Abstract
The aim of this study was to estimate possible differences in the chemical composition and relaxation of orthodontic aligner materials. Four commercially available thermoplastic materials CAM (Scheu-Dental, Iserlohn, Germany), COP (Essix, Dentsply Raintree Essix Sarasota, FL, USA), DUR (Great Lakes Dental Technologies, Tonawanda, NY) and ERK (Erkodent Erich Kopp, Pfalzgrafenweiler Germany) were included in this study. Rectangular strips from each material were prepared according to the manufacturer’s instructions and subjected to attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and stress relaxation characterization. The reduction in applied stress (RAS) after one week was estimated and statistically analyzed by one-way ANOVA at the 0.05 level of significance. All specimens were subjected to optical microscopy before and after stress relaxation testing under transmittance polarized illumination. ATR-FTIR microscopy revealed that all materials are made of polyethylene terephthalate glycol (PETG) while no significant differences were identified in RAS values among materials tested, which ranged from 6%–10% (p ≥ 0.05). All samples illustrated the developments of shear bands during relaxation testing according to optical microscopy findings. The tested materials illustrated similar chemical composition and relaxation behavior and thus no differences in their clinical efficacy are anticipated.
Highlights
The introduction of Invisalign system at the end of 1990s by Align Technology (San Jose, California, USA) was a breakthrough for orthodontic aesthetic treatment with removable appliances.The fabricated sequential positioners were based on polyurethane [1] and had the ability to move teeth in small increments without causing major patient discomfort [2] nor undermining their oral hygiene [3,4,5,6,7]
All materials tested showed identical ATR-FTIR spectra and Figure 1 (A) shows a representative spectrum which includes the characteristic bands of CH (2931, 2741, 1403, 723 cm-1 ), C=O (1716 cm−1 ), aromatic CH (875, 1505 cm−1 ) and C-C-O-(1254, 1103 cm−1 )
For all curves a short first stage (Stage I) of approximately 2 hours defined by a high rate decrease of stress was noticeable, which was followed by a second stage with a low rate decrease of stress up to the end of the recording time (7 days)
Summary
The fabricated sequential positioners were based on polyurethane [1] and had the ability to move teeth in small increments without causing major patient discomfort [2] nor undermining their oral hygiene [3,4,5,6,7]. The majority of manufacturers have changed the initially utilized thermoplastic material with polyethylene terephthalate glycol (PETG) and fabricate aligners of variable thicknesses. Every positioner is subject to deformation upon intraoral placement, and the initial magnitude of force placed on the teeth depends both on material stiffness and thickness as well as the intended tooth displacement prescription [11,12]. Upholding the magnitude of force over the applied time, which usually does not exceed 1 to 2 weeks, is a crucial aspect for a predictable orthodontic tooth movement. Force relaxation, defined as the reduction of force over time at standard deformation, is of great importance when considering aligners and Materials 2020, 13, 1868; doi:10.3390/ma13081868 www.mdpi.com/journal/materials
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