## 1 Introduction  
In orthodontics, patient records are used for documentation, diagnosis, planning, and evaluation of the outcome of treatment [1; 2; 3; 4]. Although 2D panoramic and cephalometric radiographs have been the standard procedure in orthodontics for a long time, they present limitations such as distortions, superimposition of bilateral structures, and variable enlargement ratios [5; 6; 7]. In more complex cases, such as surgical or asymmetry cases and cases involving impacted teeth, patients, and orthodontists benefit from a three-dimensional (3D) evaluation [8; 9; 10]. In the late 1990s, Cone Beam Computed Tomography (CBCT) technology made it possible to generate 3D skull images efficiently at reasonable cost and radiation levels [3]. It also provides high-resolution images, volumetric assessment of structures, options for 3D reconstruction, and easier access to bilateral structures than a two-dimensional (2D) image [3]. Application of CBCT is expected to increase [8], and although the radiation dose requires critical consideration [6], drastic reductions are possible by modulating CBCT parameters such as the field of view [11; 12; 13]. To analyze and quantify changes in the morphology of the dentofacial complex brought by growth or treatment results, radiographs are taken at different stages and are then evaluated and might even be superimposed [14]. CBCT scans are an adequate choice for this purpose because they are reproducible and consistent regarding landmark identification and measurements, with similar or even higher reliability than 2D cephalometrics [6; 15; 16]. Regarding image superimposition, CBCT scans are just as reliable as 2D cephalometrics [14]. However, reliability in determining landmarks is dependent on the chosen anatomic point or structure, on the examiner's expertise, and on the scan orientation [17; 18; 19]. In a cephalometric radiograph, the physical position of the patient's head during the acquisition permanently affects the final image [20; 21]. During the acquisition of the CBCT image, the patient's position (standing or lying) and the resultant physical orientation of the patient's head are important, as they can affect the mandibular position and, consequently, the airways. However, slight variances in head position may have a limited effect on the final image and still influence the ability of the examiner to locate an anatomical structure efficiently. This problem is easily solved in the CBCT imaging technique because the final 3D image can always be virtually oriented after being taken. In order to measure changes as a result of treatment or growth, CBCT images should be similarly acquired and oriented. However, it is virtually impossible to deliver two CBCT images in which the skull is exactly and precisely positioned in the same direction. Different orientations of CBCT images might influence the operator's ability to identify landmarks and perform measurements, affecting the vertical measurements [22]. The orientation of the scan also affects 3D model segmentation [23]. Therefore, it is essential to virtually orient the CBCT images in order to improve the reliability in landmark detection, measurements, and segmentation. Traditionally, this orientation can be performed with the assistance of a reference plane, and the standard reference plane for 2D cephalometric tracings is the Frankfort Horizontal Plane (FHP) [24]. Therefore, the FHP was automatically applied as a reference for the orientation of 3D images. This plane is delimited by the right and left Porion points (the most superior-posterior margins of the most external point of the external acoustic meatus) and the most inferior points of the lower margins of both Orbitas. Although this plane has been considered a reliable orientation reference in multiplanar reconstructed magnetic resonance images [25; 26], it is highly dependent on the Porion points.