Stress Relaxation in Orthodontic Aligner Plastics; An In Vitro Comparison Study

Kristopher J Keller · 2020 · 4 citationsRead the paper

The purpose of this study was to ascertain if repeated stresses from extension cycling would result in accumulated damage in aligner materials and affect force delivery. A secondary goal was to identify potential differences in mechanical behavior present among orthodontic aligner polymers. Four thermoplastic polymers (Essix ACE, Taglus, Zendura, and Zendura FLX) were thermoformed, cut into strips, and extension cycled to 0.4 millimeters in a 37C water bath. Force decay, maximum and minimum force, and elastic modulus were measured during cycling. Additional samples were subjected to controlled tension until failure, allowing the measurement of yield strength, ultimate strength, and elastic modulus. Our results demonstrated that extension cycling resulted in decrease in maximum force and force decay, and an increase in modulus. It was postulated that stress relaxation and recoverable creep contributed to changes in force, while increased elastic modulus was attributed to the polymer network stiffening. Material property changes caused by extension cycling to 0.4 millimeters were completely recovered to initial levels after one week of storage, suggesting that low levels of repeated deformation do not generate permanent fatigue damage. Therefore, this amount of deformation incurred during removal and reinsertion of an aligner is unlikely to introduce permanent damage. Zendura FLX exhibited the least amount of force decay, whereas Zendura, Taglus, and Essix Ace exhibited similar force decay properties. Zendura exhibited highest strength, yield strength, and toughness.

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