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Acta Stomatologica Croatica logoLink to Acta Stomatologica Croatica
. 2023 Dec;57(4):286–299. doi: 10.15644/asc57/4/1

Differential Stability of One-layer and Three-layer Orthodontic Aligner Blends under Thermocycling: Implications for Clinical Durability

Luka Šimunović 1, Antonija Jurela 2, Karlo Sudarević 2, Ivana Bačić 3, Senka Meštrović 1,✉
PMCID: PMC10812913  PMID: 38283309

Abstract

Objectives

To optimize the therapeutic usefulness of aligners, it is crucial to understand how their mechanical properties alter with time.

Materials and methods

Specimens from four different brands, including Duran+, CA® Pro, Zendura A, and Zendura FLX, were produced for material testing of thermoplastic orthodontic aligners (TOA) using dimensions measuring 4mm x 10mm. Each brand's 24 samples were split into three groups as follows: G1 being thermoformed, G2 being thermoformed and underwent 500 thermocycles (simulating 7 days), and G3 being thermoformed and underwent 1000 thermocycles (simulating 14 days). Surface roughness, modulus of elasticity in bending, and spectrophotometry were used to assess the effect of aging on TOAs.

Results

After 1000 thermocycles, Duran+ had the highest modulus of elasticity and differed statistically from all other groups. The intragroup comparison showed that only Duran+’s elastic modulus significantly changed after 1000 thermocycles in comparison with the control group. Surface roughness values (Ra), did not statistically differ among brands or thermocycling group measures. The change in chemical properties was not significant in any brand.

Conclusion

One-layer PETG (Duran+) failed to demonstrate stability after in vitro aging, thus suggesting that clinicians should be aware of the change in mechanical properties when using one-layer PETG (Duran +) in a 2 weeks regime.

Keywords: MeSH Terms: Orthodontic Appliances, Materials Testing, Mechanical Tests

Author Keywords: Aligners, Orthodontic appliance, Mechanical properties, Chemistry

Introduction

Clear aligners have gained immense popularity in orthodontic treatments, particularly among adult patients who appreciate their enhanced comfort and aesthetics (1). While aligners are effective in achieving proper leveling and alignment, they encounter challenges in certain tooth movements, such as extrusion, rotation, and torque control, which may not always be predictable (2-6). Additionally, aligners have shown limited effectiveness in correcting overbite and anteroposterior discrepancies, although they improve the alignment and interproximal contacts in certain malocclusions. However, compared to fixed appliances, clear aligners can yield comparable outcomes, especially for mild to moderate malocclusions. Studies show that aligners achieve about 50% accuracy in total types of tooth movements, with rotation being the least accurate (7). The effectiveness of clear aligners is influenced by various factors, including the used materials and the manufacturing process. Conventional, thermoformed orthodontic aligners (TOAs) are composed of thermoplastic resin polymers such as polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyethylene terephthalate glycol (PETG), and polyurethane (PU) (8). Modifications in the physical characteristics of aligner materials, such as their hardness and elastic modulus, can affect the force delivery, and subsequently impact treatment effectiveness. Processes of thermoforming and aging have been shown to affect the properties of aligner materials, but there is a lack of consensus on their specific influence (9-13). To comprehensively understand the behavior of aligners, it is important to consider the mechanical and thermal stress they experience during daily wear. Short-term mechanical stress occurs during insertion and removal, while long-term stress results from the interaction between the aligners and misaligned teeth, as well as the pressure during chewing (14). Additionally, aligner materials may exhibit a force drop in the initial hours of use (15, 16). Previous studies have shown that thermal and mechanical loading significantly affect the mechanical properties of aligner materials, thus leading to changes in hardness, elasticity, and strength (13, 17-19). Also, studies have indicated that aligners may undergo changes in shape and composition in the oral environment due to factors such as temperature, humidity, and salivary enzymes (10, 20, 21). Understanding the mechanical behavior of aligners under various stresses is essential for optimizing their performance. While it has been demonstrated in earlier research that the chemical structures of EX30 aligners remain unchanged following intraoral aging (10), to our best knowledge, there have been no studies examining the alterations in the chemical structures of PETG, PU, and copolyester aligners. Due to the critical importance of the physical attributes of materials uti-lized in the manufacturing of TOAs, relying solely on technical data provided by suppliers may not be sufficient. Experimental assessment of these materials under various conditions is needed for precise evaluation of their efficacy. Following the thermoforming procedure, thermoplastic polymers may experience alterations in their mechanical properties, pointing to the need for testing post-thermoforming (9). Additionally, when used by patients, aligners are exposed to a harsh environment of the oral cavity that can potentially degrade their properties, thereby negatively affecting treatment effectiveness (10). This study aimed to determine changes in elastic modulus, surface roughness, and chemical composition of single and multi-layer TOAs after thermocycling.

Materials and methods

Sample preparation

The 4mm x 10mm dimensions for aligner material testing were used to create specimens of four brands: Duran+ (thickness 0.75mm), CA® Pro (thickness 0.75mm), Zendura A (thickness 0.76 mm), and Zendura FLX (thickness 0.76) (Table 1.) (22) Duran+ is known as PETG, a clear copolymer with strong mechanical properties, formability, and resistance to fatigue and deformation. Unlike many thermoplastics, it does not require pre-drying for thermoforming due to its low hygroscopy. Zendura®, categorized as thermoplastic polyurethane (TPU), stands out for its versatility, abrasion resistance, elasticity, and clarity. TPU’s unique two-phase microstructure enhances its durability under stress. While, CA Pro and Zendura FLX are considered coplyester blends (20, 23-25). A total of 24 samples from each brand were prepared for the study, with 8 samples allocated to each in vitro aging group.

Table 1. Manufacturers’ information on the tested materials.

TOA brand Product data sheet material Manufacturer
Duran + Polyethylenterephthalat-Glycol Copolyester (PET-G) Scheu Dental GmbH, Iserlohn, Germany
CA Pro ABA three-layer material consisting of Copolyester (A) and thermoplastic elastomer (B) Scheu Dental GmbH, Iserlohn, Germany
Zendura A Thermoplastic polyurethane Bay Materials, Fremont, CA, USA
Zendura FLX Thermoplastic polyurethane and polyester (3-layer) Bay Materials, Fremont, CA, USA
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Thermoforming process

To shape the TOA materials, they were initially heated and exposed to a vacuum process, following the guidelines provided by the manufacturer. This process was carried out on a circular SS (stainless-steel) plate with a diameter of 110 mm and a thickness of 10 mm. The thermoforming procedure was conducted using a BioStar VI vacuum forming machine (Scheu-Dental GmbH, Iserlohn, Germany).

In vitro aging

To assess the durability of the four TOA materials, they were subjected to thermocycling after the thermoforming process. In this study, the samples underwent 500 and 1000 thermocycles using the thermocycler 1100 (SD-Mechatronik, Westerham, Germany) to simulate 7 and 14 days of intraoral use. Gale and Darvell's proposal posits that 10,000 cycles could reasonably replicate around one year of in vivo functionality, with a range of 20 to 50 cycles being analogous to the wear endured within a single day (26). Since orthodontic aligners are worn for 7 days, this translates to an equivalence of 350 thermal cycles. Furthermore, following the ISO 11405 guidelines, the application of 500 thermal cycles spanning temperatures from 5°C to 55°C is recognized as suitable for mimicking the short-term aging of dental materials. Taking this into consideration, our study adopted 500 and 1000 thermal cycles to simulate the intraoral usage of orthodontic aligners over periods of 7 and 14 days respectively, ensuring both high reproducibility and methodical consistency (27). Before thermocycling, the samples were submerged in distilled water at a temperature of 37 °C for 24 hours. During the thermocycling process, the samples were exposed to temperatures of 5 °C and 55 °C for a dwelling time of 15 seconds, followed by a dripping time of 10 seconds. Each brand's 24 samples were split into three groups as follows: Group 1 being thermoformed, Group 2 being thermoformed and underwent 500 thermocycles (simulating 7 days), and Group 3 being thermoformed and underwent 1000 thermocycles (simulating 14 days).

Modulus of elasticity in bending (Flexural modulus)

The elastic modulus is a measure of a material's ability to resist temporary deformation, also known as elastic deformation. When subjected to stress, materials initially demonstrate elastic behavior, wherein they deform in response to the stress but return to their original shape once the stress is removed (28). The elastic modulus of the materials was determined through a three-point bending test, conducted on a Mark-10 testing machine with IntelliMESUR® software (Mark-10 Corporation, Copiague, NY, USA), with a span length of 8 mm (22). The specimens were loaded at a speed of 1 mm/min, reaching a maximum deflection of 5 mm (29). Utilizing a mathematical framework grounded in the Euler–Bernoulli beam theory, we conducted calculations for various parameters within the scope of linear elasticity. These calculated values were then contrasted with the measurements taken, enabling a comprehensive comparison between theoretical predictions and actual observed data (30). The elastic modulus, expressed in gigapascals (GPa), was calculated using the equation:

E = F1 * L3 / (4 * b * h3)

F1 represents the highest load observed in the linear portion of the load-deflection curve, d corresponds to the deflection magnitude at F1, l indicates the span length between the supports, b denotes the width of the test sample, and h represents its height measured right before testing.

Surface roughness

Surface roughness measurements were made using a high-precision profilometer, the Mitutoyo SJ-210 surface roughness tester (Mitutoyo, Japan), following the ISO 4287:1997 standard (31). The top sides of the specimen were assessed to determine the roughness parameter Ra. The vertical roughness parameter Ra, considered in this study, represents the average arithmetic deviation of the profile, calculated by dividing the total roughness amplitude by the unit length of the surface. Three replicates per specimen were performed at different locations within a diameter of 5 mm from the center, from which the mean value was calculated and used as the statistical unit.

ATR-FTIR analysis

Infrared spectra were obtained using an Alpha ATR-FTIR spectrometer (Bruker Optics, Germany) coupled with attenuated total reflectance technique (ATR) with diamond crystal as a single-reflection element. The spectra were acquired over the 4000-400 cm-1 range using a resolution of 4 cm–1, and the final spectra were obtained by averaging 10 scans. For the instrument control, baseline correction (concave rubber band correction), spectra normalization, and automatic determination of band wavenumbers (peak picking) OPUS 7.0 software were used. The spectrum of each sample was recorded at least two times to check measurement reproducibility. We employed a precise and meticulous method to isolate the central and outer layers of the material for Fourier Transform Infrared Spectroscopy (FTIR) analysis. To achieve this, we utilized a high-quality precision scalpel manufactured by Fisherbrand (Fisher Scientific Company L.L.C., Pennsylvania, USA). Prior to dissection, the sample was demarcated using a fine-tipped marker to establish a clear guide for the incision. Utilizing a high-precision scalpel, an incision was initiated through the middle at a predetermined point on the sample. During this process, care was taken to exert steady pressure to circumvent tearing or misalignment, thus ensuring the integrity of the sample. After separation, a marker was used to mark outer and inner layer.

Statistical analysis

A priori statistical analysis was performed to ensure adequate statistical power in the study. Considering the study design, the comparison of Young’s modulus and surface roughness of 4 brands of TOAs after thermoforming, thermoforming and 500 thermal cycles, and thermoforming and 1000 thermal cycles indicated F test groups for sample size analysis: ANOVA: Fixed effects, special, main effects, and interactions, effect size f (0.4), α err prob. (0.05), and power (0.8) = 93. Every TOA brand should have 24 samples, 8 in each in vitro aging group to achieve the appropriate power. An analysis of data normality using the Shapiro–Wilk test and asymmetry tests revealed a non-normal distribution of Young’s modulus and surface roughness values. Change in flexural modulus after 1000 thermal cycles revealed non-normally distributed data, while the change in surface roughness was normally distributed. A comparison between brands was made using the Kruskal-Wallis test and one-way ANOVA test with post-hoc Dunn’s and Tukey HSD test, respectively. This analysis was conducted using the program Statistica (TIBCO® Statistica™ Version 14.0.0.15, Palo Alto, CA, USA).

Results

Modulus of elasticity in bending (Flexural modulus)

The in vitro aging effect on flexural modulus after 500 and 1000 thermocycles of thermoformed orthodontic aligner (TOA) materials is presented in Figure 1. In the initial values, Duran + had significantly higher modulus values than Zendura FLX (p<.001) and CA Pro (p=.005). After 500 thermocycles, neither brand showed a significant change in modulus (p=.394). Still, significant differences were observed between Duran + and all other brands, Zendura A (p=.048), Zendura FLX, and CA Pro (p<.001). After 1000 thermocycles, as well, no change in modulus was observed in either brand, but Duran + modulus values were significantly higher when compared to the control (initial values) (p<.001). When comparing change from initial values and values after 1000 thermocycles, the highest change in modulus had Duran + (0.78 GPa, IQR 0.41 - 0.95), and significantly differed from all others: Zendura A (p=.026), Zendura FLX (p=.023) and CA Pro (p=.016) (Figure 2). As presented in the figure, Duran + increased, Zendura A slightly decreased and Zendura FLX and CA Pro showed stability.

Figure 1.

Figure 1

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Flexural modulus (GPa) after 500 and 1000 thermocycles.

Figure 2.

Figure 2

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Change in flexural modulus (GPa) after 1000 thermocycles. (Median and interquartile range)

Surface roughness

The in vitro aging effect on surface roughness after 500 and 1000 thermocycles of TOA materials is presented in Figure 3. Initial values (control group) of surface roughness were highest in Zendura A (0.24, IQR 0.2 - 0.27), followed by CA Pro (0.24, IQR 0.22 - 0.25), Zendura FLX (0.2, IQR 0.18 - 0.23), and Duran + (0.18, IQR 0.16 - 0.18). Surface roughness values (Ra), did not statistically differ among brands or thermocycling group measures. The highest change in surface roughness was presented in Duran + after 1000 cycles (0.04 Ra, SD 0.058), while the lowest one was in CA Pro (0.003 Ra, SD 0.11).

Figure 3.

Figure 3

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Surface roughness values (Ra) after 500 and 1000 thermocycles.

Spectrophotometry

FTIR spectra of control aligner samples and the samples subjected to the aging process are shown in Figures 4-9. Zendura FLX is a three-layer aligner material for which the FTIR technique confirmed that the outer layers are made of the same copolymer material based on polyethylene terephthalate glycol (PETG). The FTIR spectrum (Figure 4) is dominated by strong absorption bands attributed to stretching of carbonyl group at 1714 cm-1, vibrational modes of C (=O)-O esters groups visible as weakly separated maxima at 1261 and 1245 cm-1, symmetrical stretching vibrations of C-O glycol bonds at 1097 cm-1 as well as out of plane bending vibration of C‒H bonds in aromatic ring at 726 cm-1. The in-plane C–H stretching band of the aromatic ring is also present in the spectra and is located at 1017 cm-1. Weak but informative peaks at 1407 cm-1 and 1375 cm-1 are associated with the in-plane deformation of the aromatic ring and wagging of glycol CH2 groups in gauche conformation, respectively. Medium absorption at 956 cm-1 is characteristic for C–H stretching of cyclohexylene ring and represents an important difference between FTIR spectra of PETG and PET that does not contain this band. Other important features of PETG spectra are stretching vibrations of the C–H bonds in methylene groups visible at 2926 and 2854 cm-1 (32, 33).

Figure 4.

Figure 4

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FTIR spectra of the outer layer of Zendura FLX.

Figure 5.

Figure 5

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FTIR spectra of the central layer of Zendura FLX.

Figure 6.

Figure 6

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FTIR spectra of Zendura A.

Figure 7.

Figure 7

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FTIR spectra of Duran +.

Figure 8.

Figure 8

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FTIR spectra of the outer layer of CA Pro.

Figure 9.

Figure 9

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FTIR spectra of the central layer of CA Pro.

FTIR spectrum of the central layer in Zendura FLX (Figure 5) shows a profile of polyurethane-based (PU) materials, where the most prominent peaks are connected with characteristic urethane –NH–CO–O– group. The spectrum contains a broad absorption in the range 3420–3210 cm-1 with the maximum at 3318 cm-1 which originates from stretching vibrations of the N–H bonds, while the non-hydrogen bonded and hydrogen bonded carbonyl groups are represented by weakly separated bands at 1728 cm-1 and 1700 cm-1, respectively. Polyurethane systems are also characterized by the bending of the N–H group, which, coupled with the stretching of –C–C and –C–N bonds, contributes to the band at 1527 cm-1. The medium band at 1597 cm-1 arises from C=C stretching vibrations in the aromatic ring. Intense vibrational mode at 1219 cm-1 is assigned as stretching vibrations of the C–O bond, followed by a region characteristic for C‒O‒C stretching vibrations (1102 and 1064 cm-1). The C-H stretching region between 3000 and 2800 cm-1 exhibits absorptions of antisymmetric and symmetric vibrations of methylene groups (2939 and 2855 cm-1) (32, 34).

FTIR analysis of other aligners revealed that Zendura A consists of a single-layer material based on polyurethane (Figure 6), Duran + is PETG (Figure 7), while three-layer CA Pro aligner, as well as Zendura FLX, consists of outer PETG layers (Figure 8) and the central polyurethane-based material (Figure 9). The FTIR spectra of all polyurethane and PETG materials coincide to a large extent.

The FTIR spectra of all PU-based and PETG materials coincide to a significant extent but still with certain differences that could affect their properties. One of them is observed in the spectra of the Duran + aligner and outer layers of the CA Pro aligner. Namely, in comparison with the outer layer of the Zendura FLX aligner, new bands at 1340 and 1044 cm-1 are observed. According to the literature, the band at 1340 cm-1 originates from the wagging of glycol CH2 groups in trans conformation, while the small peak at 1044 cm-1 is assigned as gauche C–O asymmetric stretching (35).

FTIR spectroscopy was also used to monitor possible chemical changes in materials during the aging cycles. Comparing the spectra of control aligner samples with the spectra of samples that have aged in 500 and 1000 thermocycles it was found that none of the analyzed material underwent changes in chemical composition that could be measured by the applied FTIR spectroscopy technique.

Discussion

The importance of having reliable and affordable aligners cannot be overstated. Due to the critical importance of the mechanical properties of materials used in the production of clear aligners, relying solely on technical data provided by suppliers may not be sufficient. Experimental assessment of these materials under various conditions is needed to accurately evaluate their efficacy. The thermoforming process can induce alterations in the mechanical properties of thermoplastic polymers, underscoring the necessity for conducting tests after the thermoforming stage (9). Additionally, when used in the oral cavity, aligners are exposed to a harsh environment that can potentially degrade their properties, thereby negatively affecting treatment effectiveness (10). The aim of this study was to investigate the effects of thermocycling on four commonly used TOA materials: Duran+®, Zendura A®, Zendura FLX, and CA Pro® through various tests such as three-point bending, spectrophotometry, and surface roughness measurements. The materials were subjected to thermoforming followed by thermocycling to simulate real-world conditions. By conducting thermocycling, the goal was to identify the materials that maintained optimal mechanical properties even after undergoing in vitro aging, to minimize costs and ensure the effectiveness of orthodontic treatment. Previous research has offered valuable insights into the mechanical characteristics of various thermoplastic materials used in aligners and retainers. However, comparing these studies is challenging due to the diverse array of experimental designs and methodologies employed (22, 26, 36-41). Furthermore, there is a lack of ISO specifications or national standards particularly addressing the assessment of mechanical properties of TOA materials. While some test standards, such as ISO 20795-2 for orthodontic base polymers (42), have been published and utilized in the literature, they mainly focus on stiffer orthodontic materials that experience more uniform material stress and possess greater thicknesses. An example is the use of polymethyl methacrylate (PMMA) in the fabrication of Hawley retainers (22).

In a previous investigation by Iijima et al. (19), the impact of thermocycling on the mechanical characteristics of TOA materials (PETG, PP, and PU) was examined. The aforementioned TOA materials underwent thermocycling for 500 and 2500 cycles, with temperature variations between 5 and 55 °C. The results showed that after 500 cycles, the hardness values of the materials remained relatively unchanged, while there was a significant decrease in their elastic modulus values. Nevertheless, in contrast to the present study, where most materials exhibited an increase in Young’s modulus values after thermocycling, the previous research observed a significant decline in elasticity after 2500 thermal cycles for the majority of thermoplastics. A recent study by Albilali et al. (43) who studied the effects of thermocycling on the mechanical properties of PETG and PU materials, revealed results in agreement with ours, namely that thermocycling leads to an increase in the elastic modulus of the material. Furthermore, the investigation conducted by Dalaie et al. (29) examined the impact of thermocycling on the flexural modulus and hardness of PETG aligner materials, specifically Duran and Erkodur. Their findings revealed a slight increase in the flexural modulus after thermocycling, although this increase did not reach statistical significance. It is worth noting that the limited number of cycles (200) employed in their study might have contributed to that outcome.

Our study indicates that only the PETG material (Duran+) showed a dramatic change in the modulus of elasticity, compared to single-layer and multi-layer PU as well as copolyester, which showed stability, regardless of the number of cycles. Duran + is an amorphous transparent copolymer of polyethylene terephthalate (PET). It possesses excellent mechanical properties, dimensional stability, fatigue resistance, optical qualities, and formability (23). It has low hygroscopicity and is easily manufacturable, as pre-drying is typically not necessary before thermoforming. On the other hand, Zendura is classified as thermoplastic polyurethane (TPU). It is recognized as one of the most versatile engineering thermoplastics, offering high abrasion resistance, elasticity, good transparency, and excellent shear strength (20, 24). TPU possesses a two-phase microstructure composed of soft and hard segments. Under stress, the soft segments tend to orient perpendicularly and subsequently break into smaller pieces, enabling further deformation (25). Elevated temperatures and moist conditions can result in polymer oxidation, which some researchers suggest is the cause behind the observed increase in elastic modulus or stiffness (44). However, copolyesters exhibit low resistance to hydrolysis (45). PET polymers are susceptible to elevated temperatures ranging from 100 to 130 °C, which can result in warping, bending, and deformation (45). Aforementioned, could be a possible explanation for the change in elastic modulus presented in PETG only. Numerous studies have examined the impact of thermal cycling on various mechanical properties of PETG TOA materials (13, 29). However, there is a scarcity of research focusing specifically on the impact of thermal cycling on surface roughness (13, 29). Measurements of surface roughness were conducted after undergoing thermoforming alone and thermoforming followed by thermocycling. In the control group, which involved only thermoforming, no statistically significant differences in surface roughness were observed among the materials, with Zendura A having the highest values and Duran + the lowest. After thermocycling with both regimes, Duran + (PETG) exhibited the highest change in surface roughness compared to single or multilayer PU or copolyester. Moreover, the surface roughness of all materials did not increase significantly after thermocycling. A recent study found that the PETG material demonstrated susceptibility to external factors that can cause instability such as thermocycling and brushing. Thermocycling increased both roughness and mass while brushing predominantly led to an increase in roughness and a decrease in mass (40). A possible explanation for significant results could be a greater number of cycles used (1500) compared to this study. Thermocycling is a method employed to replicate challenging conditions similar to those experienced in the oral environment, involving frequent temperature fluctuations and increased moisture levels over a specified duration (46). When materials are subjected to thermocycling, their mechanical properties change due to both extreme temperature variations and water absorption (13, 46). Thermoplastic materials, including PETG, have been reported to exhibit higher water absorption rates, in comparison to copolyesters, which further contribute to alterations in their mechanical properties (11, 13, 23, 46). When the water absorption of a material increases, it becomes more susceptible to degradation, exhibiting more noticeable signs of deterioration (47). Additionally, research has demonstrated that higher temperatures facilitate greater permeation of water molecules into the material. Consequently, as the intraoral temperature rises, the extent of water absorption also increases (48). In addition to facing challenges posed by salivary enzymes, high humidity, and intermittent and continuous forces, retainers are exposed to fluctuations in temperature (49). Following the consumption of hot beverages or foods, the temperature within the oral cavity can reach up to 57 °C, and it may require several minutes for it to revert to its initial values (49). The mechanical properties of the TOA material can be negatively affected by these temperature fluctuations (11, 19, 29, 49, 50). When a material is exposed to a moist environment, it undergoes a chemical reaction known as hydrolysis, where water reacts with the polymer matrix. This process results in the deterioration of the material through hydrolytic reactions and causes swelling (48, 51). Additionally, water permeates the polymer's structure, acting as a spacer between polymer chains and resulting in hygroscopic expansion (48, 52). In consequence, the weight and volume of TOA specimens increase (53). Previous studies have shown that water primarily penetrates the amorphous regions of polymers, while the crystalline regions remain relatively unaffected (51). PETG is considered to exhibit more stability than other materials in humid environments due to its high degree of crystallinity (53). Furthermore, Zhang et al. (54) suggest that a modified blend of PC, PETG, and TPU material exhibits a lower water absorption rate compared to PETG alone, which is an explanation for greater change in Duran + than in Zendura A, Zendura FLX, and CA Pro in our study. In this study, the impact of aging on clear aligners was examined. However, it is important to consider that factors such as wear (55), brushing (40), and other cleaning protocols (41) have the potential to modify the surface properties of TOA materials. In a recent study (41), the surface roughness of PETG material was investigated after exposure to chemical and mechanical cleaning procedures, namely alkaline peroxide tablets (Corega+), a toothbrush, and a combination of the two. The findings indicate that these cleaning procedures have the potential to alter the mechanical properties of TOA materials. Furthermore, almost all of the tested cleaning methods resulted in a significant increase in the surface roughness of the PETG material. While most of the tested materials, excluding Duran +, demonstrated stability in the modulus of elasticity during in vitro aging, it has been confirmed that the outer layers of the three-layer blends consist of PETG, while the inner layers consist of PU. However, it is important to consider the potential factors contributing to the varying modulus of elasticity between Zendura FLX and CA Pro materials. Specifically, Duran + and the outer layers of the CA Pro aligner exhibited a new band at 1340 cm-1, originating from the wagging motion of glycol CH2 groups in the trans conformation. This band is commonly used to assess sample crystallinity, assuming that only trans chains can form crystals and that amorphous regions primarily consist of gauche conformations. However, a detailed analysis of the 1340 cm-1 band revealed that using the equations of Belali and Vigourex (56) to extract crystallinity values from oriented PET films is erroneous (57). These equations assume that the crystalline regions are unoriented and the amorphous regions mainly consist of gauche conformers. In the case of oriented specimens, the number of trans conformers in the amorphous regions significantly increases, leading to an artificially high crystallinity reading for the 1340 cm-1 band (57). Since increased crystallinity is known to enhance Young's modulus and strength in polymers, this discrepancy explains the difference in modulus of elasticity between CA Pro and Zendura FLX, both of which are PETG/PU blends.

In this study, the wear of the TOA material was simulated through thermocycling for 7 and 14 days. However, it is crucial to acknowledge that this may be a limitation of the study as the number of thermal cycles may not precisely correspond to the exact number of days of aging of the TOA material. A limitation of this study is the potential presence of changes in the materials that, while detectable at a microscopic or molecular level, may not have a discernible clinical impact or affect patient outcomes. The use of standardized rectangular samples in this study is a limitation, since in clinical practice, TOAs are individually formed based on a plaster or 3D-printed model that replicates the patient's teeth. The absence of evaluating TOAs shaped to match the patient's dentition may have overlooked potential differences resulting from sample shape variations. Despite incorporating the respective thicknesses of 0.75 mm and 0.76 mm into our calculations for the modulus, a limitation of our study remains the assumption that this minor variation in thickness between materials does not substantially affect the overall mechanical property results. Moreover, thermocycling was carried out without any external force or loading. In reality, however, the aging process of the orthodontic aligner will occur under stress. Thermocycling treatments might not offer sufficient data to simulate real-world problems. Further research is required to provide more comprehensive findings. Additionally, it is important to assess the materials' tear strength, tensile strength, and creep. Furthermore, clinical studies should be conducted to investigate the effects of intraoral aging on the properties of diverse materials employed in the fabrication of TOAs.

Conclusion

One-layer PETG (Duran+) failed to demonstrate stability after in vitro aging, suggesting that clinicians should be aware of the change in mechanical properties when using it in a 2-week regime. Although both three-layer blends consist of PETG, while the inner layers consist of PU, the wagging of glycol CH2 groups in trans conformation are contributing factors to the varying modulus of elasticity between Zendura FLX and CA Pro materials.

Footnotes

Conflict of interest

The authors report no conflict of interest.

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