# Engineering and Clinical Study of Surface Geometry of Clear Aligners at the Nanoscale
## 3 Results and Discussion
### Comparison to Similar Studies  
As mentioned in the preceding sections, there is still a significant information gap concerning elastic, geometric and mechanical changes occurring in the aligner material under the influence of various factors associated either with manufacturing procedures, pre-use handling or therapy treatment. Most of this is due to an inconsistency in the results obtained in studies on degradation and wear processes of the surface of the aligners from the point of view of changes in its texture, i.e., its well-defined shape. [PERSON] et al. reported that the surface morphology of the aligners under study showed some defects after clinical use (2 weeks), however, there was no significant difference in mechanical properties [22]. However, [PERSON] et al. found that the mechanical properties of thermoplastic materials were influenced by both structural factors, such as molecular and crystal structures, and environmental factors, such as temperature, humidity, pressure and heat history. Therefore, the mechanical properties varied under the influence of both forming conditions and the conditions of use [23]. A similar conclusion was drawn by [PERSON] et al., who did not find any detectable chemical changes in their aligners; however, they revealed that intra-oral aging adversely affected the mechanical properties of the Invisalign(r) appliance [13]. Another study conducted by [PERSON] et al. brought the conclusion that Invisalign(r) aligners retrieved after 14 days of wear displayed optical, chemical and morphological changes compared to new specimens; however, further studies are required to evaluate how intra-oral conditions may influence the optical properties and chemical stability of aligners [24]. This problem is not solely limited to the thermoformed polymer of Invisalign(r). In the work by [PERSON] et al., it was reported that on three different materials from which the clear aligners are manufactured (Duran(r), Biolon(r) and Zendura(r)) the impact of the operating conditions on the mechanical properties could vary according to the specific polymer [11]. The common point in all of the mentioned works was studying the properties  
Figure 6: Graphical comparison of raw data and descriptive statistics of functional parameters: (**A**) thicknesses of the main tribological layers: peaks Spk, core Sk and valleys Svk, (**B**) material shares of individual surface layers, i.e., the boundaries between the layers of peaks and core Smrk1 and core and valleys Smrk2 and (**C**) volumetric specific shares of the layers: peaks Vmp, core Vmc and void valleys Vvv. The latter parameters comprehensively combine the thicknesses of the tribological layers with their material shares.  
of the bulk material or the surface properties at the micro- or even macro-scale. Our approach is therefore different, and the obtained results cannot be directly compared due to allometricity (non-linear variations in some properties depending on the magnification).  
The results presented here reveal the flattening occurring in the upper layer of the surface of the aligners, but the adhesion force Fadh remains the same regardless of the daily use of the aligners. Moreover, it is only the Young's modulus that statistically decreases from 1010 MPa to 917 MPa in new and worn aligners, respectively. Despite that, several other parameters describing surface geometry do not change significantly. Therefore, only the elastic properties and selected geometric properties measured at the nanoscale are found to vary under the influence of the conditions of use, but the majority of other measured parameters remain intact.