Individual variation of the distance between nasion and Frankfort horizontal plane—an error factor of facebow in semi-adjustable articulators

Statement of problem: Semi-adjustable articulators use an average axis facebow with a nasion relator at a fixed distance of 22 mm or 23 mm from the orbitale anterior reference point to locate the axis orbital plane, which is a major factor in measuring the sagittal condylar inclination. Lateral cephalometric radiograph allows the measurement of the distance between the nasion and Frankfort horizontal plane, which has shown great variability among patients. Purpose: Measure and describe individual variation of the distance between nasion and Frankfort horizontal plane, and error factor of facebow in semi-adjustable articulators, in order to assess the importance of an accurate transfer of the axis orbital plane of the patient to the articulator. Material and methods: This cross-sectional analytic clinical study used 637 lateral cephalometric radiographs from the database of 2 private clinics in Santiago de Chile. The distance between the nasion and Frankfort horizontal plane was measured. Then, a correction of -7.1 mm was applied to determine the distance between N and the axis orbital plane, and this corrected distance was analyzed and compared with the one used by Panadent and Whip Mix articulators. In 150 cephalometric radiographs, the measurements were classified according to facial biotype and skeletal class. ANOVA Bonferroni test was applied to recognize how the N to FHP measurements behave according to biotype and skeletal class (P ≤.05). Results: The average corrected distance from nasion to axis orbital plane in the present study was 24.7 mm, 2.7 mm higher than the distance used by Panadent articulator and 1.7 mm higher than the distance used by Whip Mix articulator. No statistically significant differences were found among biotypes or the different skeletal classes. Conclusions: For the studied population, a fixed-value nasion relator is a factor that introduces error.