## 4 Discussion  
The early diagnosis and treatment of canines at high-risk of impaction are essential to providing interceptive treatment and minimizing the risk of more severe complications, such as resorption of adjacent lateral incisors, which can have significant esthetic and functional consequences [47]. Although many studies have been conducted on the possible association between maxillary canine impaction and transverse maxillary contraction, they have led to controversial and contradictory results. To the best of our knowledge, the present study is the first one to investigate the risk of canine impaction in children with transverse maxillary arch constriction and bilateral crossbite, compared to sex- and age-matched patients with normal maxillary width and absence of crossbite. If an association is present between canine impaction and maxillary width, a clear difference would be observed in the analyzed sample. However, our findings did not reveal any statically significant differences in the risk of maxillary canine impaction between children with transverse maxillary deficiency and bilateral crossbite and those without. These results contrast with findings from some available studies that indicate a higher risk of canine impaction in the presence of transverse maxillary deficiency [13; 36; 48; 49; 50]. However, a careful review of the results of these studies [36; 48; 49; 50] highlights that, even in the presence of a statistically significant difference, the clinical significance of such difference is minimal. On average, the difference observed in the maxillary width between subjects with impacted canines and controls accounts for a range between 1.3 and 2.5 mm, which is smaller or similar to the standard deviation reported, thus limiting the clinical utility of their results. Contradictory findings could also be due to the differences in the considered sample size, which could fail to reveal any statistically significant differences if underpower, in the different sex distribution of the included participants [12; 51], and in the radiographic method of measurement collection (i.e., panoramic radiographs vs. CBCT). This latter has been suggested to differ in accuracy in the measurement of canine crown size, [52] angular to the occlusal plane [52], mesio-distal location of the apex of the impacted canine (i.e., sector location in the present study) [53], and buccal-palatal location of both apex and crown [53]. Notably, this difference in the variable measurement influencing canine impaction according to imaging type (2D vs. 3D) was also found in the subsequent treatment provided to manage such cases. For example, orthodontis evaluating their patients by means of a panoramic radiograph were most likely to adopt a more conservative approach, such as watchful waiting; conversely, those utilizing a 3D radiographic examination were more likely to consider the case as more difficult and favor active intervention [53]. When differentiating between bucally and palatally displaced canines, many of these studies supported an association between a decreased transverse maxillary width and bucally displaced upper canines [49; 50], but not with palatally displaced canines [37; 54; 55; 56], which could also explain some discrepancies. The present results are, however, consistent with what was reported by other studies that failed to demonstrate any association with intermodular maxillary width, whether measured on dental casts or through CBCT [23; 30; 33; 37; 38]. Most of the available studies identifying factors related to canine impaction were conducted by comparing measurements of skeletal or dental parameters between subjects with already impacted canines and controls [57]. Conversely, the current study utilized a different approach by comparing cases and controls according to the presence of bilateral crossbite and skeletal transverse maxillary deficiency, and not based on the diagnosis of canine impaction.