## 4 Discussion  
The variation in methods of defining a plane depends on the selected points while transferring the definition of a 2D plane to a 3D plane. The variations in the different FH plane definitions arise from the number of landmarks involved and the orientation preferences. However, the rationales behind the choice are either not stated or are circumstantial, thus leading to an element of contention. Several studies have reported on such variations in the past decade. They have focused on identifying a new reference point or new horizontal plane of reference [14, 20]. However, the abovementioned studies differ from the practical application of 3D CBCT analysis, one that can be adapted in a clinical setup. Researchers have drawn their conclusions using customized setups that necessitate the extraction of threshold-based stereolithographic (STL) models. This, in turn, would not be viable in a typical clinical setup [3, 20]. [PERSON] et al. suggested the use of an internal acoustic foramen as an alternative landmark for superior reproducibility, compared to Po [20]. However, in contrast to their study that involved the use of dry human skulls, soft tissue interaction and the limitation of exposure dose pose challenges to the effective visualization of intracranial landmarks in a clinical scenario. Similarly, [PERSON] et al. suggested substituting the inion (tip of the external occipital protuberance) for Po, thus making it essential to capture a large FOV image, with an increase in the anteroposterior dimension [11]. We used a commercially designed software for clinics without additional customization in the acquired images or code. Furthermore, our subjects included the cases indicated for a CBCT scan, using FOV in the ethical guidelines [21] and abiding by the \"as low as diagnostically acceptable\" (ALADA) [22] principle. [PERSON] et al. reported higher discordance and lower specificity while using alternate planes for cephalometric evaluation [14]. This suggests the importance of the FH plane and the significance of its evaluation for its basic possible permutations (FH1 to FH4). This was in agreement with our objectives. Previous studies have also reported a similar objective to compare the influence of different horizontal reference planes in cephalometry. [PERSON] et al. [3] compared all four definitions, FH1 to FH4, used in the present study, along with the constructed medial points. However, their study was limited to common anteroposterior measurements. In contrast, [PERSON] et al. [12] and [PERSON] et al. [23] included only FH3 or FH4 and limited the comparison to the occlusal cant and landmark deviation, respectively. [PERSON] et al. [20] conducted a pairwise comparison for the absolute angular differences for the horizontal planes. However, the included planes comprised the derived landmarks, and they did not consider the standard definition (FH1). Notwithstanding the cephalometric comparison reported in  
Figure 4: Influence of the different FH plane definitions on the (**A**) Me deviation and (**B**) bilateral Ag to MSP (error bars represent standard deviation). Me, menton; Ag, antegonion; MSP, midsagittal plane. Differences in the lower-case letters indicate a significant difference between the groups. * \(p<0.05\), *** \(p<0.001\). the abovementioned studies, they did not report an objective assessment of the deviation between the planes (SOL), unlike the present study. Alternate definitions for the FH plane in 3D cephalometry have often advocated the use of a median landmark for either porion or orbitale [24]. Adherence to such a definition will incorporate a constructed point, calculated based on the underlying algorithm. Thus, it will present a tendency to vary within the range of anatomical coordinates. In other words, a median-constructed landmark will offer one of the multiple possibilities of a plane within the range of the defined anatomic landmarks (Or and/or Po). The SOL comparison validates the presence of significant differences, even with anatomical landmarks.