## 4 Discussion  
The aim of this study is to investigate the retention force of 3D-printed Noxi aligners by analyzing different margin designs, thickness gradients, and grip points and comparing  
them with thermoformed aligners. The goal was to enhance the understanding of the factors influencing the effectiveness of orthodontic treatment with aligners and to explore the potential of this innovative device. This in vitro study evaluated the combined impact of margin designs, thickness variations, and grip points on aligner retention. These parameters were compared across different occlusal models, simulating a variety of clinical conditions. Retention is a critical aspect of orthodontic treatment with aligners, as it counteracts vertical displacing forces and ensures greater predictability of tooth movement, thereby enhancing adherence to planned therapeutic objectives. Several factors influence aligner retention [7; 8; 9; 10; 11; 12]. [PERSON] et al. [10] identified margin finishing levels as a key determinant. Their study highlighted that the most retentive aligner design featured straight margins positioned 2 mm above the gingival zenith. They also observed that straight margins increase the rigidity of thermoformed aligners, thereby improving retention and enabling more complex dental movements. A comparison of these findings with a study conducted by the University of Ferrara [31] underscores a similar emphasis on the importance of margin configuration. The results revealed that scalloped margins were the least retentive, followed by straight margins, while straight margins positioned 2 mm above the gingival zenith achieved the highest retention levels. Similarly, [PERSON] et al. [12] investigated the role of grip points in aligner retention. They observed that ellipsoid grip points did not significantly enhance retention, whereas inclined grip points showed a notable improvement, particularly when combined with stiffer materials like CA-hard. In contrast, when using Essix ACE material, ellipsoid grip points were observed to reduce retention. These studies demonstrate that aligner retention is influenced by a combination of factors; however, no universal solution has been identified. The selection of the most suitable aligner design must be tailored to the specific clinical requirements and the materials available. The results of the present study confirm this complexity, showing significant differences in maximum force among various aligner configurations. The null hypothesis was rejected. Comparison of 3D-printed Noxi aligners in polyamide and thermoformed F22 aligners in Evoflex allowed for analysis of retention-related aspects. Notably, scalloped-margin aligners exhibited significantly lower maximum force values compared to Noxi aligners with high margins with horizontal and vertical gradients (\(p\)-value = 0.029). This result suggests that scalloped margin designs compromise retention, while configurations with more rigid and stable margins, such as those in Noxi aligners, are more effective. Similarly, a comparison between scalloped margin aligners and F22 aligners revealed that the maximum force values of the former were significantly lower than those of the latter (\(p\)-value = 0.008). This result confirms that, although the Evoflex material used in F22 is less advanced than polyamide, the straight margin design provides greater adhesion than scalloped margins. Furthermore, a comparison between Noxi aligners with zenith margins with uniform gradients and F22 aligners showed significantly lower values for the former (\(p\)-value = 0.040). This result may indicate that uniform configurations, while effective in certain clinical situations, do not guarantee the same retention level offered by gradient configurations. At a marginal significance level (\(p\)-value = 0.0866), Noxi aligners with high margins with uniform gradients had slightly lower values than F22 aligners.