INTRODUCTION

Over the past decade, there has been a significant rise in interest in clear aligners, particularly among adults. Their most imperceptible appearance and unparalleled comfort have made them a preferred choice. Moreover, they contribute to a decline in emergency visits, improving the overall treatment process. Adult patients are increasingly conscious of their appearance during orthodontic treatment. These aligners not only blend seamlessly into their busy lives but also offer greater comfort and time efficiency compared to traditional braces.

The manufacturing of clear aligners can be broadly divided into thermoformed and directly printed techniques. Thermoforming involves heating a polymer sheet until it becomes pliable and then molding it over a dental model to form the aligner.[1] The characteristics of aligner materials are crucial in defining their mechanical and clinical performance. Traditional thermoformed aligners are typically constructed from a single layer of plastic, which may include materials such as polyvinyl (PV), PV chloride, polyethylene terephthalate glycol (PETG), polypropylene, polystyrene, or polyurethane (PU). To address the shortcomings of single-layer materials, multi-layer hybrid materials have been developed. These materials combine resilient outer layers with softer inner layers, enhancing both mechanical performance and patient comfort.[2]

The 3D printing resin is unique in the market for its shape memory function, as claimed by TC 85. The production process eliminates the need for printing models or vacuum forms, resulting in significant time and cost savings. In addition, this method reduces carbon emissions and waste production.[3] The additional steps involved in manufacturing thermoformed aligners compared to direct-printed aligners heighten the potential for errors.[4] The optical properties of materials are defined by how they interact with light, specifically in terms of absorbance and transmittance. Transmittance refers to the proportion of incident light at a given wavelength that is transmitted through a material.[5] Higher transmittance means greater transparency, whereas higher absorbance leads to decreased transparency. Over time, aligner materials degrade, causing their optical properties to diminish. Polymer materials absorb water from humidity in the air or when immersed in water. This water absorption typically leads to expansion and alterations in the mechanical properties of the materials. Dimensional changes in these materials caused by hygroscopic expansion in the oral environment can affect the fit of orthodontic appliances, potentially altering the forces they deliver.[6]

There is a lacuna in the understanding of the mechanical and optical properties of various clear aligner materials, particularly those that are directly printed. This study aimed to evaluate the changes in mechanical and optical properties of nine commercially available clear aligner materials after being aged in vitro in artificial saliva for 14 days at 37°C. To date, no studies have compared such a wide range of commercially available clear aligner materials.

MATERIAL AND METHODS

This study involved nine different clear aligner materials, with five samples each from the leading manufacturers. The material thickness ranged from 0.7 to 0.8 mm and included Scheu Duran+, Erkodur, Scheu CA Pro+, Zendura FLX, Taglus PU Flex, Zendura A, Taglus Standard, Erkodur AL, and TC 85 [Table 1]. All materials, except for TC 85, were thermoformed; TC 85 was directly printed. Among the samples, Zendura FLX and Scheu CA Pro+ were multilayered. The materials were immersed in artificial saliva at 37°C in an incubator for 14 days to simulate the oral environment. Measurements were taken both before and after incubation. The rectangular samples of clear aligner material, each measuring 5 × 4 cm, were prepared for testing. Measurements were conducted at a temperature of 30°C using a universal testing machine (AG-X Plus 10 kN, Shimadzu, Japan), as shown in [Figure 1]. The crosshead speed was set to 10 mm/min to generate stress-strain curves. From these curves, the elastic modulus and ultimate tensile strength (UTS) were calculated. The optical properties of each aligner were evaluated by measuring transmittance and absorbance using an ultraviolet (UV)-visible spectrophotometer (UV–2600 240 Volt EN A11665302576 Shimadzu), which is a double-beam single monochromator instrument, as shown in [Figure 2]. Absorbance was measured across the 200–800 nm range. To ensure accuracy and minimize errors due to varying thicknesses, the clear aligner material was exposed to the light source at the buccal surface of the canine.

Figure 1: Universal testing machine with a sample during a tensile test.

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Figure 2: Ultraviolet -visible spectrophotometer.

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Table 1: List of samples in this study

Sl no. Brand name Material Manufacturer Thickness (mm) 1 TC-85 No official information GRAPHY, Seoul, Korea 0.7 2 SCHEU DURAN+ PETG Scheu-Dental GmbH, Germany 0.75 3 ERKODUR PETG Erkodent Erich Kopp GmbH, Germany 0.8 4 SCHEU CA PRO+ Copolyester and thermoplastic elastomer (3 layer) Scheu-Dental GmbH, Germany 0.75 5 ZENDURA FLX Thermoplastic polyurethane and polyester (3-layer) Bay Materials, LLC, California 0.76 6 TAGLUS PU FLEX Polyurethane Taglus - Aligners and Retainer Material, India 0.76 7 ZENDURA A Thermoplastic polyurethane Bay Materials, LLC, California 0.76 8 ERKODUR-AL Copolyester
(No further detail about composition available. Erkodent Erich Kopp GmbH, Germany 0.8 9 Taglus Standard PETG Copolyester Taglus - Aligners and Retainer Material, India 0.76 Statistics

Data were analyzed using the Statistical Packages for the Social Sciences (SPSS) software version 26.0 (SPSS Inc., Chicago, IL), and the level of significance was set at P < 0.05. Inferential statistics to find out the difference between the groups were done using the paired t-test.

RESULTS

[Table 2] shows the results for the within-group comparison of transmittance of aligner materials at 400 nm before and after in vitro aging.

Table 2: Comparison of transmittance at 400 nm

Aligner material PRE POST Variation % Graphy 93.58±0.21 93.055±0.25 -0.56 Erkodur AL 96.237±0.5 94.393±0.45 -1.95 Taglus PU flex 95.536±0.72 91.991±0.60 -3.71 Zendura FLX 93.88±0.41 90.526±0.32 -3.70 DURAN+ 95.089±0.46 90.668±0.47 -4.64 Zendura A 95.722±0.26 95.206±0.34 -0.53 CA Pro+ 94.551±0.31 91.344±0.35 -3.51 Taglus Standard 92.979±0.54 92.715±0.44 -0.28 Erkodur 95.935±0.48 94.015±0.39 -1.92

Materials with higher transmittance values are more transparent. The findings indicate that Erkodur AL is the most transparent, followed by Erkodur, Zendura A, Taglus PU Flex, DURAN+, CA Pro+, Zendura FLX, TC 85, and finally, Taglus Standard 0.762 mm, which shows the lowest transparency. The greatest percentage variation in transmittance is observed in Duran+, while the smallest is in Taglus Standard. Paired t-test between the pre- and post-transmittance groups yielded a P = 0.003, signifying a statistically significant reduction in transparency after 14 days of immersion in artificial saliva at 37°C. The above-mentioned P-values represent the overall comparison between the pre- and post-groups and not between the individual materials tested.

[Table 2] presents a comparison of transmittance values measured at 400 nm. As the transmittance increases, the transparency of the material improves. A graphical representation of transmittance at 400 nm is displayed in [Figure 3]. There is an inverse relationship between transmittance and absorbance, meaning the material with the highest transmittance shows the lowest absorbance. Absorbance values at 400 nm are shown in [Table 3]. Since absorbance is the inverse logarithm of transmittance, this article primarily focuses on transmittance graphs before and after the aging process. A paired t-test between pre- and post-aging absorbance groups yields a P = 0.000, indicating statistical significance.

Table 3: Comparison of absorbance at 400nm

PRE POST Variation % Graphy 0.04±0.02 0.037±0.01 7.5 Erkodur AL 0.03±0.01 0.027±0.01 10 Taglus PU flex 0.037±0.01 0.03±0.02 18.9 Zendura FLX 0.026±0.01 0.024±0.01 7.69 DURAN+ 0.031±0.00 0.025±0.01 19.35 Zendura A 0.031±0.01 0.026±0.01 16.1 CA Pro+ 0.039±0.02 0.034±0.01 12.8 Taglus Standard 0.035±0.01 0.033±0.01 0.05 Erkodur 0.022±0.01 0.02±0.01 9.09 Figure 3: Pre and post transmittance at 400 nm.

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[Table 4 and Figure 4] display the changes in UTS before and after aging. For the aligners Taglus PU Flex, Erkodur AL, Erkodur, and Zendura A, statistically significant differences were observed. Among all the materials, TC 85 had the lowest UTS, while Taglus PU Flex had the highest. A paired t-test comparing pre- and post-aging UTS values showed a statistically significant P = 0.004. [Table 5 and Figure 5] illustrate the changes in elastic modulus before and after immersion. Before immersion, Taglus Flex had the highest elastic modulus, while CA Pro+ had the lowest. After immersion, both materials experienced a decrease in their elastic modulus values. A paired t-test for pre- and post-immersion elastic modulus yielded a P = 0.000, indicating statistical significance.

Table 4: Comparison of ultimate tensile stress

PRE (MPa) POST (MPa) Pvalue Taglus PU Flex 33.87±5.3 29.77±6.4 0.01* Zendura FLX 15.83±3.7 14.91±4.5 0.34 Erkodur AL 19.71±4.2 13.69±4.9 0.01* Erkodur 18.79±5.1 15.53±4.5 0.04* Taglus Standard 17.34±4.7 16.52±5.1 0.54 CA Pro+ 13.21±5.3 12.66±4.5 0.59 Zendura A 27.67±6.1 23.09±4.7 0.02* Duran+ 17.34±4.8 15.81±3.1 0.19 Graphy 13.03±4.1 11.30±3.8 0.16

Table 5: Comparison of elastic modulus

PRE (MPa) POST (MPa) Pvalue Taglus Flex 2671.24±46.4 2650.17±41.3 0.28 ZenduraFLX 1538.31±38.1 1506.47±36,5 0.25 ErkodurAL 1437.24±42.7 1410.25±41.3 0.36 Erkodur 1709.41±47.8 1690.18±44.8 0.49 Taglus Standard 2111.54±59.5 2097.34±53.2 0.46 CA Pro+ 1394.47±34.3 1360.58±31.5 0.25 ZenduraA 1713.18±37.2 1681.43±33.6 0.23 Scheu Duran+ 2416.28±48.1 2373.07±52.7 0.15 Graphy 1462.31±45.9 1457.63±33.5 0.58 Figure 4: Pre and post comparison of ultimate tensile strength.

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Figure 5: Pre and post comparison of elastic modulus.

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The above-mentioned P-values represent the overall comparison between the pre- and post-groups, not between the individual materials tested.

Overall, a reduction in transmittance, absorbance, UTS, and elastic modulus was observed in the post-immersion samples.

DISCUSSION

Thermoplastic appliances have gained popularity in recent years, but severe malocclusions have presented new difficulties for aligners. These challenges are distinct from those associated with traditional fixed appliances, especially in terms of the materials used and the methods by which force is exerted.[7] The mechanical properties of thermoplastics are affected by their molecular structures as well as the surrounding environmental conditions.[8] The presence of cracks and delamination can substantially reduce the mechanical strength of clear aligners, potentially compromising the effectiveness of treatment.[9] Furthermore, cracks in some aligners have been associated with the release of elements such as aluminum, nickel, and zinc, which could pose a risk to individuals with sensitivities or allergies to these metals.[10]

Daniele et al.[11] observed that thermoplastic materials available on the market exhibit a wide range of mechanical properties. UTS refers to a material’s ability to endure stretching loads without breaking. It is determined as the maximum stress a material can withstand when subjected to uniaxial tension. Materials with higher UTS are less prone to fracture under stress. The penetration of water molecules into the aligner’s structure in the oral environment can result in physicochemical changes, leading to mechanical degradation and swelling.[12] An ideal aligner material should exhibit low water absorption. Daniele et al.[11] found that Essix ACE plastic had the lowest water absorption, while aligners made from PU, such as Zendura FLX and Invisalign (reported to be primarily PU), performed the worst. In our study, mechanical degradation was observed in Taglus PU Flex, Erkodur AL, Erkodur, and Zendura A aligners. Among these four materials, there was a statistically significant difference in UTS before and after immersion. Of these, Taglus PU Flex and Zendura A are made of PU, while Erkodur is composed of PETG, and Erkodur AL is a copolyester. The manufacturer did not provide further details about the specific type of copolyester used in Erkodur AL. Chen et al.[13] reported that Zendura FLX sheets did not exhibit significant changes after being immersed in artificial saliva, which differs from the findings of our study. Tamburrino et al.[14] observed a reduction in the tensile yield stress of Duran, although the decrease was not statistically significant.

The elastic modulus, which indicates the stiffness of a material, showed significant variation among different aligner materials. For conventional aligners, the Young’s modulus has been reported to range between 0.5 and 2.2 GPa.[15]

Ranjan et al.[16] reported that Erkodur aligners have lower stiffness compared to Duran, which aligns with the findings of our study. In our comparison of nine different aligner systems, Taglus PU Flex emerged as the stiffest, while CA Pro Plus was identified as the least stiff. For efficient tooth movement, material properties must lie within an ideal range. A higher elastic modulus indicates greater stiffness and resistance to deformation, while materials with superior tensile strength show minimal distortion under stress, making them better suited for precise and controlled orthodontic tooth movement.[17]

Visible light wavelengths start at 400 nm. Therefore, we analyzed the changes in transmittance and absorbance after in vitro aging at 400 nm. The material that shows the maximum transmittance value is more invisible. Cremoni et al.[5] observed a decrease in the transmittance of clear aligners after in vitro aging in artificial saliva. Lombardo et al.[18] observed that the optical properties of orthodontic aligners differ depending on the brand and material composition, but consistently decline with in vitro aging which aligns with our study. During clinical use, the light transmittance may be further modified. Occlusal forces from chewing and bruxism are found to contribute to the opacity of the aligners.[9]

Considering that TC 85 is the only directly printed aligner available in India, a more comprehensive evaluation of its properties, especially its elastic modulus, is essential. In our study, among the materials tested, TC 85 was the third least stiff, with CA Pro Plus being the least stiff, followed by Erkodur AL. This lower stiffness could potentially improve tooth movement due to greater flexibility. Atta et al.[2] found that TC-85 exhibits outstanding shape memory at oral temperatures, which enhances its adaptation, minimizes force decay, and, combined with its greater flexibility, allows for significant tooth movement with each step. Moreover, it retains microhardness comparable to thermoformed sheets, ensuring the durability and effectiveness of the dental aligners. However, in terms of UTS, TC 85 showed the lowest value among all materials tested, suggesting it may be less capable of handling heavy loads or high stress compared to the others. On the positive side, the material exhibited minimal changes after immersion, which is encouraging. Regarding transparency, TC 85 was ranked 8th out of the materials tested. Although transparency differences among the aligners exist, these are minor, implying that, from a clinical perspective, there would be negligible differences in transparency when viewed with the naked eye.

Changes in the optimal properties of aligner materials may be attributed to physical staining and chemical interactions, such as enzymatic activity within the intraoral environment. In addition, exposure to extreme temperature fluctuations, warming to body temperature, and water absorption can decrease the material’s Young’s modulus, potentially reducing the effectiveness of the orthodontic forces applied.[19]

A limitation of this study is its in vitro design, which does not fully replicate the conditions of the oral environment, making it difficult to accurately predict optical performance during intraoral aging. Additionally, since the samples were not subjected to acidic beverages, enzymatic activity, or mechanical stress, the complete scope of changes in their optical and mechanical properties remains uncertain. Our study included nine different aligners; however, to the best of our knowledge, there is currently insufficient literature available to compare our results with other studies.

CONCLUSION

This study evaluated nine commercially available clear aligner materials of comparable thickness. The materials tested included PET-G, PU, and polyester-based types, although the exact composition of some aligners remains unspecified.

There is an overall decline in UTS, elastic modulus, and transparency after immersion, irrespective of the type of clear aligner material used.

Mechanical degradation was observed in Taglus PU Flex, Erkodur AL, Erkodur, and Zendura A aligners. Among these four materials, there was a statistically significant difference in UTS before and after immersion.

TC 85 exhibited the lowest UTS among all the materials tested, indicating it may be less capable of withstanding heavy loads or high stress compared to the others. However, it showed the smallest change in UTS following immersion.

SCHEU CA PRO+ is the least stiff, while Taglus Flex is the stiffest. Erkodur AL was found to be the most transparent clear aligner material, whereas Taglus Standard exhibited the lowest transparency.

The transparency of all nine types of aligners tends to decrease with in vitro aging. Additional research is needed to evaluate the changes in the optical and mechanical properties of clear aligners after they have gone through a cycle of in vivo wear, as this would provide the most accurate results.