Estimated natural head position and facial morphology

Estimated natural head position, or natural head orientation (NHO), has been proposed as a preferred reference position for assessing facial morphology. However, because this position is subjectively defined by the orthodontist, it could be influenced by facial form, which would be an undesirable attribute. The aim of this study was to assess whether NHO is influenced by facial morphology. Lateral photographs of 14 patients were used. Each was warped to produce 2 new images, with the chin positioned backwards or forwards relative to the original (+/-2degrees change of the soft tissue N-Pg line). All 42 images were placed in circular frames and shown to 7 experienced orthodontists, who were asked to orient them to NHO. Image orientation between the 3 chin positions was measured and compared. The results showed that NHO depended on chin position. Images with protrusive chins were positioned with the head rotated more downwards (Frankfort plane 3.10degrees relative to horizontal) than were images with retrusive chins (Frankfort plane 4.98degrees). The difference in head orientation measurements was half of the difference in chin position (1.88degrees for a chin change of 4degrees). These findings call into question the validity of NHO for diagnosis because it depends on the same factor it alms to assess. Use of NHO would result in underestimating the true skeletal relationships.