The Frankfort horizontal as a basis for cephalometric analysis.

A random sample of 79 British 12-year-old children was studied from tracings registered in the natural head position (NHP). The angle between the Frankfort horizontal (FH) and the horizontal at right angles to the plumb line was measured. Two experienced assessors checked every tracing; for those children perceived to show unnatural head position this was adjusted to what they considered to be the natural head orientation (NHO). The NHO was defined as the head orientation of the subject perceived by the clinician, based on general experience, as the NHP in a standing, relaxed body and head posture, when the subject is looking at a distant point at eye level. The FHK horizontal angle was then measured, now related to the corrected head position. The standard deviation for the latter angle was smaller than that of the uncorrected, but still too large for the FH to be considered reliable as a basis for clinical cephalometric analysis. No statistically significant difference in variability as found between FH and the sella-nasion line. The extracranial horizontal plane related to NHO was recommended as the least variable of the references studied.