## 4 Discussion
In terms of effectiveness, most authors agree that aligners produce primarily dentoalveolar changes, characterized by buccal crown tipping of posterior teeth [11; 15; 33; 31; 32], with the expansion being more effective in the premolar regions and less in the amines [10; 15], and less predictable for the upper arch than for the lower arch [33]. In our study, the expansion achieved greater changes at cusps than at the cervical levels, indicating buccal crown tipping too. In the maxillary arch, the greatest expansion was observed at the first premolar area and the lowest at the canine area. Similarly, in the mandibular arch, the highest expansion rate was observed at the second premolar and the lowest at the canine in both groups. Our data revealed mean transverse expansions at the cusp level of 2.78 mm for the GT group and 2.44 mm for the CA group in the maxillary first premolars, with both systems demonstrating similar efficacy in achieving dentoalveolar expansion at the cusp and cervical levels. These findings align with other clear aligner studies, which reported expansions ranging from 2.6 to 3.7 mm in mixed dentition and 2.2 to 3.2 mm in adults, primarily through controlled tipping [33]. However, these values are lower than those achieved with skeletal techniques such as SARPE and MARPE. For instance, SARPE achieved mean interpolar expansions of 7.0 mm, with skeletal contributions of approximately 3.3 mm [34], while MARPE demonstrated mean expansions of 5.67 mm in interpremolar and 6.18 mm in interpolar distances, offering more parallel expansion patterns and reduced dental tipping [35]. Nonsurgical, tooth-borne appliances like the Haas expander achieved expansions of 4.6 mm for molars and 5.5 mm for second premolars, accompanied by slight buccal tipping (3\({}^{\circ}\)) and alveolar displacement [36]. Lingual appliances achieved comparable dentoalveolar expansions, correcting up to 5 mm for posterior crossbites [37]. Unlike these methods, our findings focused exclusively on intra-arch expansion and did not address crossbite correction. Our study aligns with the consensus that aligners are effective for controlled dentoalveolar expansion, particularly in the premolar region. However, their efficacy for skeletal corrections or crossbite resolution remains limited. This highlights the importance of selecting expansion techniques based on patient-specific requirements and objectives. The predictability of the aligner's expansion is usually assessed by comparing the virtual plan with the post-treatment digital models [30]. Predicted expansion varies between different studies; some authors found significant differences between the results planned on the virtual plan [10; 11; 12; 16; 29; 31], while others did not find significant differences [15; 38; 39], even showing a high degree of predictability [32]. As the virtual plan, i.e., Clincheck(r), tends to overestimate the expansion, many authors also plan an overcorrection during the expansion movement [2; 31]. In our study, the virtual setup of GT and CA treatments showed similar predictability, although with a high degree of variability in both groups, ranging from 42.41% for lower canine cervical width to 95.99% for upper canine cervical width. The variability in the prediction of treatment outcomes was even higher in the GT group. The high degree of variability in the prediction of the results offered by the virtual setup is a common finding among the different authors [10; 11; 12; 13; 14; 29; 30; 31; 32; 33], with percentages of predictability ranging from 45% [38] to around 98-100% [32]. The study findings offer valuable insights for orthodontic treatment planning. Clinicians should consider dental expansion efficacy, treatment duration, and predictability when selecting aligner systems. The GT system is effective for dentoalveolar expansion, offering clinical advantages like shorter first treatment phases and fewer aligners.