Which intracranial plane can be used instead of the true horizontal plane?

<strong>Purpose:</strong> The aim of this study was to evaluate the reliability of the Frankfort horizontal (FH), sella-nasion horizontal (SN-h), optic, and orbitooccipital planes by assessing their variabilities relative to a true horizontal line (TrH) in Class 1, 2, and 3 patients. <strong>Materials and Methods:</strong> Eighty-one pre-treatment lateral cephalometric radiographs (LCRs) (27 each from Class 1, Class 2, and Class 3 based on ANB (°) were taken in the natural head position (NHP). NHP was created using a laser level creating the true vertical line and the TrH perpendicular to the true vertical. The inclinations of the anatomic reference planes were compared with the TrH. <strong>Results:</strong> The orbitooccipital and FH planes were closest to the TrH, with mean values of -0.55 ± 3.26 and -0.60 ± 3.67, respectively. The mean value for the SN-h was 3.33 ± 4.40, whereas the mean value for the optic plane was 4.46 ± 4.58. The ranges were high for all anatomic planes: -9.03° to 8.22° for the FH plane, -8.79° to 6.49° for the orbitooccipital plane, -9.87° to 13.16° for the SN-h, and -4.21° to 16.43° for the optic plane. No significant differences were found in relation to skeletal patterns (orbitooccipital plane; p=0.05, FH plane: p=0.115, SN-h; p=0.156, optic plane; p=0.063, respectively). Regarding sex, there was a significant difference in the optic plane in only Class 1 females (p=0.024). <strong>Conclusion:</strong> The FH and orbitooccipital planes are not the same TrH, but they are the closest reference planes. Variations in reference planes affect diagnosis and therapy.