## 1 Introduction  
In the nonextraction orthodontic treatment of Angle Class I, II, or both malocclusion(s), the treatment procedure occasionally involves distalization of the maxillary molars to ensure an ideal occlusion [1; 2]. The use of temporary anchorage devices (TADs) techniques in recent years has made maxillary molar distalization efficient and feasible [3; 4; 5; 6; 7]. However, orthodontists are often faced with anatomical limitations, such as the maxillary sinus, that may compromise successful orthodontic tooth movement [4; 8; 9].  
The maxillary sinus is the largest paranasal sinus, and histology has shown that most radiographically observed roots protruding into the maxillary sinus are surrounded by a thin cortical bone layer [10]. The maxillary sinus is known to prevent orthodontic movement; therefore, for the purpose of planning an appropriate treatment strategy, it is important to establish the anatomical relationship between the maxillary teeth roots and maxillary sinus floor [8]. When the latter protrudes into the maxillary sinus, apical root resorption and tipping may occur as the tooth horizontally moves across the sinus floor [11]. Furthermore, when the roots of the maxillary teeth intrude into the maxillary sinus, root resorption may be induced when the teeth vertically move [12]. In contrast with this general knowledge, several studies have demonstrated that teeth can be moved through the maxillary sinus without inducing any root resorption [4; 9; 13]. In other words, there is still no consensus on the association between the maxillary sinus and orthodontic treatment. It is thus necessary to evaluate the results of orthodontic treatment through the maxillary sinus using a reproducible, quantitative evaluation to establish an evidence-based safety protocol [8].  
In orthodontics, the evaluation of treatment results, such as tooth movement and root resorption, is generally performed using two-dimensional (2D) panoramic radiography and lateral and anteroposterior cephalometric measurements. Given the limitations of 2D radiography in assessing the topographic relationship between the roots of the maxillary teeth and sinus floor, it has been difficult to accurately establish the relationship between the three-dimensional (3D) tooth movement and root resorption [14; 15]. Although some studies have reported successful orthodontic treatment through the maxillary sinus, only a few researchers have used a 3D evaluation method [4; 9]. Instead, 3D analysis using cone-beam computed tomography (CBCT) images can provide a detailed and accurate assessment of root apices of the maxillary teeth and maxillary sinus [15; 16].  
In this report, we describe the treatment of a man with a skeletal Class I pattern and demonstrate the possibility of moving teeth through the maxillary sinus, which was confirmed on 3D evaluation using CBCT. We are the first to evaluate orthodontic tooth movement through the maxillary sinus using CBCT superimposition. Additionally, the results of this study support the usefulness of the pretreatment, post-treatment, and superimposition CBCT images for diagnosis and evaluation. Our findings provide valuable insights and can be used to avoid orthodontic treatment-related complications.