# Impact of Dental Model Height on Thermoformed PET-G Aligner Thickness--An In Vitro Micro-CT Study
## 4 Discussion
Therefore, aligner thickness is not the only factor influencing resulting forces and moments. Scientific disagreement persists concerning the capability of aligners to perform bodily tooth movement [29; 31; 33; 34], which necessitates application of a counterbalancing moment for control of root position [35]. In the present study, aligner specimens were thinnest at the facial/facial-cervical aspects, which is in line with a recent study revealing a 50% reduction of layer thickness in buccal-gingival regions [36]. Previous research demonstrated that low thickness values can lead to aligner deformation at the gingival margin [15], eventually resulting in reduced root control. However, besides the rigidity of aligners, application of attachments and shape modification like power buttons or ridges are also relevant to control tooth movement [37; 32]. Thus, the impact of the facial/facial-gingival thinning should be further explored in future investigations. The impact of dental model height has not been addressed in previous studies, despite its potential association with aligner thickness values, and therefore, the resulting force systems. In the present investigation, aligners thermoformed on the higher model showed increased homogeneity of material thickness values, but also a decreased overall thickness. Therefore, it may be speculated that adopting raw material thickness and height of 3D-printed models may be advantageous to better control the material thickness properties of thermoformed aligners, and eventually the resulting force systems. Additionally, as crown height varies among patients [38], clinicians should also carefully consider that higher crowns may lead to reduced thickness values, and vice versa. Posterior bite opening is one of the common adverse effects in aligner therapy. Also, perfect occlusal finishing is immanently complicated by the interposition of aligner material between occluding teeth [39; 40; 41]. The results of this study demonstrated that aligners were particularly thick at cusp tips, which constitutes the almost inevitable bite-lowering effect of aligners. Manual fabrication of aligners inevitably leads to a certain degree of error: manual trimming of the aligner edges causes minor length discrepancies, and manual removal of the aligner from the thermoforming model may lead to shape deformations. Therefore, the thermoforming process was repeated 10-times for each group, and minor differences in thickness were seen at the various measurement locations. However, the quartile ranges were much smaller at the respective measurement points compared to the within-group variability among measurement locations, thus confirming the precise fabrication of aligners in the present investigation. To avoid deformation during micro-CT scanning, the aligners were held in place by polyurethane foam. Nonetheless, minor aligner deformations due to contact to the foam cannot be ruled out. Despite these minor shortcomings, it could be demonstrated, that micro-CT-scanning is an effective method to assess aligner thickness three-dimensionally. In contrast to previous studies, thickness values were automatically computed upon successful segmentation by means of a 3D-distance transform [42]. This method has the advantage of being resistant against oblique measurements which may occur whenever the micro-CT slices are not orthogonal to aligner surfaces. Limitations associated with the present analysis include that only upper models were utilized. Due to differences in arch- and tooth-shape, future studies should also investigate thickness homogeneity and the impact of model height for both dental arches. Additionally, only one raw material of the same thickness was utilized. Therefore, it remains unclear whether the results of the present study can be transferred to different aligner polymers and other material thicknesses. Eventually, no a-priori sample size calculation could be performed, as no eligible study could be identified at that time.