Reference structures for assessment of frontal head posture.

The aim of the study was to test (a) the validity of the supra-orbital line as a substitute for the interpupillary line, and (b) the reproducibility of two different approaches of assessing cranio- and cervico-vertical relationships on frontal cephalograms. The material consisted of natural head position frontal cephalograms of 21 healthy Finnish students and 11 Danish young adults prepared according to a method described previously, modified in the Danish sample by addition of spectacles with a wire to indicate the interpupillary line. The cephalograms were analysed manually twice by two investigators with an interval of one week. The following reference lines were selected: (1) orbital line (ORB): a tangent to the extreme cranial point on the supra-orbital margins; (2) cranial line (CR): the line drawn through crista galli and anterior nasal spine; (3) cervical line (CER): a line drawn through the midpoints of atlas (half the distance between the most median points on the tubercle of the transverse ligament), and C4 (half the distance between the most concave points of the lateral masses); (4) the main course of the upper cervical spine (SPINE): a subjective impression of the spinal inclination; (5) the interpupillary line (IP); and (6) the true vertical and horizontal lines (VER, HOR). The angles were measured to the nearest 0.5 degrees. Intra- and inter-examiner reproducibility was calculated. The supra-orbital line almost coincided with the interpupillary line as the mean inclination between the lines was only 0.5 degrees. Both intra- and inter-examiner errors were less for ORB and CER than for CR and the line indicating the main course of the upper cervical spine (SPINE). The intra-examiner reproducibility s(i) for ORB/HOR and CR/VER was 0.4 and 0.8 degrees respectively, and for CER/VER and SPINE/VER 0.8 degrees and 1.0 degrees. Based on these results, the supra-orbital line and the cervical line (defined as the mid-transversal line between the atlas and the fourth cervical vertebra) are recommended as reference lines for assessing frontal head posture.