## 4 Discussion  
Previous studies on conventional aligners have shown that the final thickness of the device may depend on the thermoforming process and might be related to the geometry of the 3D-printed dental cast used for the production, which can vary according to different clinical conditions such as tooth shape and size, arch length, palatal vault depth, and alveolar bone height. Particularly, it was emphasised that within a single thermoformed aligner, different regions of the arch may present different thicknesses [17]. In the present study, no significant association was found between thickness values and curing time/conditions. Moreover, no differences in material heterogeneity were noticed  
Figure 11: Representative renderings visualising the local material thickness of micro-CT scanned direct-printed aligners with a 50 min curing time (blue: 0.00 mm, red: 1.20 mm). Figure 10: Representative renderings visualising the local material thickness of micro-CT scanned direct-printed aligners with a 40 min curing time (blue: 0.00 mm, red: 1.20 mm). Figure 9: Representative renderings visualising the local material thickness of micro-CT scanned direct-printed aligners with a 30 min curing time (blue: 0.00 mm, red: 1.20 mm). among the groups. Despite this, and in line with previous studies, significantly higher thickness values were found at specific locations, i.e., central incisors and the mesiodistal centre of the central fissure. These observations confirm that direct 3D-printed aligners achieve a quite homogeneous output of the set initial thickness whereby they might present isolated locations of increased thicknesses. [PERSON] et al. assessed the accuracy (trueness and precision) of orthodontic models and used four different 3D printers obtained from crowded and spaced dentition for the thermoforming of clear aligners. They concluded that accuracy was affected by the 3D printer employed as well as by the anatomical characteristics of the dental arches [18]. Interestingly, also in the present study, constant variation in thickness values was noted. Hence, also in the direct printing case, different thicknesses were found within single aligners, and further observations are needed to understand to what extent the thickness patterns remain the same for more irregular dentitions. According to another study based on micro-CT scanning, aligners thermoformed over a higher model exhibited lower material thickness values, especially at facial and palatal surfaces [19]. In the cited study, differences in thicknesses between teeth ranged from 0.001 to 0.011 mm, whereas differences between locations ranged from 0.005 to 0.184 mm. In the present study, inter-reference point differences showed higher values, with a maximum of 0.27 mm (estimated difference between the central incisor and the canine). However, when related to the mean aligner thickness, in the worst case this difference shows a smaller weight than in the case of the thermoformed aligners (32.8% of the mean thickness rather than 57.5%), which were overall chosen to be much thinner. Importantly, the aligner thickness can affect not only optical and aesthetical properties, but also the forces and moments expressed by the device, and exert an important influence on the orthodontic force applied to the teeth [12]. Particularly, the lack of homogeneity in thickness at different locations of the arch may lead to a reduced predictability of tooth movements. On the other hand, the homogeneity of thicknesses of 3D-direct-printed aligners has not yet been investigated and, in the present study, no biomechanical testing was performed to assess the clinical impact. Additionally, the intra-aligner heterogeneity may be less pronounced compared to the thermoformed ones, and with adequate 3D planning, it would be possible to modify the local thickness at specific sites for treatment purposes. Coloured dimensional deviation maps (Figures 6-11) were generated in the present study to visualise positive and negative deviations from the planned thickness.