## 4 Discussion  
In this patient, we performed distal movement of the maxillary molars, premolars, and canines to reduce the crowding using TADs [1,2,4]. In addition, we performed uprighting of the mandibular molars to eliminate mandibular anterior crowding and established Angle Class I relationships [1]. Because we had extracted the third molars, there was enough bone in the posterior maxilla and mandible to induce molar distalization [1,2,5]. Previous clinical reports have suggested a treatment plan that includes distalization of the mandibular or maxillary dentition to correct Class I or Class II malocclusion is preferred because this method avoids extraction [1,2,4,23]. However, in this patient, the maxillary sinus floor was inferiorly located and appeared to be in the vicinity of the roots of the maxillary molars, according to the pretreatment panoramic radiograph [14]. Furthermore, 3D analysis based on the CBCT images indicated that most roots of the maxillary molars contacted or protruded into the maxillary sinus floor [15,16]. Thus, the risks of tipping and root resorption during orthodontic treatment were high [10,11]. The maxillary floor and bone-deficient areas of the sinus are known barriers to successful orthodontic tooth movement [4,8,10,11,12]. Tipping and root resorption may be induced while the tooth horizontally moves across the sinus floor [11]. Therefore, orthodontic treatment through the maxillary sinus is considered challenging [10,11,12]. However, several recent case reports have shown that bodily movement can be achieved during orthodontic treatment through the maxillary sinus to close a tooth extraction space [4,9,13,24]. Additionally, a previous animal study indicated that bone reduction of the maxillary sinus and root resorption could be avoided by applying an optimum force [25]. In contrast, excessive orthodontic force was suggested to induce root resorption during orthodontic treatment through the maxillary sinus [26]. In general, the degree of root resorption depends on both the orthodontic force strength and the duration [27]. The application of heavy forces during orthodontic tooth movement causes increases in the severity and incidence of root resorption [25,26]. According to previous reports, applying a constant force of 50-200 g is recommended during orthodontic treatment through the maxillary sinus [4,9,13,24]. Therefore, we used the ISW for the initial leveling of maxillary posterior crowding, especially for the buccal inclination of the second molars. An ISW can maintain the shape into which it is molded, is superelastic, and is amenable to applying light, continuous orthodontic force [2,3,28,29]. After leveling the maxillary teeth using the ISW, distalization of the maxillary molars was initiated with 100 g and 50 g nickel-titanium open-coil springs (the first and second molars, respectively) on both sides, using the SSW. As a result, tooth movement was achieved through the maxillary sinus using light force throughout the orthodontic treatment, as referred to in previous reports [4,9,13,24]. However, evidence to date related to orthodontic treatment through the maxillary sinus is based on only a few case reports [8]. In this case, based on the superposition of CBCT, we confirmed that the roots of the maxillary molars not only distally moved through the maxillary sinus but also moved in the palatal direction without inducing any considerable root resorption. Changes in the surrounding tissues of the roots were also confirmed with the tooth movement