In this study, different thermal and mechanical properties of two aligner materials, Duran and Erkodur, were evaluated through experimental methods after exerting thermoforming and in vitro aging. Given that there was a change in all parameters, the null hypothesis was rejected.
Duran and Erkodur aligners are both made of PETG, but they have different thicknesses, which is 1 mm in Duran and 0.8 mm in Erkodur. So, it made this possible to compare the different thicknesses of same composition in addition to evaluating two different materials. In general, Duran had greater thermal and mechanical properties than Erkodur, which can be attributed to its greater thickness.
In both materials, flexural modulus significantly decreased after both thermoforming and thermocycling processes compared to the control group, although only thermoforming caused a significant decrease in flexural modulus, and the changes after thermocycling were not significant in both materials. In other words, thermoforming has a more dominant role to weaken the aligner materials in comparison to in vitro aging, simulating the intraoral aging of aligners for 2 weeks. Considering the significant decrease in the thickness of aligners after thermoforming [11, 15], the reduction in flexural modulus seems justifiable.
In another study by Ryu et al. [11], it was observed that Duran aligners with 0.75-mm thickness showed an increase in flexural modulus after thermoforming, but the ones with 1-mm thickness showed a decrease after this process. The Duran material that was used in the present study also had 1-mm thickness. Furthermore, this study showed that flexural modulus decreases with increasing material thickness; however, in our study Duran, which had greater thickness than Erkodur, showed higher flexural modulus in all groups.
Elkholy et al. [18] evaluated different methods of three-point bending test for measuring flexural modulus for PETG aligners. They suggested that, given that aligners apply forces on teeth through very small deflections and with small distances between force application points on tooth surfaces, the lesser distance between two supports in three point bending test is preferred. In the present study, 11 mm was set as the distance between two supports in UTM, almost similar to the distance between two points of force application in aligners. Also, they have suggested that cracking in the material during the test leads to distorted results. In our study, none of the specimens had macroscopic cracks after the test. Furthermore, they observed a significant decrease in flexural modulus in all groups after thermoforming which is consonant with the present study results.
Another mechanical property evaluated in our study, was Vickers hardness. A previous study by Kohda et al. [13] indicated that there is a strong relation between the hardness of different aligners (Duran, Erkodur, and Hardcast) and the amount of applied force by them. So, changes in hardness can properly indicate the changes in applied force and consequently the efficiency of aligner therapy. In this study, in both materials, hardness significantly decreased after thermoforming. However, a previous study [11] showed that thermoforming does not influence the hardness of Duran.
There is controversy about the effect of aging on hardness in previous studies. Bradley et al. [12] showed a decrease in hardness of Invisalign after using by patients for 44±15 days, but Schuster et al. [10] showed an increase in hardness of the same aligners after 14 days of intraoral aging. It must be considered that the composition of Invisalign and utilized aligners for the present study are different. In the current study, thermocycling did not cause significant change in the hardness of Duran, but caused a significant increase in hardness of Erkodur. This increase in the hardness may be attributed to changes in crystal and amorphous structures or release of plasticizers after exerting intermittent thermal cycles, which should be evaluated more accurately in future studies. Although previous studies have shown no changes in chemical structures of Invisalign aligners after intraoral aging [12] and also in vitro aging [10], to the best of our knowledge, no study has evaluated the changes in chemical structures of PETG aligners. Same as before, thermoforming had a more obvious effect on hardness rather than thermocycling.
On the other hand, Iijima et al. [23] claimed that the hardness of different aligners, such as Duran, does not change significantly after 500 thermal cycles but significantly decreases after 2500 cycles. Since every aligner is frequently used about 2 weeks, we exerted 200 thermal cycles, and similar to the mentioned study, it did not lead to changes in the hardness of Duran.
In the present study, glass transition temperature of aligners was analyzed by both DSC and DMTA. According to DSC, Tg of untreated aligner sheets was 76.3°C in Duran and 76.6°C in Erkodur. Furthermore, previously, this temperature was measured at about 80°C in pure PETG [24], 75.3°C in Duran [23], and 77.2°C in Erkodur [19], which are in line with the results of our study. In this study, the dynamic Tg evaluated by DMTA, significantly decreased after thermoforming and also thermocycling in both materials, indicating the attenuation of thermomechanical properties after these processes.
It was confirmed that the influential factor on the exerted force by aligners is their Tg and not their crystal structure, and glass transition temperature can be an appropriate representative of the efficiency of aligners. Also, they showed that Duran aligners have higher mechanical stability due to their higher Tg than other aligner materials [23], which consequently leads to its higher stability at the maximum rate of increase in oral temperature after consumption of a warm drink (57°C) [25]. Similarly, in the present study, both materials had the same dynamic Tg in the control group, but it was higher in Duran than Erkodur after thermoforming and aging, which confirms the results of the mentioned study.
Now it is well-known that DMTA determines Tg with higher sensitivity and reliability compared to DSC [22] and the resultant temperature has higher values in the first method [19, 26]. In the present study as well, the resultant Tg by DMTA had higher values than DSC in both materials and all groups.
In the current study, elastic modulus was assessed by DMTA. Previously, tensile test or three-point bending test were utilized to determine elastic modulus. In these conventional methods, loading is applied constantly, but in DMTA besides sinusoidal loading, a gradual increase in temperature is exerted, so the thermomechanical properties of viscoelastic materials are evaluated with greater accuracy [21]. Formerly, the importance of assessment of elastic modulus in aligner materials have been indicated, that there is a strong relation between exerted force by aligners and their elastic modulus [13]. Also, elastic modulus, or equivalently storage modulus, stands for the amount of energy that is stored inside the viscoelastic material [21]. So, we can boldly say that the amount of energy remains stored inside the aligner will be then expressed as the force exerted on the teeth.
This analysis showed that elastic modulus decreases significantly by increasing temperature and the intensity of this decrement is greater at higher temperatures, which emphasizes diminished mechanical properties at higher temperatures. In Duran at all three temperatures (25, 37, and 55°C), thermoforming and thermocycling individually did not cause a significant decrease in elastic modulus, but their cumulative effect was significant, which caused about 10.2% decrease in elastic modulus. However, in Erkodur at all three temperatures, elastic modulus decreased significantly after thermoforming and did not change significantly after thermocycling. Therefore, when the aligner is placed over the teeth and starts force application, at constant strain the amount of stress they exert will be less than expected. Two materials had similar elastic modulus in the control group, but then it decreased greater in Erkodur, which reemphasizes the greater mechanical properties of Duran.
In this study, the mean of elastic modulus in untreated sheets at 25°C was 2198.1±118.1MPa in Duran and 2234.9±93.7MPa in Erkodur. A previous study by Daniele et al. as well [19] defined this variable as 2160 to 2430MPa in PETG aligners at 25°C. Also, another study by Ryu et al. [11] observed attenuation of elastic modulus after thermoforming, which was measured by tensile test and was in accordance to our results. On the other hand, Ryokawa et al. [15] observed an increase in elastic modulus of Duran, measured via tensile test, after thermoforming and also after immersion in 37°C distilled water. The difference can be attributed to various evaluation methods of elastic modulus and also simulation of intraoral aging.
Previously, Ihssen et al. [17] evaluated elastic modulus of PETG aligners by tensile test at 22 and 37°C and assessed the effect of thermocycling with 1000 thermal cycles. They observed a significant decrease in elastic modulus after thermocycling; however, in our study, it did not change significantly after 200 cycles. Furthermore, they observed a lower elastic modulus at 37°C, which is consonant with the results of the current study.
Also, viscous modulus and loss factor were evaluated by DMTA. Both of them represent the loss of thermomechanical properties. In other words, the higher the loss factor, the less mechanical stability [21]. Both of these factors had a similar pattern of changes at 25, 37, and 55°C. In both materials, they increased with increasing temperature, and the intensity of this increase was greater at higher temperatures. In both materials and at all three temperatures, these factors increased after thermoforming but did not change significantly after aging, again indicating the prominent role of thermoforming. Also, after thermoforming and especially after thermocycling Erkodur had a higher viscous modulus and loss factor than Duran.
The present report evaluated the influence of aging on clear aligners. However, it should be taken into account that wear [27] or brushing [28] can alter the surface characteristics of the orthodontic materials. Therefore, further studies are needed in order to also consider the possible effects of other unexplored variables.
Generally, considering the normal distribution of data and remarkably low values of standard deviation, DMTA can favorably be utilized for evaluating the different aligner materials in future studies.
The limitations of our study were that intraoral aging was simulated only by thermal cycles, and fatigue caused by loading of occlusal forces was not considered. Also, only aligners composed of PETG were evaluated, and thermomechanical properties of different aligner materials were not compared. It is suggested that in future researches, different aligner compositions such as polyurethane, polyethylene terephthalate, or copolyester and also their different thicknesses be evaluated and compared through DMTA. It is also implicated to evaluate changes in chemical and crystal structures of aligners besides their thermal and mechanical properties, so the cause of the attenuation of these properties after different processes might be understood in basic structures of these materials and get improved in the future.