## 4 Discussion
While the thickness of the incisors varies between the NOXI models, they all show a uniform thickness of 0.95 mm in the molars. Despite this uniformity, the models show different forces on the molars, which can be explained by the structural design: different thicknesses in the anterior section also influence the force in the posterior areas. For example, NHVH generates greater forces than NHH, despite having the same thickness in the molars, due to its horizontal and vertical gradients. NHZ, on the other hand, despite having the same thickness as the other models in the molars, exerts lower forces, such as thermoformed scalloped aligners, which produce significantly lower forces. In summary, the NOXI configuration responds to the biomechanical requirements of incisors and molars, ensuring effective force distribution for orthodontic movement. The average force was higher on molars than on incisors for all aligners and measuring points, with some exceptions: scalloped cut models and NOXI models with a high horizontal gradient showed lower values on molars. This finding is consistent with that described by [PERSON] et al. [21], according to whom, molars, having a larger root surface, require more intense forces for orthodontic movement. The NOXI configuration, with increased thicknesses of up to 1 mm in molars, addresses this biomechanical need. With the advent of 3D printing, concerns about mechanical material properties, chemical stability, and technical processes have emerged [30; 31]. [PERSON] et al. [32] found that the mechanical properties of 3D-printed aligners did not deteriorate after one week of intraoral use, while [PERSON] et al. [33] observed more stress relaxation than aligners thermoformed in PET-g at 37 \({}^{\circ}\)C. A subsequent study on 3D-printed materials, Material X (Envisiontec, Inc; Dearborn, MI, USA), and OD-Clear TF (3 DResyns, Barcelona, Spain) showed that the residual stress after 2 h of 2% deformation was less than 10%, indicating a significant reduction in the initial stress, speculating that longer tests would further reduce stress [34]. [PERSON] et al. [23] compared thermoformed aligners, Zendura FLX (Bay Materials LLC, Fremont, CA, USA) and Duran (SCHEU, Iserlohn, Germany), and 3D-printed aligners, Tera Hartz TC-85 (Graphy Inc., Seoul, Republic of Korea) and polyamide (PA, Sweden&Martina, Due Carrare, Padova, Italy), by subjecting them to stress relaxation tests for 8 h at 37 \({}^{\circ}\)C and controlled humidity. The results showed significant differences between Tera Hartz TC-85 and PA; the former showed the lowest stress relaxation values, with a decay from 90% to 100% in both tests. The PA aligners were characterized by a greater ability to maintain strength, with 32% relaxation in test 1 and 23% in test 2, compared to the thermoformed aligners, values that were, however, lower than Duran and Tera Hartz TC-85. In the current study, the different aligners showed no significant differences in stress relaxation, maintaining stable and constant forces comparable to those of existing aligners but with higher initial forces. This stability ensures controlled orthodontic movement, reduces the risk of microtrauma, and improves patient comfort [20]. Discrepancies with previous studies can be attributed to differences in materials, molds, and methodologies used. This study provides valuable insights into the force application of different aligners, particularly NOXI, on the upper central incisor and first molar. Although NOXI exhibited higher force values in vitro, further clinical studies are needed to determine its actual effectiveness in patient treatment. The introduction of PA material and differentiated thicknesses enabled the development of aligners capable of exerting greater forces than traditional thermoformed aligners. While this could potentially compensate for the reduced wear time, its impact on tooth movement remains to be clinically validated. Additionally, aligner performance in real-world conditions may differ from in vitro simulations.