This study aims to evaluate the position and displacement tendency of the unerupted maxillary canines in orthodontic patients with non-syndromic CD compared to healthy controls, using predictors like sector classification of the crown position and angulation of the canine in the pre-treatment PAN. In addition, the association of the tooth position and tooth agenesis, cleft side, and sex were determined. Thus, a null hypothesis was defined, stating that patients with CD have an increased displacement tendency compared to patients without congenital disorder. This is a novel approach regarding the sample group of patients with CD, e.g. RS and CL/P in this study. Currently, there are no literature reports about canine position and displacement tendency in patients with CD.

Considering the sector classification, no displacement tendency for the maxillary canine was observed in both groups. Meanwhile, the inclination angle yielded a statistically significant higher displacement tendency in patients with CD compared to the healthy controls. Thus, the null hypothesis was fulfilled. Several studies have shown that the sector score parameter in combination with the angulation of the canine to the lateral incisor and the midline were powerful and reliable values [30, 40]. Thus, they recommend that the linear angle measurement in PAN should be combined with the additional assessment of the canine sector position [21, 41, 42]. Despite this, the results of the current study showed a higher correlation between the inclination angles than the sector classification. The inclination angle was proved to be a reliable predictor for the displacement tendency of the maxillary canines, certainly in the CD group given the presence of a CL/P. In this scenario, the authors recommend prioritizing the use of inclination angles as a predictor of the canines’ impaction and using the sector classification as a supplementary. The combination of both methods should increase the estimation of a tooth impaction at the start of orthodontic treatment [34].

In the current literature, the alpha angle according to Ericson and Kurol [30] is the most frequently used parameter to determine the angulation of the canine in patients with CL/P [17, 43]. Hereman et al. set the predictive cut-off value in patients with CL/P at an angle between the canine and the midline higher than 23.8 degrees [44]. The group of Russel and McLeod even increased this value with an angle of more than 45 degrees [45]. Rizell et al. used a modification of the angulation and sector measurements of Ericson and Kurol adapted to dental anomalies occurring in patients with CL/P [30, 46]. They have shown that cleft canines and especially the ones requiring surgical exposure had an increased angulation. These results are in line with this study, as increased alpha angles in patients with CD, particularly on the cleft side were observed. This could be the result of the angle building axis which is the midline of the maxilla in between the central incisors. As this midline is frequently affected by deviations caused by the cleft or tooth agenesis, it can influence the angulation of the line, resulting in an increased alpha angle. In contrast, the beta angle is not affected by a midline deviation, since it is defined by the canine position and the lateral incisor. Therefore, applying the alpha angle proved to be more accurate than the beta, certainly in patients with a CD. This finding goes in line, by regarding the difference in interrater reliability for alpha and beta angles. The beta angle had a superior degree of agreement compared to the alpha angle, meaning that it was easier for the examiner to mark on the radiograph or that it was less affected by the used method. In contrast to this, the alpha angle was more affected by the examiner and, therefore more sensitive considering the presence of anomalies like a cleft or angulation misalignment of the angles’ building involved tooth. Concerning the reliability of the kappa coefficient in the interrater agreement in radiology, it is described as a meaningful value, as it does not account for agreements due to chance alone [47].

Generally, in this study cleft-sided canines have shown a higher risk for dislocation and malposition compared to the non-cleft-sided ones. Not only this latter has to be considered in orthodontic treatment of patients with CD, also the correlation with tooth agenesis [48]. Several authors described that displacement and impaction coincide with tooth agenesis, specifically regarding the lateral incisor [21, 45, 49]. On the one hand, following the guidance theory, this tooth has a crucial role in guiding the maxillary canine to its correct position. On the other hand, the absence of the lateral incisor is frequently associated with the occurrence of a cleft, certainly on the cleft side. This tooth agenesis is more a characteristic sign of a CD and could consequently lead to a displacement of the maxillary canine.

In the current study, the location of the canines was assessed using visual diagnosis of the examiners without an exact standardized scaling of the PAN. In a previous study, tooth agenesis of patients with CL/P or RS were assessed by the same two examiners, and comparably to the current study, the kappa value showed a moderate degree of agreement [48]. This study was performed under the same conditions as the current one. The PANs were analyzed by both examiners independently, in separate rooms, and at different times. To assess the exact canine position, regardless of the reliability and bias of the examiner, three-dimensional images seem to be the more reliable and ideal radiographic technique. In patients with CD, three-dimensional images are performed routinely to evaluate the bone dimensions of the alveolus and palatal cleft. Indications for three-dimensional radiographic imaging are the assessment of bone volume before and after bone grafting or setting skeletal bone anchorage screws for the use of an orthodontic appliance [50]. In combination with these examinations, it is useful to assess the position and the potential agenesis of the teeth to avoid additional images, increased radiation exposure, as well as costs. This means that interdisciplinary therapy can be planned based on one three-dimensional image by providing benefits to the patient and therapists.

Considering the study design, certain limitations need to be mentioned. Regarding the age distribution of the compared patient groups, subjects with CD were statistically significant younger than the healthy ones. This fact could be a bias in the correlation between both groups. Unfortunately, as there is a higher tendency for an earlier orthodontic treatment for patients with CD, this limitation cannot be overcome [51]. The orthodontic treatment starts around the age of four to five years and therefore, the pre-treatment PAN is taken at a younger age compared to orthodontic patients without CD. However, the recommended therapy for patients without CD is set at the beginning of the first transitional stage, which is around the age of six to seven years. Aside from that, the literature describes that the displacement of a maxillary canine is regularly detected at an age of around ten years [52]. Before this age, it can be diagnosed by a missing lateral incisor, asymmetry in tooth eruption of the dental arch, conspicuous familial history, and, also while considering the above-mentioned information, also a radiographic diagnostic. This radiographic diagnosis can happen at a later time point during orthodontic treatment by an attentive orthodontist. Therefore, eliminating the mentioned bias is complex, as more importance was given to employ a pre-treatment PAN that exposed the position of a non-treated canine for the current study. After the therapy starts, e. g. extraction of the primary canine or reducing the crowding of the dental arch, approximately sixty to eighty percent of the canines with a displacement tendency erupt spontaneously [31, 53, 54]. This is a fact, which would have biased the results of the current study, when PANs during orthodontic treatment would have been taken for study purpose. Finally, the obtained sample size was limited but this was given the monocentric character of this study and the low prevalence of the studied patient population.

This is a cross-sectional retrospective study, based on data obtained from the clinical records evaluating the final canine displacement. In the future, the same canine displacement parameters should be evaluated in a prospective study design, both at the beginning of the observation period and towards the end, to evaluate the eruption position and the potential occurrence of a displacement.