A 3-DIMENSIONAL NONINVASIVE STUDY OF HEAD FLEXION AND EXTENSION IN YOUNG NONPATIENT SUBJECTS

Head flexion and extension movements near the natural head position (N HP) were analysed for the location of the mean instantaneous centre of rotation (ICR). Forty-six healthy young adults (30 women and 16 men) with sound dentitions, free from cranio-cervical disorders, performed habitual movements that were automatically detected and measured by an infrared three-dimensional motion analyser. ICR and curvature radius were calculated for each movement and subject. In both extension and f lexion, ICR position changed during the motion. The movement was symme trical in all subjects. No gender or flexion/extension differences wer e found for both ICR position and relevant curvature radius. On averag e, ICR relative to NHP soft-tissue nasion was located at about 150% of the soft-tissue nasion-right tragus distance, with an angle of about 220 degrees relative to the true horizontal. Results suggest that head flexion or extension is always performed with a combination of rotati on (atlanto-occipital joint) and translation (cervical spine) even in the first degrees of motion. Moreover, NHP at rest seems to be some de gree more flexed and anterior than head position during movements. The se relative positions and their muscular determinants could also influ ence mandibular posture at rest and during functional movements.