Variation in cephalometric reference lines

The use of intracephalic reference lines in cephalometric assessment is valid only if these lines bear some constant relationship to the true horizontal and to each other. Otherwise, the use of a true horizontal would give the benefit that cephalometric assessment would be based on the same standards as clinical judgment. In using a true horizontal, however, the radiograph must be taken with the head in a reproducible natural posture. A study was carried out to assess the reproducibility of a natural head posture using a simple positioning method in a standard cephalostat, and to assess the variation in the relationship of the sella-nasion, Frankfort, maxillary and mandibular reference lines, to a true horizontal and to each other. Though in their mean inclinations the Frankfort and the maxillary reference lines were almost parallel to each other and to the true horizontal, the range of variation for all the reference lines was considerable. The range of variation in the reference lines was greater than the error of reproduction of head position, and the range of variation of the slope of the mandibular line was no greater than that for the slope of each of the other three lines. The sella-nasion line was reproduced more accurately that the other reference lines, using a digitizer. The possibilities of a simple cephalometric analysis based on a true horizontal are outlined.