# A Modified Hyrax-Type Expander Is Effective for Distal Bodily Movement of the Maximally First Molar
## 7 Discussion  
### Treatment Planning Focusing on the Presence of the Third Molars  
Because the maxillary amines play an important role both esthetically and functionally, it is recommended that the maximum effort be made to guide them into the dental arch even when they are erupting in abnormal positions or impacted [18]. In the present case, the crown of the maxillary left canine had erupted near the labial root surface of the left lateral incisor, while panoramic radiographs and CBCT images showed that its root apex was located between the lateral incisor and first premolar. We therefore assumed that if the left maxillary canine could be distally inclined and tractioned, it could be aligned between the maxillary left lateral incisor and first premolar. A plan was thus made to traction the ectopically erupted maxillary left canine to align it in the dental arch. However, because of the lack of room for eruption due to the mesial positioning of the maxillary left molar, it was necessary to create space for the eruption of the maxillary left canine. The well-known methods of creating eruption space include expansion of the dental arch and extraction of anterior teeth and premolars. In the present case, however, the maxillary space shortage was 7.5 mm, which was considered difficult to obtain only by expanding the dental arch. Tooth extraction was thus considered necessary to gain eruption space.  
According to previous reports, in patients with a maxillary third molar of proper size, shape, and position, extracting the maxillary second molar to allow for the eruption of the maxillary third molar is effective for improving Angle Class II malocclusion [21; 22; 23]. [PERSON] et al. noted that the advantage of extracting a maxillary second molar is that it increases the efficiency of distal movement of the maxillary first molar and allows the first molars to achieve a Class I occlusal relationship [24]. It has also been reported that extraction of the maxillary second molar results in eruption of the maxillary third molar with a likelihood of 96.2% to 99% [25; 26]. To allow the maxillary third molar to erupt in the appropriate position, it is recommended that the second molar be extracted when the third molar reaches the root-crown junction of the second molar or when calcification of the third molar crown is almost complete [27]. In the present case, panoramic and lateral cephalometric radiographs showed that the crown of the maxillary left third molar was upright and almost as large as that of the ipsilateral first and second molars, and calcification of the crown was almost complete. The patient also had a straight facial profile, a labially impacted maxillary canine, and Angle Class II malocclusion with a slightly under-spaced mandibular arch. These features made maxillary left second molar extraction a viable treatment option. During treatment, extraction of the maxillary left second molar allowed for efficient distal movement of the maxillary left first molar, and subsequent successful alignment of the maxillary left canine.  
These results suggest that gaining eruption space for the maxillary canine by extracting the second molar and distally moving the first molar, taking into consideration the presence of the maxillary third molar and the position and size of its tooth germ, is an effective treatment method.