresin model were measured at three time points: before aging (day0), after 2days and after 14days of aging. Before aging, extrusion-intrusion forces were 0.6-3.0 N, orovestibular forces were 1.7-2.3 N, and moments as mesiodistal rotation were 0.3-42.1 Nmm. In all directions, multilayer Invisalign® exhibited the lowest force/moment magnitudes. After aging, all materials showed asignificant force/moment decay within the first 2days, 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). Thermomechanical aging significantly reduced forces and moments during the first 48 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.