Variability of the inclination of anatomic horizontal reference planes of the craniofacial complex in relation to the true horizontal line in orthognathic patients

Introduction: The purpose of this study was to assess the reliability of the Frankfort horizontal (FH), sella-nasion horizontal, and optic planes in terms of their variabilities in relation to a true horizontal line in orthognathic surgery patients. Methods: Thirty-six consecutive presurgical orthognathic patients (13 male, 23 female; age range, 16-35 years; 30 white, 6 African Caribbean) had lateral cephalometric radiographs taken in natural head position, with a plumb line orientating the true vertical line, and the true horizontal line perpendicular to the true vertical. The inclinations of the anatomic reference planes were compared with the true horizontal. Results: The FH plane was found to be on average closest to the true horizontal, with a mean of -1.6 degrees (SD, 3.4 degrees), whereas the sella-nasion horizontal and the optic plane had means of 2.1 degrees (SD, 5.1 degrees) and 3.2 degrees (SD, 4.7 degrees), respectively. The FH showed the least variability of the 3 anatomic planes. The ranges of variability were high for all anatomic planes: -8 degrees to 8 degrees for the FH, -8 degrees to 15 degrees for the sella-nasion horizontal, and -6 degrees to 13 degrees for the optic plane. No significant differences were found in relation to patients' sex, skeletal patterns, or ethnic backgrounds. Conclusions: The clinically significant variability in the inclinations of anatomic reference planes in relation to the true horizontal plane makes their use unreliable in orthognathic patients.