This study aimed to classify the radiographical characteristics of impacted maxillary canines treated by orthodontists and to investigate the correlation between these radiographical characteristics and the traction time required for impacted maxillary canines. The distance from the maxillary canines to the occlusal plane and to the midline, and the angle between the midline and maxillary canines showed a statistically significant correlation with traction duration. The distance from the maxillary canines to the occlusal plane showed the strongest positive correlation with traction duration. Hence, the null hypothesis of this study was rejected.
Orthodontists often encounter impacted maxillary canines in clinical practice. The treatment for impacted teeth increases the duration and difficulty of the overall orthodontic treatment8. Impacted maxillary canines can cause various complications in the surrounding tissues if orthodontists fail to properly intervene at an early stage9. Before treating impacted maxillary canines, practitioners need to understand the various aspects of impaction associated with impacted maxillary canines, and it is important to use panoramic radiographs to identify radiographic information, such as the angle and location of the impacted canines.
Ericson and Kurol argued that before the age of 10 years, the determination of impaction was significantly early because of variations in eruption pathways11 and that the ectopic position of impacted maxillary canines should be identified before the age of 11. If the palatally impacted maxillary canine is on the distal side of the maxillary lateral incisor’s midline, 91% of these will change their path of eruption spontaneously if the deciduous canine is extracted at the age of 10–13 years16. Based on normal eruption time of the maxillary canines at the age of 11 years, 48 (64.9%) patients were treated with surgical exposure before the age of 11 years; this percentage was higher than that in patients aged over 12 years17. This shows that, in a clinical situation, orthodontists decide the appropriate timing of intervention for these impacted teeth by evaluating impaction patterns and clinical findings, which are best evaluated by panoramic radiographs rather than expecting spontaneous change of eruption path based on the patients’ age.
CBCT findings associated with impacted maxillary canines were mostly distributed in the following order: cystic-appearing lesion (26.4%), root resorption of adjacent teeth (23.0%), supernumerary teeth (4.6%), and odontoma (4.6%). A cyst is an epithelium lining cavity that contains fluid material. Pathologic analysis of the epithelial lining is mandatory to make a definitive diagnosis18. The term ‘cystic-appearing lesion’ is used to distinguish cyst-like lesions on radiographic imaging19. Since we were unable to perform pathologic analysis of all of the impacted canines, we chose cystic-appearing lesion as one of the factors that would represent the radiographical characteristics of impacted canines. The ratio of cystic-appearing lesion to resorption of adjacent roots was as high as 49.4% of the total teeth. For orthodontists, presence of pathological signs (cystic-appearing changes and root resorption) is an important factor to determine whether or not to intervene early in the case of impacted canines. In cases of supernumerary teeth and odontoma, orthodontic traction is rarely performed because normal eruption can be expected only by eliminating the cause.
Several studies have confirmed the reliability of panoramic radiographs for diagnosing impacted maxillary canines when compared to that of CBCT and have stated that the sector, encompassing the canines, is an important factor in predicting the normal eruption of the impacted canines11,12,14. The impacted maxillary canines treated with surgical exposure and orthodontic traction were mostly distributed in the lateral incisor area (60.9%) and least in the deciduous canine area (8.1%), possibly owing to the primary distribution of impacted canines in the area of maxillary lateral incisors. However, if the impacted canines are close to the deciduous canine area, the chances of intervention by orthodontists are reduced in anticipation of a natural eruption. Conversely, owing to anatomical limitations, the chances of orthodontic traction being performed are reduced if the impacted canines are located close to the maxillary central incisors. Location of maxillary lateral incisors in the middle is suggestive of earlier intervention by orthodontists. The ratio of palatal impaction according to the sector showed a statistically significant correlation. Accordingly, as the impacted maxillary canine was located mesially, the proportion of palatally impacted teeth increased possibly because of the increase in the ratio of palatally impacted canines as they moved to the mesial side. However, it is also possible that orthodontists prefer palatally impacted canines to buccally impacted canines because of the latter’s anatomical limitations, such as contact with adjacent roots when they are on the maxillary central incisor area. A statistically significant correlation was found only in the ratio of resorption of the adjacent roots with respect to the ratio of CBCT findings associated with impacted maxillary canines according to the sectors. In the maxillary central incisor area, half of the 18 impacted canines in sector A showed root resorption in adjacent teeth. This indicates that the proximity of the impacted canine on the mesial side causes root resorption in the adjacent teeth.
In a previous study involving the use of panoramic radiographs to perform various measurements on maxillary canines, the angle of the long axis of the impacted canine and the occlusal plane was 48.7 (± 19.3)°20. Similar results were found in the current study at 49.6 (± 17.7)°. Moreover, in previous studies, the distance between the cusp tip of the canine and the occlusal plane was 17.4 mm13 or 15.7–20.2 mm21. However, a small value of 12.0 mm was found in the current study. This can be due to the magnification rate of the radiographic device used; however, we believe that it can be due to the fact that the samples in this study were impacted canines treated with surgical exposure and orthodontic traction. All the measurements showed statistically significant differences between the sectors, except for 3c-OP. The angle between the long axis of the canine and the occlusal plane decreased when the impacted canine was located mesially. The angle between the long axis of the canine and the midline decreases as the impacted canine is located closer to the deciduous canine area and then increases as it moves away from the deciduous canine area. Based on this aspect, it can be inferred that the dental germ of the impacted canine is mostly located in a certain position near the canine area and only the crown portion of the impacted canine is tilted to the mesiodistal side.
In previous studies involving the treatment period of impacted canines, treatment periods of 5.5 (± 3.4) months for 23 impacted canines22 and 8.40 (± 3.26) months for 45 impacted canines in adolescents23 were reported. The average duration of traction in the current study was 13.9 (± 8.9) months, which is a longer period than that reported in previous studies. This can be attributed to the fact that the position and traction difficulty of the impacted canines were different from those reported in previous studies. In addition, the definition of treatment completion for impacted canines differs. Since there are many factors involved in the total orthodontic treatment duration, we only limited the traction duration related to impacted canine. We chose a 0.016 × 0.022-inch NiTi wire because when it is engaged, the location, tip, and the rotated state of the impacted canine are aligned roughly. For each sector and the entire tooth, the differences between the traction durations of the buccally or palatally impacted canines were not statistically significant. Hence, only the simple average values were compared. Examination of the entire impacted canines revealed that impacted canines on the palate side (16.2 [± 11.2] months) had a longer traction duration than those on the buccal side (12.9 [± 7.5] months). In sector A, compared to the average of 17.3 (± 5.9) months for palatal impaction, buccally impacted canines took an average of 22.7 (± 17.9) months. This shows that if the maxillary canine is impacted at the position of the maxillary central incisors, anatomical limitations (e.g., contact with adjacent teeth) will result in a longer time and effort taken to perform the traction of the buccally impacted canine.
A statistical analysis of the correlation between the traction duration and the measurements in panoramic radiographs showed that all measurements showed a statistically significant correlation, except for the angle between the long axis of the impacted canine and the occlusal plane. Sajnani and King reported that the distance from the occlusal plane to the cusp tip of the canine is the most important factor for predicting the impaction state21. Similar to the results of a previous study, of the four measurement values in this study, the distance from the cusp tip of the canine to the occlusal plane showed the strongest correlation with traction duration (p < 0.01). However, this was the only measurement value that did not show statistically significant differences based on sectors. This indicates that when estimating the traction time for treating impacted maxillary canines, the most important consideration is how far the impacted maxillary canine is from the occlusal plane, rather than the angle of the impacted canines.
A limitation of this study is that it was dependent only on diagnoses at a single center when selecting the sample for this study. In addition, we only selected patients treated with surgical exposure and orthodontic traction and did not have additional information when we set up a control group. However, most previous studies focused on predicting the normal eruption of unerupted canines. Thus, there is a lack of research on impacted canines requiring orthodontic intervention, including angle and position on the panoramic radiograph. If a follow-up study is to be conducted in the future, it will be necessary to set up a control group using the diagnosis of a multicenter clinician to focus on the analysis of the characteristics and factors of impacted maxillary canines and to determine whether orthodontic intervention is essential. Furthermore, if we combine the results of this study with recent advancements in artificial intelligence technology, in the near future, simple panoramic images can reveal the expected traction time duration and whether the unerupted canine requires orthodontic intervention.
In conclusion, we found that impacted maxillary canines treated with surgical exposure and orthodontic traction showed increasing possibilities of palatal impaction and resorption of the adjacent root as they were located mesially. Moreover, the distance from the occlusal plane to the impacted maxillary canine showed the strongest positive correlation with traction duration. When establishing treatment plans for patients with impacted maxillary canines, distance from the occlusal plane to the impacted canines, rather than the angle, should be considered in predicting the duration of treatment.