# A Modified Hyrax-Type Expander Is Effective for Distal Bodily Movement of the Maximally First Molar
## 1 Introduction  
Impaction and ectopic eruption of permanent teeth are frequently encountered in orthodontic practice. The prevalence of maxillary canine impaction is reported to be 0.8% to 2.9% [1], which is the second most common after third molar impaction [2, 3]. In Asians, including Japanese people, about 70% of canine eruption disturbances occur on the labial side [4], and this condition is also common among patients consulting orthodontists. Various etiologies have been reported for maxillary canine impaction, including an abnormal position of the tooth germ, lack of space, lack of guidance, and genetic factors [1, 5]. Furthermore, compared with other teeth, the maxillary canines are formed in a deeper layer and thus have a longer eruption path, making them more prone to eruption disturbance [6]. [PERSON] et al. [7] reported that the maxillary lateral incisors and first premolars erupt prior to the canines, and as long as the canines are in their normal eruption position and there is sufficient space, they can erupt unimpeded. However, if the adjacent first premolar erupts with mesial buccal rotation, its root can intrude into the eruption path of the canine. Such an abnormal alignment of the first premolar may result in impaction of the permanent maxillary canine. These findings suggest that maxillary canine impaction is strongly influenced by environmental factors. [PERSON] et al. [8] also stated that because the canines erupt last, a lack of arch space due to premature loss of the deciduous teeth or other causes may result in canine impaction. [PERSON] et al. suggested that the most important point for preventing maxillary canine impaction is to detect tooth malposition as early as possible and predict possible subsequent eruption disturbances [9]. The mean age at which maxillary canines erupt is reported to be 10.10 \(\pm\) 1.01 years [6], and [PERSON] et al. [10] reported that 10-13 years is the optimal age to check for abnormal eruption of the maxillary canines. [PERSON] [11] stated that patients9-10 years of age who do not show a labial bulge should be suspected of having an impacted canine, which should be confirmed radiographically. Furthermore, given that 80-85% of canine eruption disturbances are unilateral [12; 13], it is extremely important to perform a close radiographic examination in patients with a marked left-right difference in canine eruption status [1]. However, in some patients, such examinations are delayed, treatment is delayed, and an impacted maxillary canine may be left in place until the permanent dentition stage. Depending on the severity, impacted teeth and abnormally erupted teeth can cause root resorption and displacement of adjacent teeth, eruption disturbance, and deterioration of the occlusal relationship. It is therefore important to accurately determine their position and applicable treatment methods, and to implement the appropriate treatment [14]. [PERSON] et al. [15] argued that extracting a labially impacted maxillary canine to improve malocclusion should be contraindicated because although it may temporarily improve aesthetics, it may also reduce the patient's functional occlusal force and lead to compromised outcomes in orthodontic treatment. In addition, they state that the most desirable approach is to align the maxillary canines through surgical fenestration and orthodontic treatment. This report presents a case of a patient with eruption disturbance of the maxillary left canine in which the maxillary left second molar was extracted and the maxillary left first molar was distally moved with a Hyrax-type expander (Veltri Monolateral Screw(r); Leone, Firenze, Italy) designed to enable the distal movement of a unilateral first molar in order to create eruption space for the maxillary left canine, resulting in smooth distal molar movement and alignment of the left maxillary canine in the dental arch. Furthermore, the maxillary third molar was also aligned to achieve a good occlusal relationship without reducing the number of teeth.