Effect of head posture on cephalometric sagittal angular measures.

The definitions of cephalometric landmarks include relative terms that are dependent on the orientation of the head (e.g., lowest, most posterior) and head orientation differ widely during cephalometry (e.g., Frankfort plane horizontal, natural head posture). The purpose of this study was to quantify the effect of head posture changes on the commonly used sagittal angular measures SNA, SNB, and SNPogonion (SNPg). The sample comprised 30 Chinese skulls. Cephalograms were taken in a purpose designed skull holder with the Frankfort plane horizontal and at +10 degrees, +20 degrees, +30 degrees, -10 degrees, -20 degrees, and -30 degrees. A geometric grid was used to locate the deepest landmarks, and the landmarks were digitized followed by computer superimposition on the S-N line. To compare the values of the SNA, SNB, and SNPg angles in the different head postures, t tests were used. Head (skull) posture changes produced significant effects on the cephalometric values even at +/- 10 degrees of tilting. Overall, the angles were underestimated by approximately 1 degrees. In general, the angular differences and their standard deviations increased as the skull rotation increased with most of the differences for the SNB and the SNPg angles being clinically significant and also statistically significant at the 0.1% level. It was concluded that head posture needs to be standardized during cephalometry. Changes in posture significantly affect the location of some landmarks and the subsequent data obtained.