## 4 Results  
Sixteen landmarks were used to assess the possible benefits of orientation with the Acta plane concerning reliability in oriented versus non-orientated scans. The inter-examiner Intraclass Correlation Coefficient (ICC) was 0.997 *. Intra-examiner reliability was also excellent for both examines 1 and 2 (ICC of 0.992 * and 0.999 *, respectively). For all the accessed landmarks in Table 4, reliability was mostly excellent in the non-oriented group (ICC \(>\) 0.90), except for the following coordinates (axis):  
* \(\;\)\(\;\  
When determining the internal reference of the skull, the aim was to determine an origin point in the midsagittal plane, as previous studies have confirmed the consistency of the landmarks in the midsagittal plane [17; 34]. At first, the midpoint between the left and right anterior clinoid processes was considered as the origin point. However, the left and right anterior clinoid processes have been reported as a reference with low reliability [35], and they were asymmetric in dry skulls as well as in many CBCT images from this sample. The second option was to use the most anterior border of the dorsum Sella. However, the Sella landmarks (Sella, Sella inferior, Sella posterior) are highly dependent on the DICOM orientation [17; 35; 36]. On the other hand, the most superior-posterior point of the dorsum Sella seemed more accessible and less orientation-dependent on the CBCT images. However, after using this point to align the CBCT scans, the final head position rolled too far backward. This issue was probably caused by the excessively high position of the dorsum Sella relative to the F-points. To address this discrepancy, the origin point was set in a lower position. One of the options considered was the Basin point. However, its position was too low in regard to the F points, which resulted in a forward-rolled skull after reorientation. As a result, the midpoint between the Basin and the dorsum Sella was adopted as the origin point, named point O. The main advantage of the Acta plane is that it allows the virtual orientation of already existing CBCT scans that, either by accident or insufficient field of view, do not include the image of both Porion points. Using the Porion points as references presents several disadvantages. First, locating the Porion points in CBCT images is challenging, especially in the sagittal plane [25; 37]. Second, because the FHP was first used in 2D, the usual asymmetry between the right and left Porion landmarks was not entirely evident due to the superimposition of bilateral structures. However, the irregular anatomy of the Porion points often means that the discrepancy between the left and right Porion is high [29], and it might cause a tilt in the head when used to orient the 3D images. Also, because the reference points of the Acta plane are more central to the skull, the new plane might allow CBCTs to be taken with a smaller field of view whilst still being able to be oriented afterward, subsequently resulting in a reduction in radiation dose [11; 12; 13]. This would allow orthodontists to obtain scans with lower radiation that would still be acceptable to use for diagnosis (ALADA principle--as low as diagnostically acceptable) [38]. For these reasons, the Acta plane is a reliable option to virtually orient an existing scan without the image of Porion. The main disadvantage of the plane is the high dependency on orbital symmetry. Therefore, in cases in which there is a severe asymmetry, the Acta plane's accuracy still has to be further tested. Also, the stability of point O through growth and time still has to be evaluated. In addition to introducing the Acta plane, this study also aimed to verify whether the reorientation of a CBCT scan with the Acta plane would affect the examiner's reliability in locating landmarks or performing measurements. Without orientation, some of the landmarks chosen to test the plane, such as Porion, Basin, and lateral Orbitas, did not have excellent ICC scores.