Craniofacial reference plane variation and natural head position

Commonly used craniofacial reference planes such as Frankfort Horizontal (FH) and sella-nasion have shortcomings, including their variable interindividual orientation when related to true horizontal (HOR). Therefore, the aim of this study was to evaluate the potential usefulness of a range of craniofacial reference planes to HOR, including those which have not been investigated previously: the Krogman-Walker (KW) line, the neutral horizontal axis, the foramen magnum line, and the posterior maxillary plane. A sample of 57 (38 female, 19 males) consecutive, pre-treatment orthodontic subjects aged 12-18 years were photographically recorded in a standing mirror-guided natural head position (NHP). Cephalograms taken at the same time were traced, orientated to a plumb line (true vertical) transferred from the photograph, and measured. Descriptive statistical analysis including means and standard deviations (SDs) were used to describe average orientation and variability. Thirty-nine of these subjects were photographically recorded 2 months later to test the reproducibility of NHP. The results showed that the variability of the 11 selected craniofacial reference planes related to HOR was generally high. The planes illustrating the lowest variability to HOR were FH and the KW line with SDs of 4.6 and 4.7 degrees, respectively. These, however, showed approximately double the variation in NHP reproducibility (mean square error 2.1 degree). The KW line and palatal plane were also on average orientated closest to HOR. Therefore, the KW line and palatal plane are potential substitutes for the commonly used reference planes in the absence of a reliable NHP. However, NHP still represents a more valid craniofacial reference system than the investigated reference planes.